Tungurahua

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  • Ecuador
  • Ecuador
  • Stratovolcano
  • 2014 CE
  • Country
  • Subregion Name
  • Primary Volcano Type
  • Last Known Eruption
  • 1.467°S
  • 78.442°W

  • 5023 m
    16475 ft

  • 352080
  • Latitude
  • Longitude

  • Summit
    Elevation

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2 April-8 April 2014

IG reported that seismicity at Tungurahua steadily increased from 2-4 April. On 2 April two small explosions, at 0757 and 2305, were accompanied by roaring and incandescent blocks rolling down the flanks. The second explosion ejected incandescent blocks and produced an ash plume that rose 600 m. Ashfall was reported in Cotaló (8 km NW) and Chacauco (NW). Although cloud cover often prevented visual observations, an ash plume generated by an explosion at 1455 on 4 April rose 2 km above the crater and drifted SW; ash fell in Choglontus (SW). On 4 April an explosion at 1810 lasted five minutes and generated pyroclastic flows that descended the NW and N flanks. An ash plume rose 10 km above the crater and drifted SW. Another explosion at 1816 lasted four minutes and possibly generated pyroclastic flows. Tephra up to 7 cm in diameter fell in Cusúa (8 km NW) and Píllaro. Constant tremor continued, interspersed with explosions. Strombolian activity was observed during the morning of 5 April. Steam-and-gas emissions with small amounts of ash rose less than 1 km and drifted W. At 1040 an ash plume rose 2 km. On 6 April ash plumes drifted W, and Strombolian activity ejected material that was deposited 1.5 km down the flanks. Ashfall was reported on 7 April in Bilbao (W) and Cevallos (23 km NW). On 8 April steam emissions with some ash rose 200 m and drifted SW. Minor ashfall was reported in Bilbao, El Manzano (8 km SW), Juive (7 km NNW), Mocha (25 km WNW), El Manzano. Large lahars descended the Achupashal (NW) and Confesionario drainages (WSW).

Source: Instituto Geofísico-Escuela Politécnica Nacional (IG)



Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

09/1999 (BGVN 24:09) Elevated seismicity and SO2 fluxes led to an eruption on 5 October

10/1999 (BGVN 24:10) Crisis continues with few earthquakes, abundant tremor, high SO2, and explosions

11/1999 (BGVN 24:11) Explosions continue; 1998-99 summary; ~25,000 people displaced for months

01/2000 (BGVN 25:01) More than two-fold increase in long-period earthquakes during December

04/2000 (BGVN 25:04) Frequent early-2000 explosions with loud reports; ash plumes to 10 km altitude

07/2000 (BGVN 25:07) January-July volcanism possibly decreased; lava fountains and many lahars

07/2001 (BGVN 26:07) Summary of August 2000-August 2001 eruptive activity

01/2002 (BGVN 27:01) Powerful tremor, plumes, 600-m-high lava fountains, and lahars during 2001

08/2002 (BGVN 27:08) High seismicity, lava fountains, and explosions during January-June 2002

12/2002 (BGVN 27:12) Summary of 2002 activity includes several episodes of intense seismicity

11/2003 (BGVN 28:11) Ash eruptions and other activity throughout 2003, but elevated after August

01/2004 (BGVN 29:01) Frequent ash plumes, prompting occasional ash falls through January 2004

06/2005 (BGVN 30:06) Ash plumes and LP earthquakes still common in 2004-2005

07/2006 (BGVN 31:07) 14-15 July 2006 eruption, the strongest since onset of eruption in 1999

12/2006 (BGVN 31:12) Intense ongoing activity in 2006; new bulge on the N flank

04/2007 (BGVN 32:04) Post-eruptive quiet spurs return of residents, but activity increases again in 2007

08/2007 (BGVN 32:08) During March-July 2007, many lahars; variable eruptive behavior

06/2008 (BGVN 33:06) Explosions up to 14 km altitude during July 2007 to February 2008

07/2009 (BGVN 34:07) Eruptions in 2008-2009; two fatalities in sudden flood on 22 August 2008

08/2009 (BGVN 34:08) Vascún River map; lahar concerns; decreased activity in mid-2009

03/2013 (BGVN 38:03) Return of explosions and earthquakes through at least October 2010


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

All times are local (= UTC - 5 hours)

09/1999 (BGVN 24:09) Elevated seismicity and SO2 fluxes led to an eruption on 5 October

In mid-September, increasing seismic activity was recorded at the volcano, continuing into the first week of October. As a result of this increased activity, instrumentation for a new deformation network was installed on the W-side of the volcano and 10 new seismic stations were installed on the N-side and at other locations on the volcano. In late September, an inclinometer was installed adjacent to the seismically active area and a Yellow alert was declared, which continued as of 5 October.

Increased seismicity started on 14 September in conjunction with increased gas emissions, with plumes rising up to 3 km above the volcano. On 1 October, a column of vapor and gas rose to a height of 1 km. COSPEC measurements on 2 and 4 October indicated elevated SO2 fluxes of ~4,300 and ~9,500 tons/day, respectively. Then on the morning of 5 October three explosions at 0721, 0738, and 0743 threw blocks of rock and ash around the crater. The largest in this sequence, at 0738, yielded a reduced displacement of 25 cm2 and explosion hypocenters 4-5 km under the crater. During the night of the 4th, seismicity had reduced considerably and the activity that followed appeared to have produced a seal, leading to the subsequent explosions.

One particularly vulnerable town, Baños, was evacuated during the current crisis.

Reference. Hall, M., Robin, C., Beate, B., Mothes, P., Monzier, M., 1999. Tungurahua Volcano, Ecuador: structure, eruptive history and hazards. Journal of Volcanology and Geothermal Research, v. 91, p. 1-21.

Information Contact: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

10/1999 (BGVN 24:10) Crisis continues with few earthquakes, abundant tremor, high SO2, and explosions

This report briefly summarizes Tungurahua's activity during the period 6-29 October 1999. During this period there was increasing seismicity, particularly tremor, which began in September. Relatively continuous gas emissions also prevailed, with most plumes rising to altitudes of 1-5 km above the summit and extending in various directions as far as 16 km from the volcano.

On 15 October, electronic distance measurements and inclinometer data on the cone's W side indicated an anomalous deformation on the W-NW flanks. That evening a strong collapse of incandescent rocks took place over the higher side of the volcano. A magmatic body was soon seen or inferred very close to the surface below the area of fallen rock. Due to the rapid change in the cone, the Geophysical Institute recommended a change of status from yellow to orange for the most vulnerable zones on the W and SW sides of the volcano. This change was implemented at 0900 on 16 October and prevailed throughout the remainder of the reporting period. That same day pilots were warned of a plume to ~5 km extending to 16 km towards the W. Light ash fell. A flight the next day, 17 October, disclosed an enlarged crater that had expanded by unspecified amounts on its E and SE sides.

On 17 October light ash fell from a plume reaching 3-5 km high. In the afternoon observers saw incandescent rocks on volcano's W side. On 18-24 October plumes were seen by pilots rising to 7-8 km in altitude. Along the highway between the cities of Penipe and Baños recent deposits of two mudflows were found in the Chontapamba and Rea valleys. On 24 October more incandescent rock was seen on the volcano's N side.

Tremor remained at a constantly high level throughout the reporting period but other kinds of seismicity remained relatively low. The most frequently occurring earthquakes, those of long period, typically occurred fewer than 20 times a day. Volcano-tectonic earthquakes and hybrid earthquakes took place less than half as often.

In addition, gas plumes appeared to rise to higher altitudes beginning in mid-October, and incandescent rocks reached distances of 1 km below the volcano summit. Tremor amplitudes grew, saturating seismic stations near the crater and thwarting the ability to recognize local events. This tremor accompanied numerous explosions, 45 being recorded during the last few days of the period.

SO2 fluxes were estimated by either daily COSPEC readings, weather permitting, or sulfuric anhydride measurements, or both. The indicated values ranged from as low as 3,100 to as much as 10,800 tons/day with wide variability (table 1). These high values were taken as indicating elevated magmatic gas, in accord with the observed emissions.

Table 1. Daily values of SO2 flux (metric tons/day) at Tungurahua, 6-27 October 1999. These estimates were based on COSPEC or sulfur anhydrite measurement or both. Plumes on additional days were not measured quantitatively. Courtesy of the Geophysical Institute.

    Date (1999)   SO2 flux (metric tons/day)

    06 October    5,600
    07 October    5,600 and 6,400
    08 October    5,700 and 9,300
    09 October    8,900 and 3,300
    10 October    9700; 10,800; 5,300; and 3,100
    14 October    4,786
    15 October    5,000
    23 October    7,800
    27 October    7,580

Information Contact: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

11/1999 (BGVN 24:11) Explosions continue; 1998-99 summary; ~25,000 people displaced for months

Frequent explosive eruptions continued at Tungurahua volcano through 30 November (figure 1 and table 2). Ash plumes rose to maximum heights of about 5 km above the summit. Daily explosions increased during the month, reaching a peak during 16-25 November before decreasing slightly (figures 2 and 3). On 19 November 0.5 mm of ash fell on Baños, 9 km NNE of the summit at an elevation of ~1,850 m. Two millimeters of ash fell on the town of Runtún farther up slope at ~2,350 m elevation and ~6.2 km NE of the summit.

Figure 1. An aerial oblique photograph of Tungurahua taken from the W during July 1974 shows the morphology of the snow-and-ice-covered summit crater prior to the current eruption. Courtesy of the Geophysical Institute.

Table 2. Explosions and other activity at Tungurahua as described in daily reports, 31 October to 30 November 1999. Courtesy of the Geophysical Institute.

    Date     Number of   Observations
             Explosions

    31 Oct       28      Night observations of incandescent emissions falling on upper cone; an
                           ash-bearing plume rose ~3 km about the summit
    01 Nov      21       A 3 km plume deposited ash in Banos and 31 km SW of the summit in Riobamba
    02 Nov      22       On two occasions, plumes rose to ~5 km above the summit; windows vibrated 12
                           km from the summit.
    03 Nov      34       Ash deposited 32 km NW of the summit in Ambato
    04 Nov      25       Plumes up to 5-km tall were produced; vibrations and explosions continued;
                           sulfur smell noted in the city of Banos; harmonic tremor recorded
    05 Nov      22       Weather clouds covered the plumes
    06 Nov      32       Dense, 3-5 km plumes were seen during the day
    07 Nov      27       ~1.5 km plume dispersed to the N and S
    08 Nov      24       Blocky pyroclastic flows with over 1 km runout distances
    09 Nov      19       -
    10 Nov      27       -
    11 Nov      29       Ash columns to 1.5 km height
    12 Nov      28       Ash column to ~10 km altitude (~5 km above the summit); blowing E
    13 Nov      15       ~2 km plume produced containing minor ash and directed NW
    14 Nov      25       Plume heights unstated but plume visible to NNW on satellite imagery
    15 Nov      37       Morning eruptions discharged ash plumes that moved NNW
    16 Nov      43       Airborne observations of an ash-rich plume ascending to 5-km above the
                           summit and blowing NW; a satellite image also showed the plume at ~5 km
                           altitude
    17 Nov      55       A consistent 1-km-tall ash plume directed to the N
    18 Nov      83       Loud booming noises; continuous series of ~5-km-tall, W-directed plumes
    19 Nov      64       Morning eruptions, plume blowing SW; in the past few days there was ~0.5 mm
                           of ash deposited in Banos and up to ~2 mm deposited in Runtun (see text)
    20 Nov      47       A strong rain produced mud flows; some crossed roads; reports stated one
                           mudflow was 20 m wide (see text).
    21 Nov      67       -
    22 Nov      55       A large quantity of blocks and incandescent material found on sides
                           of volcano; 1 to 2 km plume produced and inclined to the NE
    23 Nov      96       Emissions rose to 1 km over the summit
    24 Nov     102       Up to ~2-km-tall plumes of vapor and ash directed to the E
    25 Nov      97       Taller plumes reached 3-4 km height; ~3,300 tons/day SO2 emitted in an
                           interval with a constant 1-km-tall plume
    26 Nov      35       An explosion of vapor and ash rose 1 km above the summit of the volcano; a
                           ~5-km-tall plume formed; 1 cm ash accumulated in one sector during the
                           past weeks
    27 Nov      16       4 km ash-bearing column rose after the explosion
    28 Nov      13       A 500-m-tall vapor column was seen
    29 Nov      46       An explosion produced a 4-km-tall column; loud noises heard
    30 Nov      62       Observers noted strong thunder sounds; vibrations felt ~10 km away
Figure 2. A dark ash plume rises from Tungurahua's formerly snow-covered summit crater on 16 November 1999. Courtesy of the Geophysical Institute.
Figure 3. A histogram indicating the number of daily explosions at Tungurahua during 24 October to 30 December 1999. Explosions were most frequent during 22-25 November. Courtesy of the Geophysical Institute.

A pronounced peak in monthly earthquakes during August-September diminished rapidly in October and still farther in November (figure 4). The greatest number of monthly earthquakes were volcano-tectonic, in a pattern that became prominent in September 1998 and prevailed until October 1999. The ratio of multiphase to long-period earthquakes showed significant variability. In some months (eg., February, March, May, June, and September 1999) the multiphase events dominated. August 1999 showed the extreme reversal of this pattern with 436 long-period and 58 multiphase events. The last two months shown on figure 4, October and November, portrayed a similar though less pronounced reversal in their relative abundance of the multiphase events. These months also displayed a comparative scarcity of volcano-tectonic events.

Figure 4. A histogram for Tungurahua showing three types of monthly earthquakes occurring between April 1998 and November 1999. For any given month, from left to right the earthquakes shown are long-period (LP), hybrid or multiphase (MP), and volcano-tectonic (VT). All three types plot on the same scale, shown on the left side of the histogram. Courtesy of the Geophysical Institute.

SO2 flux during the crisis (figure 5) showed wide variability. Comparatively high fluxes were measured prior to the eruption. On the eruptions first day, 5 October, measured SO2-flux values reached 9,000-10,000 metric tons/day (t/d) (BGVN 24:09). The highest fluxes, seen during mid-September through early November, also showed rough, though inexact correlations with the seismic and explosion patterns.

Figure 5. SO2 flux measured at Tungurahua during 11 July-8 December 1999. Although error bars were not provided they are typically on the order of plus or minus 10-20%. Courtesy of the Geophysical Institute.

Two mud flows were reported on 20 November. They occurred after a strong rain that washed large tree trunks and rocks into a main highway in Baños. One of these mudflows was 20 m wide; another earlier in the day blocked part of a different highway in Baños.

1998-99 activity divided into five stages. In January 2000 the Geophysical Institute issued a summary report that divided 1998-99 activity into five stages. The first stage, December-May 1998, included swarms of small predominantly volcano-tectonic earthquakes. Tremor also continued, presumably associated with a phreatic source; this kind of tremor has been detected since 1993 and is thus here referred to as persistent or long-lived tremor.

The second stage, May 1998-15 July 1999, was an interval when seismic swarms (including volcano-tectonic (VT), long period (LP), and hybrid or multiphase (MP) earthquakes) became more energetic. Small explosion signals began to register from greater-than-shallow depths. The preponderance of VT earthquakes was interpreted as a result of stress beneath the edifice due to intruding magma. Stable-frequency tremor at that time underwent a slight increase in amplitude.

In the third stage, which began after 15 July 1999, tremor included higher frequency signals. Geophysicists noted a series of many small earthquakes of all kinds that continued until mid-December. At the end of July came the first reports of strong sulfurous odors in the vicinity of the crater. In the meantime, SO2 fluxes rose from essentially zero to 3,200 t/d (figure 5).

During 24-28 July and 8-10 September LP earthquake swarms struck with significant energy. Seismicity continued to rise considerably during August and early September. An alert was declared on 8 September 1999.

The fourth stage began 14 September 1999 when low-frequency tremor appeared, presumably associated with degassing and ascending magma. The persistent tremor increased in amplitude. On 14 September a column of vapor 2 km tall was observed. On 15 September the alert status rose to yellow. Later and until 25 October tremor reached extraordinarily high amplitudes and contained three dominant frequencies: 1, 1.7, and 2-2.5 Hz.

The first explosive activity was reported on 5 October (BGVN 24:09), when blocks and ash were ejected at 0721, 0738, and 0743 hours. This emission was associated with a comparatively big explosive seismic signal with a reduced displacement of 25 cm2 and high SO2 fluxes. The next day an ash plume rose to 2 km above the summit; small airfall ash deposits were found in Quero, Bilbao (where the thickness was given as 2 mm), and probably in Ambato. Subsequent Geophysical Institute reports described small ash-bearing or "dark" plumes to 0.5-5 km above the summit.

On 13 October observers first noted incandescence. SO2 fluxes rose to over 10,000 t/d (figure 5). Deformation at one of the tilt stations on the SW underwent significant changes. Activity increased on 16 October when an ash plume reached ~5 km above the summit and blew W. During the previous night's darkness observers saw incandescent ash and blocks deposited on the upper flanks of the volcano. On 16 October the alert status was raised from yellow to orange, prompting evacuations of Baños and settlements along Tungurahua's W and SW flanks (see below).

During the fifth stage, which began after 25 October, the persistent tremor remained near the levels seen in the third stage. Low-frequency tremor also continued. SO2 fluxes dropped to 3,500-4,000 tons/day in mid-November. Magmatic explosions became common in this stage. At night observers saw pyroclasts descending 1-2 km below the summit. Ash-charged plumes rose 3-5 km above the summit. During 1999 the Geophysical Institute tallied 2,030 explosions and emissions, 2,542 VT earthquakes, 4,086 LP earthquakes, and 1,038 MP earthquakes.

Geography and hazards. Baños sits in a narrow valley on the N margin of the volcano 75 miles S of Quito and 9 km NE of Tungurahua's summit. Baños lies along the Pastaza river (draining the N flanks) below the Chambo river (draining the W flanks over the NW to SW sector). This geography leaves Baños open to "high hazard for directed blasts and fallback pyroclastic flows" as well as lahars (Hall and others, 1999). Within this hazard zone, ~4.5 km downstream, sits the Agoyan dam, an important source of hydroelectric power.

Tungurahua is very dangerous because it has 3 km of vertical relief, 30°slopes, a record of previous sector collapses and a comparatively high propensity for future collapses, a pre-evacuation at-risk population of ~25,000 people, a major hydroelectric dam on its NNE margin, and a record of relatively violent, sudden andesitic eruptions with pyroclastic flows (Hall and others, 1999). The same authors noted that the volume of magma emitted by Tungurahua during the last 2,300 years has been ~3.45 km3. This gives it a magma flux rate similar to that at Merapi during the last century and 2- to 3-fold larger than the estimated rates seen in the Central Andes during the Late Cenozoic.

Evacuations. The newspaper El Universo reported that on 16 October when Tungurahua's volcanic activity increased and its hazard status first rose to orange, evacuations followed at cities closest to the volcano, including Baños. On 21 October the United Nations (UN) reported that the evacuations relocated "22,000 persons from some 60 locations." El Universo noted that at one point nearing the end of the evacuation one hundred buses were used.

As of late October some of the residents had moved to Ambato, 32 km NW of the volcano. Official sources indicated that 1,200-1,500 evacuees went to temporary shelters in the provinces of Tungurahua, Chimborazo, and Pastaza. Besides Ambato, individual cities that took refugees included Puyo (45 km E of the summit) and Shell (41 km E). About 100 families found shelter in a religious foundation and 200 families on a farm belonging to the Polytechnic Institute of Chimborazo. The UN further reported that ~600 military police and personnel have been deployed to the affected region to protect abandoned property. Access into this area was to be strictly prohibited.

The UN reported that 4,000 livestock, 100,000 fowl, and the animals from the zoological garden in Baños had also been evacuated. According to the Associated Press, a government census found that 40,000 chickens died from respiratory infections during early October.

According to the Associated Press, Baños had been evacuated for two months when on 13 December a caravan of residents briefly returned. During this brief visit, one resident entered his home and found it intact, although most parts of the house lay covered in ash. Residents faced an uncertain future because they did not know exactly when they would be able to return. The governor of Tungurahua province, Ignacio Vargas said, "This won't be permanent. We will have to wait until the eruption ends so that everyone can return to his normal activities."

Because of economic problems associated with leaving their homes and livelihoods, Baños area residents have been bypassing the military to plant crops and tend their farms. According to early January ABC News reports there have even been skirmishes between residents and the military. The eruptions are occurring in the context of tension and conflict between the military and some Unions and other groups as the country's economy has worsened.

Reference.: Hall, M., Robin, C., Beate, B., Mothes, P., Monzier, M., 1999. Tungurahua Volcano, Ecuador: structure, eruptive history and hazards: Journal of Volcanology and Geothermal Research, v. 91, p. 1-21.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Embassy of Ecuador, 2535 15th Street NW, Washington, DC 20009 USA (URL: http//www.ecuador.org/), United Nations Office for the Coordination of Humanitarian Affairs (OCHA), Palais des Nations, 1211 Geneva 10, Switzerland (Email: info@dha.unicc.org); El Universo, Quito, Ecuador (URL: http://www.eluniverso.com/); Associated Press, International Headquarters, 50 Rockefeller Plaza, New York, NY 10020 USA (URL: http://www.ap.org/); ABC News (URL: http://abcnews.go.com/).

01/2000 (BGVN 25:01) More than two-fold increase in long-period earthquakes during December

This report describes activity during December 1999 based on daily reports and other resources from the Geophysical Institute's website.

In comparison to other months, long-period earthquakes grew substantially during December (figure 6). The previous monthly high during the crisis, consisting of 883 events, took place in November 1999. December's count of LP earthquakes reached 1,995 earthquakes, more than double the November high. In addition, the count for December LP earthquakes stood higher than the sum for these events during all the previous months of the crisis.

Figure 6. Histogram showing three types of monthly earthquakes at Tungurahua between January 1998 and December 1999. For any given month, from left to right the earthquakes shown are long-period (LP), hybrid or multiphase (MP), and volcano-tectonic (VT). All three types plot on the same scale. Data for November 1999 has been revised from that in BGVN 24:11. Courtesy of the Geophysical Institute.

Intervals of both low-frequency tremor and repeated explosions (often 10 to 25/day) prevailed during the reporting interval. Scientists explained the frequent, ash-bearing explosions in terms of a magmatic system that maintained an open conduit to the surface.

The Instituto's daily reports stated that on the morning of 1 December an ash column from Tungurahua rose to unstated height and moved NW. Ash fell on the city of Ambato and its vicinity. The next day ash blown SW fell on the town of Riobamba.

Modest ash columns, typically under 2 km, were seen almost daily during December. Cannon-like blasts were also widely heard. For example, despite bad weather on 4 December, observers heard explosions and later saw incandescent blocks descending 1 km below the summit. A blast on 5 December was audible 8 km from the crater. An explosion on 10 December discharged a plume that rose to 3 km altitude; that same day drivers discovered small mud flows cutting across three segments of the Baños-Penipe highway. Plumes from explosions on 11-12 December sent ash 4 to 5 km in altitude; in the case of the former, winds carried the plume 20 km laterally.

At 0840 on 16 December members of TAME flight 172 reported an ash plume above Ambato at an altitude of ~8 km. It drifted N at ~30 km/hour. These events correlated with seismic and ground-based visual observations. A second example of an upper-altitude ash-bearing plume took place on 28 December when a TAME flight crew reported a 6.2-km-altitude plume directed SW. A third such report came on 31 December when a TAME crew noted an ash plume at 7.5 km altitude extending SW.

On 26 December, a COSPEC instrument was used to estimate the SO2 flux at 1,700 tons/day. Land surveys with an EDM instrument detected an absence of measurable geodetic changes. That same day, however, it was announced that in the past few days there had been some extended periods of tremor.

By mid-December 1999 the people of Baños became dissatisfied with the military occupation of their town. Some sources claimed that evacuated residents had seen looting of homes on television news. On 5 January several thousand residents of Baños bearing sticks and machetes forcibly drove the military off, re-entered the town, and proclaimed that they would be responsible for their own safety in the event of a threatening eruption. By some estimates, 25% of the town's ~ 16,000-20,000 residents had returned. Tourists, too, have come to enjoy the town's ambiance, hot springs, and resort atmosphere. Baños is located on the N-flank of the volcano, where it is vulnerable to various volcanic hazards.

Correction: In BGVN 24:11 the plot showing monthly earthquakes at Tungurahua presented values that have since been revised. The chief difference lies in the number of long-period earthquakes during November 1999 (now reported at 883 events). Figure 2 shows the corrected values.

Reference. Hall, M., Robin, C., Beate, B., Mothes, P., Monzier, M., 1999. Tungurahua Volcano, Ecuador: structure, eruptive history and hazards: Journal of Volcanology and Geothermal Research, v. 91, p. 1-21.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Embassy of Ecuador, 2535 15th Street NW, Washington, DC 20009 USA (URL: http//www.ecuador.org/), United Nations Office for the Coordination of Humanitarian Affairs (OCHA), Palais des Nations, 1211 Geneva 10, Switzerland (Email: info@dha.unicc.org); El Universo, Quito, Ecuador (URL: http://www.eluniverso.com/); Associated Press, International Headquarters, 50 Rockefeller Plaza, New York, NY 10020 USA (URL: http://www.ap.org/); ABC News (URL: http://abcnews.go.com/).

04/2000 (BGVN 25:04) Frequent early-2000 explosions with loud reports; ash plumes to 10 km altitude

This report covers the interval from 1 January to 18 February 2000. What follows are excerpts of the Geophysical Institute's daily reports. Basically, they disclose a pattern of noisy ash-bearing eruptions, tremor, and elevated seismicity. The alert status remained at Orange. As noted on table 3, some ash columns rose as high as 5-10 km; the sounds from one explosion were heard 75 km N in Quito. Tephra frequently blew towards the W (the major population center, Baños, lies N of the summit).

Table 3. Tungurahua plumes and other activity during 1 January-18 February 2000. Plume composition was further described as follows: dominantly ash plumes (a), dominantly steam plumes (s); and direction blown (e.g., N, S, E, W). Where available, selected comments on active processes also follow. Courtesy of the Geophysical Institute.

    Date   Plume height (km) and other data / comments

    01 January    2a, W. Banded tremor.
    02 January    0.5 a, W (fines fell on Rio Bamba). Banded tremor.
    03 January    "Significant explosions"a, W. Following the
                    explosions there was high-amplitude tremor for 30
                    minutes.
    04 January    5a, W.
    05 January    2a, N.
    06 January    3a, NE.
    07 January    Poor visibility, 1-2 a.
    08 January    Poor visibility, 3-4a, SW.
    09 January    --
    10 January    6,700-7,000 t/d SO2. High tremor.
    11 January    1.5s, NW.
    12 January    ~ 8a, with two plumes seen in satellite imagery,
                    trailing off S and W. 8,400 t/d SO2.
    13 January    Restricted visibility but incandescent ash seen
                    falling on upper slopes. 5,000 t/d SO2.
    14 January    ~ 0.2a; the plume was highly concentrated in ash,
                    thus the column collapsed soon after it began
                    discharging; later, incandescent tephra fell over
                    the upper sides.
    15 January    2a, strong detonation heard 10 km away; incandescent
                    blocks seen.
    16 January    Poor visibility and no explosions detected.
    17 January    Small explosion detected.
    18 January    One audible explosion heard during poor visibility.
    19 January    1a, continuous discharge blown W to NW.
    20 January    Small ash columns.
    21 January    Bad weather, falling rocks heard; mudflows noted.
    22 January    No report.
    23 January    Observers saw a short, hazy column with a moderate
                    concentration of ash.
    24 January    1a, blown W and SW.
    25 January    Seismically detected explosions with 5-8 cm2 reduced
                    displacements.
    26 January    Bad weather; small explosions.
    27 January    Seismically detected explosions with 8 cm2 reduced
                    displacements.
    28 January    Small to moderate explosions.
    29 January    Moderate to large explosions; cannon-like sounds
                    reported; two mudflows crossing roads.
    30 January    ~ 1a, but American Airlines also reported a plume to
                    ~10 km; moderate-to-large seismically detected
                    explosions, plume blowing NW to W.
    31 January    No report.
    01 February   Slight decrease in the number of explosions with
                    respect to previous days.
    02 February   2a, W; 20 min. of tremor.
    03 February   1a, W and SW; incandescent materials descended 0.5
                    km down flanks.
    04 February   ~2.5a, some dense columns; W, NW, and SW.
    05 February   0.6a, W; rains caused mudflows.
    06 February   Continued outbursts consisting of sporadic low
                    energy explosions. Tremor episode(s) associated
                    with gas emissions.
    07 February   Continual ash emission; S- and W-directed winds.
    08 February   3a, N and NW; explosion at 0438 heard by residents
                    of Quito; ashfall closed airports in Ambato and
                    Riobamba.
    09 February   No report.
    10 February   No report.
    11 February   No report.
    12 February   ~8-10a (aviation report); W and SW.
    13 February   Cloudy weather; roaring noises heard; fine ash in NW
                    sector of volcano; at 0743, an Avianca flight
                    en-route to Lima-Bogota reported a 7-km-radius
                    cloud of ash over the volcano at 8 km altitude.
    14 February   2a, SW; small amount of ash in Riobamba.
    15 February   2a, W.
    16 February   Plumes and emissions not discussed.
    17 February   3a.
    18 February   3a.

Histograms and plots on the Institute's web site tell a story of increased activity during late 1999, but the data for 2000 were generally absent. For example, BGVN 24:11 included their plot of daily explosions during 24 October-3 December 1999. This plot showed 1 event on 24 October 1999 and 1-39 events during the first half of November. A prominent peak in the latter half of November reached 105 events. The number of daily events subsequently declined for the last data shown, ending with 39 events on 3 December 1999.

Based on a reading of the daily reports for early 2000 this decline continued. An approximate tally of daily explosions suggests 300 during the month of January, an average of about 10/day. Similarly, for the first eighteen days of February, explosions again averaged about 10/day. In addition, a regional earthquake of M 3.3 took place on 12 January centered below the volcano's S flanks.

One of the Institute's plots disclosed a positive correlation between near-term rainfall and tremor during 1993-1999. Notwithstanding that longer-term observation, rainfall decreased during the course of 1999 while tremor escalated strongly, ultimately undergoing more than a 15-fold increase over background by the latter part of 1999.

Epicenters and foci mapped for August 1999 clustered near the summit and also occurred with a much lesser density along a radial band trending NW-SE extending out on either side of the summit. Thus, the main cluster of the August 1999 earthquake swarm lay directly below the summit at less than 10 km depth; with close approach to the surface the cluster appears directed toward the volcano's SE side. A plot of measured SO2 fluxes for the interval July-8 December 1999 peaked in September-October at ~9,000-10,000 tons/day (BGVN 24:11).

On 21 and 29 January, and 6 February, rainy weather caused mudflows to descend drainages blanketed by ash deposits. On many days, explosions caused unusually strong acoustical signals; the most powerful signal was generated on 8 February when the sound of an explosion was heard 75 km N in Quito.

Some of the other noteworthy events and ideas discussed in the daily reports follow. At 1544 on 3 January significant explosions occurred, followed by high-amplitude tremor lasting about 30 minutes. The emissions continued and the columns again blew W. On the 4th, undisclosed satellite images showed a column of ash 5 km tall, again blown W. The daily report for 7 January mentioned two possible eruption scenarios involving escalating eruptive vigor and increased hazards.

At 0614 on 12 January ground observers noted a dark gray column rose 3-5 km over the summit. Thereafter, NOAA satellite technicians noted two ash clouds. One cloud extended from the volcano toward the W reaching a length of 16 km and a width of 15 km; it rose to 3 km over the summit (i.e., ~8 km altitude). A larger ash cloud extended toward the S reaching a length of 75 km; it also reached ~8 km altitude (table 1).

Ash clouds remained obscure from the ground on 13 January but in the morning roars were heard associated with the continual emission of gases and ash. A light rain of fine ash fell in the NW sector of the volcano. Peculiar, very dense clouds vented on 14 January, rising only 200 m before rapidly collapsing.

At 0743 on 13 February, an Avianca flight in route to Lima-Bogotá reported an ash cloud over the volcano at 8 km altitude (table 1). In the morning on 14 February some loud explosions foretold of ash columns; they rose 2-3 km above the summit and blew SW. In the morning the next day satellite images showed an ash cloud that had reached a length of 100-150 km and width of 15 km. This cloud was directed from Tungurahua toward the NNW, passing over the towns of Píllaro, Latacunga, and Salcedo. At 10 km distance, falling ash reached a thickness of 1 mm.

Information Contacts: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

07/2000 (BGVN 25:07) January-July volcanism possibly decreased; lava fountains and many lahars

During January-July 2000 Tungurahua volcano experienced continuous but relatively mild activity with occasional lava fountaining. There were periods (hours to days) of relative calm during June and July.

The volcano continues to generate a variety of seismic events, most events being the long-period (LP) type. Two episodes of volcano-tectonic (VT) events were observed; one between late January and early March, and one less intense event between early May and mid-June. Epicenters for these events were across the top of the volcano's cone with focal depths at 3-13 km. Hybrid events, whose waveforms consist of a short, higher-frequency onset followed by lower-frequency, larger-amplitude signals, were most abundant in January and February (~50 events/week), partially coinciding with the greater VT activity. Subsequently these events diminished to 1-2 events/week, except for a brief swarm in early April.

Events of classical LP waveform were frequent, varying from ~400 events/week in January, ~600 in February, ~400 in March, ~600 in April, ~500 in May, and ~400 in June. A sharp increase to ~950 events/week was observed in July. Some of the LP events (3.7-4.0 Hz) were located tentatively at depths of 7-10 km below the crater. However, the great majority of LP events (1.5-3.3 Hz) were 3-7 km deep. They were often associated with explosion clouds or forceful emissions of ash-and-steam within 1-3 seconds of the seismic onset, suggesting a high-level origin.

Explosions, recognized principally by their impulsive onset, were more frequent during January and February (~80-90 events/week), but in subsequent months dropped to ~20-30 events/week, with many accompanied by a sonic boom. Reduced displacement values for the explosions typically were 5-10 cm2, and occasionally 12-18 cm2.

Low-frequency tremor with spectral frequencies between 0.5-1.6 Hz, but monochromatic at times, were observed in April and May, but only sporadically in June and July. During the period from the 2nd week of April through the 2nd week of May, the low-frequency episode coincided with lava fountaining in the summit crater. The fountains, comprised of the continuous ejection of incandescent material 100-500 m into the air, lasted hours; sustained roaring and surf-like noises heard 12 km away.

The constant glow of incandescent material in the crater, which was observed frequently in late 1999, was seen only occasionally during August, possibly due to unfavorable weather conditions. Better viewing conditions in late June and July confirmed that incandescent lava still remained in the crater or immediately below it.

The emissions have consisted of a permanent, grayish-white to light-gray column of steam with varying amounts of fine-grained ash that commonly rise less than 1 km above the crater. Explosions or strong emissions have consisted of blocks being thrown hundreds of meters into the air and by the formation of Vulcanian-like eruption clouds that are medium-to-dark gray in color and sometimes with a mushroom shape. The clouds have reached as high as 5 km above the summit. Primarily, easterly winds have carried the very fine ash to the W and WSW, but occasionally anywhere in the azimuthal arc between NW and SW. Both national and international flights reported the ash plume. The ash deposits were several centimeters thick on the lower W flank of the cone, but only several millimeters in the agriculturally important lands farther W.

Ballistic blocks were vesicular, black, glassy andesite containing phenocrysts of olivine, plagioclase, augite, and hypersthene, in a glassy matrix with 10-20% microlites. More recent samples had fewer olivines and larger augites. Chemical analyses of these blocks as well as collected ash gave the following typical values: SiO2 ~58.5%, K2O ~1.72%, MgO ~3.9%, Ni ~33 ppm, and Cr ~65 ppm.

COSPEC monitoring since November was hindered by heavy cloud cover. Following the consistently high SO2 flux values of 6,000-8,000 metric tons/day (t/d) during September-October 1999, values decreased to an average of 3,000-4,000 t/d in November-December 1999. Values then rose to ~8,000 t/d in January and subsequently dropped to an average of ~1,000-2,000 t/d in June and July 2000. An exception to this trend was an increase to ~4,000 t/d observed in April-May, 2000, which coincided with the lava fountaining episode. In general, higher SO2 values seem to be associated with greater tremor activity.

Monthly water analyses of hot springs at both the N and S bases of the edifice have not shown any variation in temperature, pH, conductivity, nor in the concentrations of SO4, Cl-, Na+, CO3--, Ca++, Mg++, and K+, since chemical monitoring began in 1992 and since the activity on Tungurahua began in July 1999.

Lahars coincided with the rainy season and became frequent in October and November 1999; they rapidly cut the main highway at every stream crossing along the western half of the cone (the area of greatest ash fall). Occasional rains from December to June generated flows of debris. The main highway to Baños and to the Amazon Basin was frequently blocked for hours due to lahar deposits.

In general, the activity appeared to be subsiding. However, during the 1916-18 eruptive period the volcano experienced 1.5 years of little activity between major eruptions. An orange alert is still in effect. In the past, Tungurahua typically generated both Merapi- and St. Vincent-like nuées ardentes. The W sector of Baños (17,000 inhabitants) lies at the mouth of a canyon that starts near the summit of the volcano, 9 km away and 3,000 m above the town.

Following the evacuation of Baños on 17 October 1999, the town remained abandoned until late December (BGVN 25:01). As of August 2000, about 80% of the population had returned and tourism has re-established itself.

Information Contact: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

07/2001 (BGVN 26:07) Summary of August 2000-August 2001 eruptive activity

Tungurahua was last discussed in BGVN 25:07, in a report covering the first half of the year 2000. This report was taken chiefly from available updates on the Instituto Geofísico (IG) website. Some of the graphics currently available there and discussed in this report cover the interval 1998-2000.

The subsequent part of this report focuses on activity from August 2000 through 15 August 2001. During this latter interval, aviation reports were issued daily, often several times a day. The final section of this report presents some statistics on Tungurahua's recent human impact from a report issued on 5 September 2001.

Tunguharua's continued eruptions were accompanied by varying SO2 fluxes, tremor, and earthquakes. Hazard concerns remained high, and plume heights reached over 11 km altitude (5 km over the summit) on three days in the first half of August 2001.

Observations during 2000. Between January and October 2000 Tungurahua produced significant discharges and explosions, some of which included impressive ash columns and shows of lava in the crater documenting the presence of shallow magma in the edifice. Seismically inferred magmatic intrusions took place in January, April-May, and August-October 2000. The critical seismicity included intense tremor and swarms of long-period and volcano-tectonic earthquakes.

As shown on figure 7, earthquakes of long-period (LP) and volcano-tectonic (VT) types both underwent progressive increases during the year 2000 but decreased again by November 2000. (More recent data were unavailable at the time of this writing.) Earthquakes attributed to explosions grew in number suddenly during November 1999 and then subsequently proceeded to decrease in number until becoming inconspicuous during late 2000. Elevated numbers of earthquakes, particularly LP earthquakes, correlated with eruptive outbursts (arrows). High numbers of LP earthquakes also persisted between outbursts.

Figure 7. Monthly number of earthquakes registered at Tungurahua during January 1998 to December 2000. High numbers of volcano-tectonic and long-period earthquakes generally occurred starting in middle to late 1999. Around this time, earthquakes with explosion signatures were sometimes abundant as well. Arrows indicate times of seismically inferred magmatic intrusions. Courtesy of Instituto Geofisico.

Figure 8 illustrates how during September 1999-December 2000 the energy contained in combined harmonic tremor and hydrothermally generated tremor underwent a sudden peak in January 2000, a time when the numbers of earthquakes seen on figure 1 also showed a strong rise. Two subsequent, progressively smaller peaks in tremor energy occurred at roughly 4-month intervals. Intervals of high tremor energy strongly correlated with eruptive outbursts.

Figure 8. At Tungurahua, the energy contained in tremor (both harmonic and hydrothermal types) during September 1999-December 2000. The interval of maximum tremor energy, around January 2000, developed rapidly and then decreased through time at least as late as December 2000. The intervals that included the highest tremor energy were inferred to reflect magmatic intrusions (arrows). Courtesy of Instituto Geofisico.

SO2 flux climbed to over 10,000 tons/day during late 1999 and early 2000, but dropped thereafter stabilizing in the hundreds of tons per day range in late 2000 (figure 9). A synopsis of SO2 flux has yet to be reported for 2001. A statement discussing the week of 10-16 January 2001 noted that SO2 flux had been in the 1,000 tons/day range but had risen to 2,000-2,400 tons/day. During that same week, new fumaroles were noted at an inaccessible spot on the NW flank above Baños. Plumes that week rose at least one kilometer over the summit (table 4).

Figure 9. SO2 flux measured at Tungurahua during July 1999-December 2000. After the large peaks (~10,000 tons/day) the SO2 values dropped significantly and then tended to decrease through the end of 2000. Courtesy of Instituto Geofisico.

Table 4. A summary of hazard status and plume height observations for Tungurahua, 1 November 2000 to 21 August 2001. These data were summarized from GVP / USGS WeeklyRreports derived from IG data. Some of the taller plume heights came from the Washington VAAC and were based on satellite imagery and local aviation reports.

    Interval         Description of Activity

    01-07 Nov 2000   Plumes 0.5 km above crater.
    08-14 Nov        13 November small ash cloud near the summit level blown
                       SE.
    22-28 Nov        27 November small ash-and-gas discharges reached 0.5 km
                       above the summit.
    29 Nov-05 Dec    Sporadic gas column. Plumes 0.3-0.5 km above crater.
    06-12 Dec        9 December ash cloud moving SW at summit height.
    13-19 Dec        14 December ash cloud moving NE at 0.5 km above the
                       summit.
    20-26 Dec        21 December ash cloud at 1 km above the summit but not
                       seen on GOES-8 imagery.
    03-09 Jan 2001   Plumes seen several times during this week; no ash
                       visible. Emissions on 3-4 January were moderate sized
                       and ash bearing. 2.9-km maximum plume height.
    10-16 Jan        Plumes ~ 2 km above crater.
    17-23 Jan        Plumes ~ 2 km above crater.
    14-20 Feb        19 February lahars down NW flank via Cusua Gorge; steam
                       column to 1 km.
    21-27 Feb        Plumes ~ 4 km above crater.
    14-20 Marc       13 March ash cloud moving NW at 4.6 km above the summit.
                       15 March ash cloud 3.2 km above the summit. 16 March
                       ash cloud 3.8 km above the summit.
    21-27 Mar        22 March incandescent eruption column 2 km above the
                       summit; 23 March ash cloud ~2 km above the summit
                       resulting from a half-hour emission.
    28 Mar-03 Apr    29 March ash cloud moving W at 1 km above the summit;
                       another small eruption on 2 April.
    11-17 Apr        Plumes ~2 km above crater.
    18-24 Apr        Incandescent dome followed by small steam columns
    25 Apr-01 May    25 April ash cloud at 2 km; more eruptions followed but
                       poor visibility. 29 and 30 April lahars to the Pampas,
                       Cusua, Hacienda, and Achupashal sectors; river levels
                       rose in the Ulba and Mandur sectors. Lahars in Pampas
                       sector blocked the Pelileo-Banos channel during 0710 to
                       1100 on 29 April and destroyed the highway.
    02-08 May        Small steam-and-ash plumes during the week. Possible
                       small lahar on 3 May.
    9-15 May         Heavy rainfall caused remobilization of ash deposited on
                       the upper flanks, producing several lahars. Lahars
                       went down the Cusua, Basural, Mandur, Bascun, and Ulba
                       gorges and  closed the Banos-Riobamba highway and
                       blocked a route to the town of Banos.
    16-22 May        Small 15 May eruptions sent ash up to 3 km above the
                       summit. Light ash fell in the towns of Cotalo and
                       Bilbao. 17 May  ash cloud 4 km above the summit drifted
                       SW. Intense activity suggested by seismicity but cloudy
                       conditions. 19 May ash cloud rose to 1.7 km.
    23-29 May        2-km-high ash plume on 26 May, poor visibility.
    30 May-5 Jun     Activity increased. A large number of long-period
                       earthquakes accompanied several small eruptions and
                       near-continuous ash clouds. 31 May eruption sent an
                       ash cloud up to 2.9 km above the summit, which drifted
                       W. Incandescent blocks ejected and a sound like a
                       cannon shot was heard kilometers away. Eruptions on 29
                       May at 2012 sent ash 2.2 km above the summit, on 30 May
                       at 1211 (ash plume to unknown height), and on 2 June at
                       1709 with an ash plume 2.9 km above the summit.
                       Incandescent material visible in the crater.
    06-12 Jun        Several small eruptions. 5 June ash cloud moving W at 2
                       km above the summit
    13-19 Jun        4.7-7 km maximum plume height.
    20-26 Jun        22 June eruptions at 0630 and 0652 sent ash clouds 0.8
                       and 3.8 km above the summit, respectively. No ash
                       visible on satellite imagery. Small explosions 25 June
                       at 0138 and 1328 produced ash clouds that rose ~1 km
                       above the summit and drifted W. Small amounts of ash
                       deposited in the town of Ambato, ~40 km NW.
    27 Jun-03 Jul    17 and 28 June ash clouds to 2 km above the summit; ash
                       fell W, damaging crops. 3 July W-drifting ash 0.8-2.6
                       km above the summit.
    04-10 Jul        5 July a larger-than-average ash plume rose to 2.6 km
                       above the summit; however, satellite imagery and
                       additional information suggested that a dense,
                       SE-drifting ash cloud rose to 4 km above the summit.
    11-17 Jul        12 July an eruption sent a cloud to ~3.3 km above the
                       summit; it drifted W to NW.
    18-24 Jul        Heavy rain remobilized ash deposited on the flanks,
                       generating lahars, and several small-to-moderate
                       eruptions produced ash clouds. On 19 July lahars down
                       the W flank reached the Banos-Riobamba highway. Larger
                       eruption on 20 July produced an ash cloud that rose to
                       ~2.9 km above the summit.
    25-31 Jul        25 July the highest ash cloud of the week rose ~4 km
                       above the summit and drifted SW.
    01-07 Aug        2 August until at least 3 August there was an increase in
                       activity. Continuous tremor began on 3 August; maybe
                       associated with continuous ash emission. Several
                       eruptions during the week; largest on 5 August produced
                       ash cloud  to ~7.5 km above the summit.
    08-14 Aug        Ongoing eruptions since at least 6 August, sending
                       steam-and-ash clouds to 2.5-8 km above the summit. Ash
                       clouds primarily drifted W. On 13 August three
                       particularly strong emissions at about 0630, 1200, and
                       1315. Two distinct areas of ash visible in satellite
                       imagery; one contained ash from the strong emissions,
                       rose to ~6.6 km above the summit and drifted E; the
                       other ash cloud was fed from continuous emissions and
                       possibly rose to ~5 km above the summit and drifted SW.
                       On 14 August one of about five explosions ascended to 8
                       km above the summit. It was emitted at 0746 and had a
                       reduced displacement of 13.2 cm2.
    15-21 Aug        Series of eruptions that began on 6 August continued
                       during the week. Seismicity characterized by many
                       long-period earthquakes and seismic signals that
                       represented ash emissions. Several sporadic explosions
                       occurred, with the largest explosion beginning on 15
                       August. The eruption produced an ash cloud that rose to
                       7.2 km above the summit. On 17 August volcanic activity
                       increased slightly and incandescent material was
                       ejected up to 1 km W of the crater. According to news
                       reports, as of 15 August ash affected more than 23,000
                       people, blanketed approximately 89,000 acres of crops,
                       and killed an undetermined number of livestock.

Reports noted an inferred intrusion during 9-12 October 2000. On 13 October, a debris flow occurred, but volcanism diminished considerably. The last explosion around this time took place on 23 October.

At the beginning of December 2000, IG survey crews detected a slight swelling in the EDM lines on the volcano's NW flank. An electronic inclinometer that could have helped confirm this deformation was located above the Refugio station. Unfortunately it was damaged when struck by rocks.

Summary of activity during November 2000-August 2001. Variable ash-cloud heights and other activity are summarized in table 1, which covers the time interval 1 November 2000 through 15 August 2001. Stated in terms of height above the summit, ash clouds rose to more than 7 km on two days in August; to 6 km on 1 day in August; and to 2-4 km on 38 days, mostly in June and July. Smaller ash clouds ascended 1-2 km on 28 days in the early months of 2001. Plumes ascended <1 km on 23 days, largely late in 2000. Thus, the latter part of the interval, particularly June-August 2001, had the most reported tall plumes.

During 17 October 1999-12 November 2000 ash plume heights exceeded 7 km over the summit on 8 days, chiefly during late 1999 through early 2000. In October 1999 an ash plume rose to ~13 km over the summit.

Observations during 2001. In early January 2001, two volcano-tectonic (VT) events were located 4-5 km below the NW flank. After 3 January, Tungurahua's 300-m-diameter summit crater had an increase in ash emissions, seen visually from the Guadalupe branch observatory, 11 km N of the volcano (table 1).

New fumaroles became apparent in late November 2000 at 4,400 m elevation on the NW flank, in the main drainage that feeds into the town of Baños (population 18,000). The fumaroles were located in a 100- to 150-m-long area.

During 10-12 June 2001, uncommonly intense and prolonged rains fell over the eastern provinces and the Andean foothills of Ecuador. At one pluviometer (rain gauge) that the IG operates on Tungurahua's NW flank, 120 mm of rain fell in two days. The rain-generated lahars that flowed down Tungurahua's flanks were the largest ever recorded, carrying volcanic blocks the size of small cars. The lahars closed the road between Ambato and Baños for hours and totally destroyed the road between Baños and Penipe. Other floods and lahars were recorded in rivers born on the volcano. Along the Vascun and Ulba rivers, some houses on the flood plains were inundated but not destroyed. The Rio Pastaza, on the N flank of Tungurahua, registered a record water flow rate of 1,760 m3/s.

The rains triggered a landslide that overcame two people living downstream of Baños in the vicinity of Rio Negro. Out milking cows, they were swept into the nearby Pastaza river. These two deaths, although in Tungurahua province, were not related to the lahars. As of July 2001, no one had died from the recent lahars. All together, the rainy season left a death toll of ~80 people in Ecuador, including losses from landslides and flooding away from the volcano.

An explosion on 17 June 2001 rose 4.8-7 km above the summit. Owing to clear weather, it was witnessed by many of the region's inhabitants. No pyroclastic flows were produced, and the explosion ceased after about a minute. After that time, the volcano produced about 1 explosion/day. These mid-June explosions were relatively small (their seismic signatures had reduced displacements of 2-5 cm2), but they generally came without warning.

Light ashfalls were also frequent W of the volcano. They affected many crops (including corn, peas, beans, potatoes, tomatoes, blackberries, and squash; as well as orchards of peaches and apples). A 27 August report by the Pan American Health Organization (PAHO) stated that by late August 2001 various areas had received up to 2.5 cm of ash.

Scientists came to believe that a weak seal was forming in the volcano's conduit system. The seal was thought to break under sufficient recharge pressure. In addition, this new spurt of mid-June activity could be attributed to a small injection of magma that was believed to have occurred during 17-18 May. The fresh injection rose up through the conduit and was seen as incandescence on 26 May and when Strombolian fountaining was observed. Later explosions could stem from residual gases and heat.

Earth Probe TOMS (Total Ozone Mapping Spectrometer) detected a weak ash and SO2 plume from Tungurahua on 6 August at around 0630. The plume was directed generally WSW and extended to approximately 4°S, 83°W, containing an estimated SO2 mass of <=5 kilotons. According to Simon Carn, this was the first time TOMS had detected emissions from Tungurahua.

Practice evacuation and maps. On 26 June 2001, 2,000-3,000 people in Baños conducted a simulated evacuation, the first in over a year. It was organized by "Ojos del Volcán" ("Eyes of the Volcano"), a local organization whose members include hotel owners, climbing guides, and tour operators. Other organizers included the IG, local civil defense authorities, the Red Cross, police, firefighters, and health officials. Participants walked to three previously identified zones of temporary refuge. The exercise was successful and revealed some unforseen shortcomings in the local disaster plans. Figures 10 and 11 show maps indicating topography and potential hazard zones.

Figure 10. Diagram showing Tunguharua's landscape as seen from the SW. Note N arrow along left margin. Courtesy of Instituto Geofisico.
Figure 11. Generalized Tunguharua hazards map (N is towards the top) indicating areas of relative risk. The city of Baños lies within the zone of highest potential risk (central, darker shaded area). The town of Patate lies within the zone of lowest potential risk (lightly shaded); Pelileo Nuevo and Pelileo Viejo lie just outside this zone. Lahar risks continue hundreds of kilometers off the map towards the E along the downstream portions of the Rio Pastaza (dark strand intersecting the map's E margin). The Pastaza is confined by a dam ~4 km E of Baños. Solid and dashed curves represent areas with inferred risks from airfall ash. Courtesy of Instituto Geofisico.

Human impact. A report was issued by the United Nations Office for the Coordination of Humanitarian Affairs on 5 September 2001, following a multi-agency meeting the day before. The report cited updated Civil Defense statistics on Tungurahua's impact.

As of 5 September, no ash had fallen in the previous 10 days; still, 39,000 people (8,000 families) had been affected by the volcano. Respiratory infections had increased. Ash had affected potable water supplies in some rural communities prompting more water-quality monitoring. There were 3,107 houses damaged.

A total of 53,597 hectares (ha) of farmland and pastures have been affected, of which 17,017 ha lie in the province of Tungurahua, 28,580 ha in Chimborazo, and 8,000 ha in Bolivar. Due to stress and new feed, 13,113 cattle developed health problems. Some were evacuated. The report also discussed a system for enlisting and tracking relief contributions.

Information Contacts: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Associated Press; NOAA Operational Significant Events Imagery Support Team (OSEI), NOAA/NESDIS, World Weather Building, Room 510, 5200 Auth Road, Camp Springs, MD 20748 (URL: http://www.osei.noaa. gov); Washington VAAC, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www. ssd.noaa.gov/); Volcano Disaster Assistance Program (VDAP), U.S. Geological Survey, 5400 MacArthur Blvd, Vancouver, WA 98661 (URL: http://vulcan.wr. usgs.gov/Vdap/framework.html), Simon Carn and Arlin Krueger, Joint Center for Earth Systems Technology (NASA/UMBC), University of Maryland Baltimore County, Academic IV-/a, Room 114J, 1000 Hilltop Circle, Baltimore, MD 21250; Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017 USA (URL: http://www.reliefweb.int); Pan American Health Organization (PAHO), United Nations, 525-23rd Street, NW, Washington, DC 20037 USA (URL: http://www.paho.org/).

01/2002 (BGVN 27:01) Powerful tremor, plumes, 600-m-high lava fountains, and lahars during 2001

The first portion of this report discusses some geophysical and geochemical aspects of Tungurahua's behavior during 2001, including further descriptions through August 2001 (BGVN 26:07). The latter portion of this report contains a log of behavioral data for 24 August-30 December 2001 in tabular form, and finally includes field notes from a visitor who watched the summit crater for several weeks in the later months of the year.

Instituto Geofísico (IG) scientists estimated that 10-15 million metric tons of ash were deposited during the 4-26 August 2001 eruption. By the end of 2001 the current eruptive crisis had included 8 inferred intrusive episodes. Some eruptions, including those during 2001, displayed fountaining with jets of lava rising over 500 m. Since 5 September 2000 through at least January 2002, Alert Levels have been set at Yellow for the town of Baños and at Orange for the rest of the high-risk zone.

Seismicity and SO2 flux. Long-period (LP) earthquakes dominated the seismic record since December 1999 (figure 12). Except for the anomalous month of February 2001, this trend continued in 2001, with the number of LP earthquakes largely swamping other kinds. Specifically, at the scale of the histogram hybrid (H) earthquakes are only visible during February and August; volcano-tectonic (VT) earthquakes, only during January, August, September, and December; explosion (EXP) earthquakes, only during June, July, August, and September.

Figure 12. Number of Tungurahua earthquakes recorded monthly during 1999-2001. LP earthquakes clearly dominated since December 1999, except for the anomalous month of February 2001. During the year 2001, the peaks seen around May, August, and December may have corresponded to magmatic intrusions. Courtesy of IG.

During 2001 both the seismicity and SO2 flux underwent intervals of relative quiet and intervals with elevated signals. The most dramatic quiet interval, from late 2000 into May 2001, appears on a plot of reduced displacements (RDs) from explosive events (figure 13). A comparative lull also appeared in overall seismicity (figure 12), provisionally in SO2 flux (figure 15), and to a lesser extent, in tremor energy (figure 14). Although the lull appears more equivocal on figure 14, the peaks in tremor energy during July and August, following the lull, were the largest recorded since the spike seen in January 2000. Elevated SO2 flux values appeared around about the same times as the peaks in tremor energy (figure 15).

Figure 13. Explosion earthquakes at Tungurahua during 26 November 1999-14 January 2002 were quantified as reduced displacements (RDs, unit, cm2) and plotted at roughly 2-day intervals. RDs can be computed from seismic records; larger values indicate larger events. The record used came from station Patacocha. The largest RD shown, ~19 cm2, corresponds to an explosion that took place in December 1999 (upper left-hand corner). Courtesy of IG.
Figure 14. At Tungurahua, the energy contained in tremor (including both harmonic and hydrothermal types) during 14 September 1999-30 September 2001. The two largest peaks in tremor energy yet recorded occurred in mid-2001 (July and August). Horizontal axis is labeled as day/month/year. Courtesy of IG.
Figure 15. SO2 flux measured by COSPEC at Tungurahua during July 1999-November 2001. During 2001, flux highs were measured during May and August. Courtesy of IG.

During 2001, instruments recorded two pronounced seismic peaks (figures 6 and 7). These swarms of LP events had focal depths of 5-7 km and a wide range of dominant frequencies, 308-1066 Hz. The first peak in LP events took place during May-June and was accompanied by emissions at the summit.

The second peak in LP events took place during August-September and also corresponded to increases in the number of hybrid (HB) and volcano-tectonic (VT) earthquakes, and to summit explosions. This second peak differed from seismicity during September 1998 and October 1999 (see plot, BGVN 26:07). During those earlier times, instruments recorded higher numbers of HB and VT events. More recently, both HB and VT events had been decreasing: the former since July 2000, and the latter since October 2000.

Although during early December comparatively few earthquakes occurred, the type of events recorded, tornillos, merit special discussion (see below). Beginning on 20 December the number of LP events increased from an average of 20 events per day in the first days of the month, to an average of 200 events per day. The LPs maintained that level until 26 December.

The two prominent seismic peaks of 2001 were considered as related to intruding magma. Thus, the intrusion associated with the first peak can be divided into three pulses, the first occurring during 19-20 March, the second, 17-18 May, and the third, 6-7 June (and perhaps into July).

The second intrusion occurred in two pulses, the first during 4-20 August, and the second during 4-25 September. The events related to the second intrusion produced the largest RDs (figure 13). For comparison, in 1999-2000 LP events had larger RDs, 12-19 cm2 (figure 13).

In the first inferred intrusion, the discharge of SO2 amounted to 2,900-3,600 metric tons/day (t/d), decreasing to 677 t/d by the end of June. SO2 fluxes associated with the second inferred intrusion reached 3,585 t/d by mid-August, decreasing to 175 t/d by the end of August (figure 15). The peaks in SO2 flux closely corresponded to the increases in tremor energy (figure 14). Incandescence visible during the end of March and July, during early and mid-August, and during early September confirmed that magma then lay at or near the surface.

The pulses of activity of each intrusion preceded, and in some cases accompanied, the emission of vapor and ash during explosive Strombolian activity. For example, for the first intrusion, the second pulse of seismic activity preceded the explosion of 28 May. In that pulse there was ~1 explosion per day with RDs of 1-3 cm2. During the third pulse, aboutone explosion per day had RDs of 1-7 cm2.

For the second, more energetic intrusion, the first pulse of activity had 7 explosions per day with RDs of 1-13 cm2. The next (second) pulse had ~1 explosion per day with RDs of 1-9 cm2 (figure 13). The last intrusion, during mid-June through July, was preceded by "LP de Juive", events so-named because residents in Juive felt them. These signals could have been caused by clearing of nearby subsurface passages that transport magma.

At the beginning of December the previously mentioned tornillos appeared. Tornillos ("Screw-type" events) are monochromatic LP events characterized by a long, slowly decaying coda. On a seismogram they appear similar to a screw. They may arise from fluid resonance in a cavity. It is noteworthy that they showed up for the first time in December 2001, and arrived with considerable intensity. Where defined farther N in the Andes at Galeras, have been recognized as eruptive precursors.

Although relatively small in number, the tornillo events were considered important. During 3-9 December, 43 occurred, the largest number recognized in the history of monitoring at Tungurahua. During 4-12 December the duration of these event's increased. During 4-10 December they underwent a decrease in their dominant frequency. The latter could stem from increased gas in the fluid. The tornillo signals may thus disclose physical changes in the volcano during early December. For example, the tornillos could be related to shifts in internal pressure.

The LP events began to register on 20 December, suggesting magma ascent. A lack of significant ash emissions or SO2 flux suggested that the conduit was sealed. This could allow internal pressure to rise, resulting in a series of explosions.

Deformation. During 2001, inclinometer data from station RETU, located above the Refuge, showed a drift in the positive direction of 10-15 µrad. These values are not highly anomalous considering the large diurnal variations stemming from effects such as temperature and humidity changes in the air and ground surface. On the other hand, measurements of points on the W flank lacked significant distance changes.

EDM measurements from a fixed base (the El Salado base station) were conducted periodically. They aim at two distinct points on the NE flank (in a region above the Refuge). A gradual decrease in the distance between the base and the two points began during July 2000 and implies a slight inflation of the NE flank of the volcano.

During the course of field studies, new NE-flank fumaroles were sighted at ~4,400 m elevation along fractures. Topographic movements were suspected in this sector.

Chronological observations, August-December 2001. Table 5 summarizes seismicity, and visual and satellite observations of eruptions and explosions and their ash clouds.

Table 5. Summary of activity at Tungurahua during August through December 2001. These data mainly came from IG reports. Some of the higher plume heights came from the Washington VAAC and were based on satellite imagery and local aviation reports. Courtesy of IG.

    Date (2001)   Long-period   Tremor    Observations
                  earthquakes   signals

    24 Aug            --          --      An eruption at 1755 produced an ash
                                            cloud that rose ~6-8 km and
                                            drifted E to SE.
    29 Aug            20        several   A gas-and-ash eruption at 1530.
    03 Sep            44          36      Ash cloud rose to ~ 5.8 km and
                                            drifted W.
    05 Sep            77          46      Weak emissions with low ash content.
    08 Sep            --          --      Ash cloud at 0828 rose ~10.5 km
                                            altitude and drifted SW.
    11 Sep            --          --      Ashfall to N in Pondoa, Runtun,
                                            Banos; ashfall to S in Quero and
                                            Penipe; mudflows between Puela and
                                            Bilbao.
    12 Sep            19           5      An explosion at 1632 produced an
                                            ash-bearing emission that reached
                                            2 km above the summit and drifted
                                            W; an explosion at 1830 produced
                                            an emission that reached 0.5 km
                                            above the summit and drifted W.
    13 Sep            63          11      A small explosion at 1106;
                                            continuous steam emission with ash
                                            reached 0.8-1 km above the summit
                                            and drifted W; ashfall to the W in
                                            Juive, Cotalo, and Bilbao.
    15 Sep            --          --      Incandescent material observed along
                                            with ash emissions; ashfall to the
                                            SW in Riobamba and Penipe.
    16 Sep           123          37      Small explosion at 1631; moderate
                                            explosion at 1750 (3-km-high
                                            column that drifted NW); 2 VT
                                            earthquakes.
    17 Sep            56          12
    20 Sep            62          49      Moderate explosion at 1044 generated
                                            an ash column 2 km high that
                                            drifted W-SW; the explosion was
                                            preceded by three hours of tremor;
                                            ashfall in Pillate, Juive, and
                                            Runtun; columns of gas and ash
                                            drifted W.
    21 Sep            --          --      Moderate explosion at 1625 (3-km-
                                            high ash column drifted NW);
                                            incandescence observed in the
                                            crater.
    22 Sep           212         139
    24 Sep           104         159      Moderate explosion at 1500 (ash
                                            column drifted WSW); flank
                                            rockfalls heard in Juive, Runtun,
                                            Pillate,  Pondoa.
    25 Sep           108          41      An explosion at 1230 produced an ash
                                            column 5 km high that drifted NW;
                                            Strombolian activity,
                                            incandescence, and rockfalls
                                            observed on the W and NW flanks;
                                            ashfall in Cotalo; 2 VT
                                            earthquakes registered.
    26 Sep            36          37      Some ashfall to the S in Quero.
    11 Oct            30          --
    14 Oct            --          --      Ash visible ~1 km above the summit
                                            at 1736.
    20 Oct           108           6      Fumarolic activity on the NE side of
                                            the crater with intermittent
                                            emissions of white clouds that
                                            reached 20-500 m.
    22 Oct             7           7      Fumarolic activity produced clouds
                                            with low ash content that reached
                                            0.5 km; at 1758 a gas-and-ash
                                            emission reached 0.7 km and
                                            drifted W.
    23 Oct             7           1      Fumarolic activity on the N flank
                                            (near Vazcun); ash emissions
                                            reached 1 km above the summit.
    24 Oct            42          13
    26 Oct            --          --      Hot spot visible at summit on
                                            thermal satellite imagery.
    29 Oct            24           3
    01 Nov            42           3      Gas-and-ash emissions reached 1-2 km
                                            above the summit and drifted E-NE.
    03 Nov            38           1
    06 Nov            12           1
    11 Nov            34          22      Gas-and-ash emissions at 1050 and
                                            1352 reached 1 and 3 km,
                                            respectively, both drifted W.
    14 Nov            10           3      Incandescence and sporadic gas
                                            columns observed.
    15 Nov            38          11      At 1420 a gas-and-ash emission
                                            reached 1 km high and drifted W.
    19 Nov            73          15      Emissions followed by 10-30 minutes
                                            of tremor; ash columns rose to 2
                                            km and drifted W-NW.
    22 Nov            30          --      New fumarole observed on the W
                                            flank; EDM measurements showed
                                            swelling of the N flank.
    24 Nov            21           4      Gas-and-ash column rose to 100 m.
    26 Nov            28           1
    27 Nov            18          --
    01 Dec            21           1      Constant gas-and-ash emission
                                            reached a few hundred meter's
                                            above the summit.
    02 Dec            --          --      A small ash emission at 1140
                                            remained near the summit level.
    03 Dec            23           2
    08 Dec            42          --
    10 Dec            33           2
    12 Dec             4          --
    14 Dec            12          --      Lahars traveled down the flanks of
                                            the volcano.
    16 Dec            17          --      Lahars traveled down the flanks of
                                            the volcano; 1 VT earthquake
                                            registered.
    18 Dec            --          --      A gas-and-ash column reached 1 km
                                            above the summit.
    19 Dec            16          --
    20 Dec            62          --      Gas-and-ash columns reached 100-200
                                            m above the summit.
    26 Dec            82          11      At 1500 a gas-and-ash column reached
                                            ~0.3 km above the summit; the
                                            continuous gas transmission was
                                            accompanied by sporadic pulses of
                                            gas and ash.
    27 Dec           186          12      At 0900 and 1500 white gas-and-ash
                                            columns reached ~0.2 km above the
                                            summit. At 1006 and 1427 gray gas-
                                            and-ash columns reached 2 and 1
                                            km, respectively; 1 VT earthquake
                                            registered.
    29 Dec            --          --      A mudflow at 2342 in the Juive
                                            Grande gorge affected La Pampa and
                                            Los Pajaros.
    30 Dec           202          --      An explosion at 0023; at 0027 ash
                                            from the explosion rose to ~15 km;
                                            until 1500 ashfall was reported in
                                            Guadalupe and Patate and other
                                            areas W of the volcano.

IG scientists estimated that 10-15 million tons of ash fell during 4-26 August eruptions. During 6-14 August ash clouds reached the Pacific Ocean, and on 9 August falling ash affected towns 100 km W of the volcano. The Washington Volcanic Ash Advisory Center (VAAC) reported that nearly continuous ash emissions had occurred at Tungurahua beginning on 6 August, but extensive cloudiness prohibited ash-cloud detection in satellite imagery. Officials reported that over 23,000 people were affected by ashfall. The Civil Defense of Ecuador reported that the ashfall reached ~5 cm deep in places. Volcanism also increased during mid-September. Ashfall was reported in adjacent communities during 11-13 September.

The IG reported that on 14 December heavy rain on the upper flanks of Tungurahua resulted in dangerous lahars (table 7). The rain lasted for ~3 hours and the road into Baños was blocked for more than 12 hours in the zone of La Pampa (NW lowermost flank), where the lahars are usually deposited. An emergency bridge was necessary so that traffic could continue to pass. A few cars were almost buried under the flows. Local authorities were alerted within several minutes prior to the event because of an acoustic flow-monitor instrument in the zone.

The minimum total volume of the lahar was ~55,000 m3, making it the seventh-largest recorded by the acoustic flow-monitor since April 2000. The deposit was mainly composed of coarse ash and small pebbles, but it removed blocks up to 2 m in diameter. Similar lahars were reported elsewhere, mostly on the western flank. On 16 December another short rain on the lower flanks removed part of the previous day's lahar in La Pampa, and formed another small flow that again blocked the road for awhile.

Watching the crater during parts of September-December. Jean-Luc Le Pennec of the Institut de Recherche pour le Développement and a collaborator at the IG visited Tungurahua during 10-18 September, 15-22 October, and 26 November-3 December. He made the following observations.

The volcano remained extremely quite, without visible gas escaping the crater, during the day on 10 September. Without clear premonitory signal, at 1915 a powerful lava fountain began. The first pulses of the fountain reached 700 m and progressively declined to 300 m above the crater, before stopping abruptly about 6 minutes after starting. The summit crater then resumed complete quiescence.

In a second episode at 2147, fountaining reached ~600 m above crater and decreased rapidly to ~300 m during the next 5-6 minutes. The crater returned to quiescence and was later obscured by clouds. A seismic swarm of LP events took place during the following hours. During 11-16 September activity was characterized by fluctuating but almost continuous gas-and-ash emissions. Plume height varied between 0.6 to 2 km, depending on gas pressure and wind speed above the crater. The plume usually drifted W (SW to NW). Ashfalls were reported in Guaranda (morning of 11 September), Riobamba (16 September), Pelileo (12 September), and in other localities closer to the volcano. In addition, short-lived explosions occurred at a rate of 0-2 per day, producing ballistic fallouts on the terminal cone, and ash columns reaching ~2-4 km above the crater. They were sometimes accompanied with cannon-like sounds heard 15 km away.

The ejected lava's brightness was particularly intense during the night of 16 September, and a few glowing blocks fell outside of the crater. Periods of rumbling noises were frequently heard all week long, but their intensity increased on 16-17 September. During the night of 17 September lava projections reached 100-300 m above the crater rim. This activity took place around 0300 and started declining very slowly 90 minutes later. The activity continued to decline during the day on 18 September, ending at about 1400 when no sounds were audible as close as 2.5 km from the crater. On 25 September, the volcano produced 1 explosion and Strombolian activity.

During 15-22 October, good weather conditions allowed for frequent observations of the crater. Extremely low activity prevailed, with almost no degassing from the summit crater (except for the permanently active fumaroles of the N crater rim and of the N flank at 4,400 m elevation). Light degassing was observed during the morning of 19 October, after 2 days of increased seismic activity (from ~10 to ~100 events/day). The same day, at 1327, a short-lived outburst sent an ash cloud to ~1 km above the crater. The cloud drifted rapidly to the NNE. Interestingly, the outburst occurred when seismic waves from a regional earthquake arrived at the volcano. Two small ash emissions also occurred, reaching 500-600 m above the crater. In the latter case, a lapse time of 42 seconds was measured between the onset of the seismic signal and the appearance of the ash cloud at the crater level. Light vapor venting was occasionally seen on 20 October. Four ash emissions were witnessed during 2000-2200, with ash columns reaching 0.5-1.0 km above the crater. Few other emissions occurred during the night of 21 October.

During 26 November-3 December activity was low. A fairly continuous pulsating gas plume was emitted from the summit crater. During a 70-minute period on 2 December, five small ash emissions occurred. They rose 0.5-1 km and drifted N. For the third emission, the delay between the onset of the seismic agitation and the appearance of the ash cloud at the crater was 25 seconds, perhaps indicating the release of magma relatively deep in the system.

Information Contacts: Patty Mothes and Daniel Andrade, Geophysical Institute (Instituto Geofísico, IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Jean-Luc Le Pennec, "Volcanic processes and hazards" research unit, Institut de Recherche pour le Développement (IRD), Whymper 442 y Coruña, A.P. 17-12-857 Quito, Ecuador (Email: lepennec@ecnet.ec, URL: http:/www.ird.fr); Washington VAAC, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); United Nations Office for the Coordination of Humanitarian Affairs (OCHA), United Nations, New York, NY 10017 (URL: http://www.reliefweb.int); Associated Press.

08/2002 (BGVN 27:08) High seismicity, lava fountains, and explosions during January-June 2002

This report discusses activity at Tungurahua during January-June 2002. Table 6 provides weekly totals of seismicity, which has decreased since January 2002 (figure 16). Overall seismicity (figure 17) was dominated by long-period (LP) earthquakes, emissions, few volcano-tectonic (VT) earthquakes, and occasional small explosions.

Table 6. Weekly totals of seismicity at Tungurahua during 1 January-30 June 2002. The LP data are also shown plotted on a histogram (figure 11). Courtesy of IG.

    Date (2002)     Long-Period   Hybrid

    01 Jan-06 Jan      687           1
    07 Jan-13 Jan      306           5
    14 Jan-20 Jan      112           0
    21 Jan-27 Jan      715           1
    28 Jan-03 Feb     1685           0
    04 Feb-10 Feb      444           1
    11 Feb-17 Feb      570           1
    18 Feb-24 Feb      517           0
    25 Feb-03 Mar      435           1
    04 Mar-10 Mar      595           0
    11 Mar-17 Mar      587           0
    18 Mar-24 Mar      434           1
    25 Mar-31 Mar      232           2
    01 Apr-07 Apr      198           6
    08 Apr-14 Apr       15           0
    15 Apr-21 Apr      447           3
    22 Apr-28 Apr      332           3
    29 Apr-05 May      287           0
    06 May-12 May      316           2
    13 May-19 May      184           0
    20 May-26 May      204           0
    27 May-02 Jun      248           0
    03 Jun-09 Jun      110           0
    10 Jun-16 Jun      112           0
    17 Jun-23 Jun       37           0
    24 Jun-30 Jun      248           0
Figure 16. An overview of seismic events registered monthly at Tungurahua during January 1999-June 2002. After early 2000, the record was dominated by LP events. Courtesy IG.
Figure 17. Summary of weekly seismicity at Tungurahua during January-June 2002. The number of long-period (LP) seismic events underwent a several-fold decrease later in the year. The number of emission signals was quite variable and remained abundant through the end of June 2002. Although volcano-tectonic (VT) seismic events were variable, the highest number appeared early in the reporting interval (during parts of both January and February). The number of explosion signals varied from 0-18 events/week during the reporting interval and except for a quiet period around late March to early May, continued unabated. Courtesy IG.

Most of the LP earthquakes were shallow and associated with the exit of gases. Events during February included continuous emissions, intense fumarolic activity, tremor, and clusters of LP events (reaching more than 150 events/day, some with high amplitudes). These events were probably generated by gases associated with a small volume of new magma injected during late January, when VT earthquakes occurred several kilometers below the summit. That injection, like previous ones, accompanied the emission of steam and ash, and the possible formation of a lava lake that showed Strombolian activity. Tungurahua again responded to the injection of magma during March, when a few impressive lava flows occurred.

The Istituto Geofisico reported that shallow and deep events during May probably occurred in response to another small injection of magma beginning in mid-April, when tremor was especially high. Deformation data generally showed some deflation, and SO2 values fluctuated. The Washington VAAC frequently reported plumes and hot spots visible in satellite imagery (table 7). A more detailed description of the activity during January-June follows.

Table 7. Tungurahua plumes and hot spots visible in satellite imagery during January-June 2002. Note that the table only includes reports of plumes and hot spots visible in satellite imagery; IG reported plumes more frequently though clouds often prevented their appearance on satellite imagery. IG also made seismically based estimates of explosions. Times are listed in UTC as originally reported. Courtesy Washington VAAC.

    Date    Time   Satellite observation (heights refer to altitude)
    (2002)  (UTC)

    03 Jan  2300    Possible small ash cloud near summit.
    04 Jan  1445    Hot spot.
    16 Jan  1330    Faint ash plume extending 34 km from the summit,
                      estimated up to ~7.6 km.
    27 Jan  0000    Small ash cloud SW of the summit.
    27 Jan  1700    Ash plume E of the volcano, estimated up to ~9 km.
    03 Feb  1930    Faint ash plume, estimated at ~7.9 km.
    04 Feb  0130    Continuous faint ash plume extending 20 km N from the
                      summit, estimated at ~7 km.
    08 Feb  1330    Narrow plume of ash and steam extending SE from the
                      summit; estimated up to ~5.5 km.
    09 Feb  1240    Hot spot and faint narrow plume to the NE of the summit.
    09 Feb  1600    Narrow ash-and-steam plume moving SW at a higher level
                      than the previous one.
    10 Feb  2220    Possible ash cloud W of the summit.
    11 Feb  0030    Eruption around 2230 resulted in a small ash cloud moving
                      SW near ~7.6 km.
    13 Feb  1815    Thin plume of ash extended W at ~7 km.
    14 Feb  1255    Faint ash plume extending to the W, estimated at ~7 km.
    14 Feb  2240    Possible eruption around 1930 resulted in a small ash
                      cloud towards the SE.
    15 Feb  0450    Ash no longer visible but hot-spot activity occurred
                      during the previous 6 hours.
    15 Feb  2230    New eruption beginning around 2000, ash visible extending
                      NW from the summit.
    21 Feb  1440    Very faint ash.
    26 Feb  1245    Narrow ash plume extending W, estimated at ~7.6 km.
    26 Feb  1900    Narrow ash plume extending W, estimated at ~7 km.
    27 Feb  1300    Possible narrow ash-and-steam plume extending to the NW
                      of the volcano, estimated at ~6 km.
    10 Mar  1645    Small plume moving W from the volcano.
    15 Mar  1415    Ongoing ash-and-steam emission.
    17 Mar  0430    Ongoing ash emission and hot-spot activity.
    17 Mar  1030    Ongoing ash emission, new ash visible near summit,
                      intermittent hot-spot activity.
    19 Mar  0248    Eruption began around 2130.
    22 Mar  1400    Narrow ash plume extending to the NW, estimated up to
                      ~6.7 km.
    23 Mar  2217    Very narrow ash plume sincerely of summit.
    28 Mar  0035    Ash plume extending NW from the summit.
    28 Mar  2205    Ash plume moving W from the summit.
    12 Apr  1440    Narrow steam-and-ash plume extending to the SE of the
                      summit.
    12 Apr  2100    Thin plume moving E-SE.
    16 Apr  1345    Ash moving SW.
    19 Apr  1240    Small ash plume moving NW ~7 km from the summit.
    24 Apr  1400    Narrow ash plume extending to the SW, estimated up to
                      ~7.6 km.
    28 Apr  1430    Thin ash cloud.
    13 May  2350    Ash cloud moving NW.
    14 May  2230    Possible ash moving NW.
    02 Jun  1330    Thin ash plume moving W.
    04 Jun  2200    Very thin line of ash extending to the W.
    07 Jun  1245    Narrow ash plume extending to the W.
    12 Jun  2300    Very narrow plume of ash extending to the W.
    13 Jun  0500    Hot spots.
    15 Jun  0000    Faint ash plume moving W from the summit.

Activity during January-June 2002. During January 2002 steam plumes, sometimes with a little ash, were emitted almost continuously and generally reached less than 1 km high. Emissions and explosions were accompanied by moderate ash reaching 3-5 km. SO2 measurements on 13 January revealed a value of 1,030 tons/day, and a few days later had decreased to 290 tons/day. Beginning on 15 January, incandescence was observed in the crater and roaring sounds were heard from nearby villages. On 22 January, two volcano-tectonic (VT) events occurred at 0622, located at a depth of ~4 km. The VT events preceded a small cluster of long-period (LP) earthquakes and an explosion with a reduced displacement (RD) of 9.5 cm2.

On 24 January two VT events were registered at 0504 and 0605, located at depths of 5 and 4.4 km, respectively. The events preceded the occurrence of an LP cluster with dominant frequencies around 1.8, 2.4, and 3.8 Hz. Roaring sounds accompanied some of the LP earthquakes. Steam was emitted continuously and reached a height of 1 km before drifting W.

A cluster of LP events was registered on 25 January during 0700-1000 (with frequencies of 2.6-3.2 Hz); it was followed by emission signals. About 30 minutes later LP earthquakes increased and the settlements of Juive and Cusua (WNW flank) reported rockfalls. During 1000-1440 a 20-minute-long episode of harmonic tremor occurred followed by another LP cluster.

On 26 January during 1508-1629, two VT events occurred at depths of 5 and 7.5 km. One hour later, at 1739, an explosion took place with a RD of 7.2cm2. After the explosion, tremor was recorded and LP events continued for about 2 hours. For most of the activity, LP events preceded the explosions, and the seismicity was concentrated mostly at 5-7 km depth. VT events were mostly located 4-11 km beneath the summit and were aligned in a NNE direction.

LP earthquakes during late January through early February were associated with banded tremor during 24-28 January. A total of eight episodes of banded tremor were detected in one week, lasting 1-4 hours each. The LP earthquakes continued at an increased intensity at depths of 5-7 km beneath the summit. Small-to-moderate explosions also took place. On 28 January, an explosion occurred with a RD of 8.8 cm2. During these periods of increased tremor and LP events, the crater emitted gases and very little ash.

In early February a small amount of ash was deposited on the city of Ambato. Rain of moderate intensity caused mudflows to occur four times during February, with an especially large flow on 5 February.

On 4 February observations of the summit revealed new episodes of Strombolian activity, characterized by incandescence in the crater and by the ejection of large blocks to several hundred meters above the crater. During 11-18 February strong Strombolian activity was accompanied by pyroclastic flows that traveled part of the way to Juive and Cusua located on the WNW flank. Observers noted rapidly fluctuating incandescence in the crater, possibly due to disruptions in an inferred lava lake there. On 13 February a lava flow descended from the NNW part of the crater and extended 2 km below the summit. Beginning on 14 February both seismicity and incandescence decreased in intensity but ash emissions reached up to 2 km above the crater. During 18-25 February incandescence and roaring sounds decreased.

On 24 February a few large LP earthquakes occurred with 5 km focal depths and with RD values of more than 5 cm2. A tremor episode (with frequencies of 1.2-2.8 Hz) lasted ~25 minutes. During the night impressive Strombolian outbursts generated a flow of incandescent blocks. Banded tremor was associated with emissions of blocks and ash. This was followed by LP earthquakes associated with several hours of low activity. The intensity of the Strombolian activity diminished greatly with the absence of the LP earthquakes. Most of the LP events were located in a column 2-8 km beneath the summit. Some of the tremor episodes were located in the first 5 km beneath the summit. The explosions occurred at depths of 2-10 km.

During the last days of February roaring noises accompanied several cycles of Strombolian activity that were associated with lava fountains, and small ashfalls occurred on the W flank. By the end of February COSPEC measurements revealed an average value of 1,344 tons/day.

During the first weeks of March, most of the LP events were located 2-7 km beneath the summit, while the VT events were located 4-10 km beneath the summit. Some of the occasional explosions were impressive with large plumes, and some were preceded by important LP activity, suggesting the explosions were related to the ascent of gas bubbles that accumulated in the magmatic conduit. March emissions included steam, ash, and lava flows. Roaring sounds occurred almost continuously until the third week. Incandescence was observed in the crater twice during the last week of March, and a steam-and-ash plume was then visible reaching up to 2 km above the crater. Ashfall occurred in the cities of Ambato, Quero, Latacunga, Cusua, Chacauco, Penipe, Puela, Patate, Pelileo, Cotaló, and Pillate. COSPEC measurements revealed an increased average value of SO2 (>2,000 tons/day) compared to February (1,344 tons/day).

April was characterized by low seismicity, and nearly continuous tremor related to emissions of steam and ash. On 3 and 8 April two explosions occurred, with RD values of 7.3 cm2 and 4.5 cm2. Most of the LP events registered during April were small and rather sporadic. On 22 and 23 April, VT events occurred that were located at depths of 7.7 and 5.6 km, presumably related to the movement or evacuation of magmatic fluids. Following the VT events, strong steam-and-ash emissions occurred, accompanied by roaring noises and incandescence in the crater. This activity continued through the end of April and was especially strong during 24-30 April. Tremor energy decreased by the end of April. LP events were located 5-7 km beneath the summit. VT events were located at depths of 5-9 km.

During April a slight deformation was detected in the N flank. COSPEC-measured SO2 revealed low values; the mid-April average was 850 tons/day. A steam-and-ash column frequently reached up to 2 km high and drifted mainly NW. At times ash was deposited on the upper flanks. Lahars were also recorded, with the largest on 7 and 8 April. The number of LP and VT earthquakes, emissions, and tremor gradually increased during mid-May. The upper conduit filled with small volumes of magma so Strombolian activity, roaring noises, and incandescence in the crater was intense and almost continuous. During the last week of May columns of steam and ash were continuously emitted and drifted mainly W, and on 13 May, NW, depositing ash on Ambato and Baños.

During the second half of May noteworthy explosions took place on the 13th and 28th when nine and five explosions occurred, respectively, with RD values up to 7 cm2. Although similar activity continued throughout May, it was most intense during 12-13 and 28-30 May. VT events were located beneath the W-SW part of the crater at depths of 4-10 km. COSPEC-measured SO2 revealed a generally low average value of 950 tons/day during mid-May.

Strombolian activity was observed in early June along with lava fountains and incandescence. During the second week of June, the number of earthquakes and tremor episodes increased slightly and explosions occurred. The explosions were accompanied by loud noises and some by lava fountains that reached ~300 m above the summit. LP events were generally located 5-7 km beneath the summit and were thought to be related to magma injection. The VT events were located below the SW part of the crater at a depth of 4-7 km.

During the last week of June seismicity increased, followed by an important tremor episode. Powerful tremor occurred during a 3-day period (28-30 June) and was noteworthy because of the temporal variation of the frequencies from 1.5-2.7 Hz. The tremor sometimes lasted up to 1 hour with one amplitude that saturated seismograms. Many large LP events accompanied the tremor (RD over 10 cm2). Little surface activity accompanied the tremor and large LP events. The lack of an increase in the volume of ash and gas emitted seems to indicate that the gas is still trapped within the volcano and that it will eventually be released through intermittent activity or explosions. COSPEC-measured SO2 flux revealed a value of 1,800 tons/day. EDM measurements around that time indicated that the volcano inflated slightly.

Information Contact: Patty Mothes, Geophysical Institute (Instituto Geofísico, IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador, Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

12/2002 (BGVN 27:12) Summary of 2002 activity includes several episodes of intense seismicity

This report presents a summary of activity throughout 2002. During 2002 several episodes of intense seismic activity occurred that shared certain characteristics: clusters of long-period (LP) earthquakes, tremor related to ash emissions, and an increase in VT events on some occasions. Magmatic intrusions during January-March 2002, were generally preceded by LP clusters with dominate frequencies of 3.8 Hz with some oscillating around 1.5-1.6 Hz. Following these clusters, increased tremor occurred, some related to the emission of gas and ash. Eruptive activity included explosions and Strombolian blasts.

In April, activity changed, LP clusters ceased including events with a dominant frequency of 3.8 Hz and began to contain frequencies of ~6 Hz. Since June, VT events seemed to precede LP events or tremor episodes. Precursors of magmatic activity changed slightly. In almost every case, fewer precursory events were registered. Instituto Geofisica (IG) stated that the present eruptive process could be more uncertain than before. In September, the acceleration of processes seemed to indicate variations in internal conditions, such as changes in magma within the conduit, increased temperatures, diminishing percentages of crystals, lower SiO2, and addition of new gases.

During October-November there was none of the intense tremor activity that usually accompanies new magma injections. Energy remained at very low levels. IG stated that a large number of VT events and their decreased influence on volcanic activity could indicate a low contribution of magmatic gases that could be mobilized and released outside the volcano by means of explosions, continuous ash emissions, or Strombolian activity as previously observed. Further details of 2002 activity follow.

Detailed activity. During the first 2 weeks of January 2002 a high number of low-energy LP earthquakes took place. Some of the LP's were associated with emissions of mainly steam with a moderate magmatic gas concentration. During the last 2 weeks of the month the number of LP's increased remarkably. The LP's occurred in clusters, most of which were preceded by VT events at depths of 4-11 km beneath the summit. Beginning on 15 January it was possible to see a glow coming from the crater, accompanied by the emission of gases. While the emissions diminished during the last week of January, explosions increased in number and magnitude. By the end of January sporadic episodes of tremor and light ashfall occurred in Ambato and Baños. These seismic characteristics, along with frequent roaring noises that occurred with the explosions, indicated possible degassing of a small volume of magma that entered the conduit beginning on 15 January.

During February magma injection apparently disturbed the system, and new gases ascended. Steam and ash emissions occurred, as well as the possible formation of a lava lake. Strombolian activity during 4-18 February was so strong that pyroclastic flows (PF's) descended the WNW flank along the Juive and Cusua valleys. Seismicity was characterized by LP's, tremor related to emissions, a few volcano-tectonic events (VT's), and small explosions.

During the first 3 weeks of March there was Strombolian activity with emissions of lava, gas, and ash, and almost-continuous roaring noises. During the third week of March, activity diminished in intensity until it disappeared almost completely by the last week of the month. Although incandescence was observed at night, it was not as intense as that observed in previous months. Ashfall occurred in Ambato, Quero, Latacunga, Cusua, Chacauco, Penipe, Peula, Patate, Pelileo, Cotaló, and Pillate.

Most of the LP's registered during April were small and rather sporadic, but frequency content changed on 17 April from 4-4.8 Hz to 6-8 Hz. On 22 and 23 April, VT events at 6-8 km depths were followed by strong gas-and-ash emissions. These became quite intense during 24-30 April.

Activity was quite intense during 12-13 and 28-30 May. On 13 May a total of 8 explosions took place, preceded by an increase in the number of LP events. The same day ashfall occurred in Ambato and Baños. On 24 May VT activity took place just before an increase in explosive activity. During 17-26 May explosions were preceded by VT events, and by 30 and 31 May were preceded by LP events. As of the second week of May Strombolian activity, roaring noises, and incandescence in the crater was intense and almost constant. Lava was present in the crater, accompanied by tremor and ongoing emissions. During the last week of the month a continuous gas-ash column drifted mainly W.

During the last week of June intense tremor registered. The tremor occurred for 3 days and contained dominant frequencies of 2.2-2.7 and 1.5 Hz. Tremor lasted up to an hour with an amplitude that saturated seismographs. Many LP's and explosions accompanied the tremor. During June VT events (4-7 km deep) occurred just before tremor and LP events. Several LP's and tremor episodes preceded explosive events. On average the LP's and tremor occurred 2-4 hours before an explosion.

Explosions occurred during the first week of July. During the first 2 weeks, deep VT earthquakes (5-10 km deep) occurred at a rate of ~1 per day and there was an increase in the number of LP's and hybrid earthquakes. VT and LP events preceded new cycles of explosions, not immediately as had previously been noticed, but in this case by about 15 days. After the new cycle of explosive activity began, most of the LP events had frequencies of 1.5-2.5 Hz. Some VT's preceded the LP's and had frequencies of 3.8 and 1.5 Hz. During the second week intense roars were heard, and increasing ash emissions mainly drifted W. There was strong persistent incandescence, and frequent explosions produced loud noises and ash columns 2-4 km above the crater.

During the first 2 weeks of July, several episodes of Strombolian activity were observed, along with continuous but light ash emissions that were accompanied by roaring noises. Ash was deposited in a thin N-S strip between Hualcango and San Pedro de Sabañag (S of Quero), extending toward the W and Igualata. Ash accumulated up to 2.5 mm thick in "El Mirador" at Cerro Arrayán. Activity decreased toward the end of the month, when small plumes were emitted.

During 5-13 September, 8-10 VT earthquakes registered. These preceded the harmonic tremor seen during 13-21 September. Strong explosions and ash emissions also occurred. Ashfalls were noted in distant cities such as Píllaro and Riobamba, located ~30 km NW and SW, respectively.

During the first week of October explosions with reduced displacements greater than 10 cm2 took place and ashfall occurred in Pillate, Ambato, Cusua, Penipe, Altar, Bayusig, Matus Alto, and Matus Bajo. During the second and last week of the month VT events preceded explosions. During the last week of the month incandescence and roaring noises were heard. Three ashfalls were noted, two in Guadalupe and one (on 29 October) in Baños (up to 1 mm), Runtún, Pondoa, and Pintitin.

On 10 and 26 November, two peaks of LP activity occurred that were very close to the peaks of VT activity. The first LP peak preceded the first VT peak by two days. This was unusual because the VT peak normally preceded the LP peak. The second LP peak took place around the same time as the VT peak, indicating that the circulation of fluids was almost simultaneous. Incandescence was observed before the VT activity on 26 November. An increase of LP activity seemed to be correlated with the increase of sounds emitted by the volcano. Frequent incandescence in the crater preceded a VT peak.

Magmatic intrusions during 2002. Five magmatic intrusions (figure 18) apparently occurred during (1) 15-29 January, (2) 15-30 April, 12-13, 24-30 May, (3) 28-30 June, (4) 3-13 July, and (5) 5-13 September. Two periods of intense activity also occurred during 8-13 and 21-27 October, and on 10 and 26 November. During April-June magmatic intrusions did not occur along with a peak of seismic activity, but VT's, hybrids, and emissions all occurred, though in smaller numbers than registered in previous years.

Figure 18. Monthly earthquakes at Tungurahua during January 1999-November 2002. Peaks indicated with arrows correspond to periods of inferred magmatic intrusion. Courtesy IG.

Tremor activity was an essential indicator of these magmatic intrusions (figure 19). Later peaks of tremor activity were always during periods of seismicity related to magmatic intrusions, although it was not clear whether the June peak was related to a possible intrusion. Tremor energy was quite variable.

Figure 19. Tremor energy at Tungurahua, 14 September 1999 through 14 November 2002. Many of these tremor episodes were related to small emissions of gas or ash. Arrows indicate 2002 peaks. Courtesy IG.

Deformation measurements. During 2002 EDM measurements on the N flank showed a slight tendency of inflation. This inflation was first noticed during the first half of 2000. During 2002 a shortening of the distance occurred between prisms and reference bases, between -2 and -6 cm with respect to values observed before the reactivation of the volcano. Although there were variations in measurements taken during the year, the overall tendency has been inflation of 4 to 6 cm with respect to that during 1998-2000.

Data from inclinometers RETU and JUIV show a positive drift of the radial axis of station RETU (elevation 4,000 m). The drift would mean a deflation in the NW sector. During September 2002, when numerous explosions occurred, inclinometer movements changed.

During 2002 measurements of the inclinometer at station JUIV5 were stable until October 2002, when there were disturbances in the radial axis and to a greater degree in the tangential axis. Since 10 November both axes showed significant changes of up to 200 µrad. The negative tendency indicated a progressive inflation. This change agreed exactly with the first LP peak on 10 November. The change lasted until 20 November and included the greater peak of VT activity during 2002. After 20 November, both axes became stabilized. The oscillations seen in this slope between September and October occurred simultaneously with other activity, possibly representing slow but continuous magma movement in the lower parts of the volcano.

Geochemistry. SO2 flux measurements determined by COSPEC during 1999-2002 were generally less than 2,000 tons/day (figure 20). The peaks took place during March and October, with values reaching 3,000-5,000 tons/day. These high values seemed to correspond with the magma injections of December 2001and January and September 2002. Other episodes of seismic activity related to magmatic injection seemed to precede the peaks in SO2 emission. The high point in August ("3 y 4" on figure 14), followed increased seismicity during June and July.

Figure 20. COSPEC-measured SO2 emissions at Tungurahua during 1999-2002. The arrows indicate the peaks of SO2 that occurred during May and August 2002.

Thermal waters generally increased in temperature ~0.5°C. A small reduction in pH occurred, with a tendency toward alkaline values. During 1998-99, when the seismicity increased, pH also increased, probably because of the magmatic unrest at the time. Conductivity did not change, and neither did geochemical characteristics such as abundances of sulfates, chlorides, and bicarbonates. IG stated that it could not yet be explained how an increase in seismicity seemed to shift the pH of thermal waters (figure 21).

Figure 21. Temperature and pH of thermal waters at Tungurahua during 1994-2002. Courtesy IG.

Future scenarios. Since 1999 Tungurahua has shown frequent, moderate volcanism with occasional lava emissions. This period can be divided into 13 magmatic intrusions of similar characteristics, although the last three injections displayed slight differences. Starting in 1916 Tungurahua displayed intermittent activity until 1918, with periods of tranquility and greater activity than at present.

The present process has been characterized by LP clusters just before and during eruptions. During October and November 2002, VT events usually preceded cycles of increased activity. Strong incandescence on 2 December was not accompanied by strong explosions, Strombolian activity, or lava emissions.

Information Contacts: Patty Mothes and Indira Molina, Geophysical Institute (Instituto Geofísico, IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (Email: pmothes@igepn.edu.ec, imolina@igepn.edu.ec).

11/2003 (BGVN 28:11) Ash eruptions and other activity throughout 2003, but elevated after August

The Instituto Geofísico (IG) provided Tungurahua reports discussing the year 2003. Ash-bearing eruptions sent plumes as high as 9.4 km altitude, with resulting noticeable ashfall 40 km distant. Lahars were common and occasionally incandescent material descended the upper flanks. Activity was low during January and February, and increased slightly in March and again in June. In August activity increased again, and for the rest of the year it generally remained elevated. IG recognized a new phase of eruptive activity beginning 20 August. That phase consisted of long-period earthquakes followed by emissions reaching up to 3 km above the volcano (~8 km altitude).

Activity during January-February 2003. During these months volcanism generally remained low, with occasional emissions of gas and ash that produced low-level plumes. Incandescence was sometimes visible in the crater at night. Seismicity was low and was characterized by sporadic long-period earthquakes and low intensity emissions. Activity increased slightly beginning 12 February with an emission that rose to low levels and drifted W. A moderate explosion on 19 February deposited a small amount of ash on the ENE flanks (Cerro de Ulba and the Ulba valley). Seismicity increased slightly during the eruption, but returned to low levels afterwards. Volcanic and seismic activity remained low through early March with continuing gas and ash emissions.

Activity during March 2003. Activity began to intensify on 5 March when lahars descended the gorges on Tungurahua's NW flank, obstructing the road between the towns of Baños (~8 km N of the summit) and Pelileo (~13 km NNW of the summit). Around 7 March ash rose to ~7 km altitude and drifted SW. No ash was visible on satellite imagery. By 9 March several low-to-moderate explosions had occurred and this activity continued. On 11 March three small-to-moderate explosions deposited ash in the W-flank village of Pillate (8 km from the summit). That day a pilot reported ash to ~8.2 km altitude.

On 16 March a fine layer of ash accumulated in Baños. Sporadic explosions continued for the rest of the month, with one on 19 March that sent incandescent material ~1 km down the flanks. Explosions during this period were accompanied by Strombolian activity, gas-and-ash emissions, and loud roaring. Seismicity was dominated by tremor and long-period earthquakes, with tremor starting to decrease after 13 March.

Activity during April-May 2003. During early April, explosions occasionally occurred at the volcano. A pilot reported seeing ash at a height of around 2.3 km over Tungurahua on 6 April. No ash was detected on satellite imagery, however. Three explosions occurred on 7 April, with the largest plume rising to ~3 km above the volcano. Very little ash was visible in the plume. Activity dropped slightly for a few days, with sporadic explosions, until a large explosion occurred on 10 April, producing a plume with low ash content to ~2 km above the volcano. Volcanic explosions, generally small, continued the following week; minor vapor columns were also noted. Cloud cover obscured the volcano on some days, but an aviation report on 16 April mentioned that IG staff reported an ash cloud rising up to ~7 km altitude (~2 km above the summit). On 17 April two ash columns rose 1.5 and 2 km above the summit and blew SW and W, respectively. The volcano generally appeared relatively placid, but concern about mudflows and sudden increases in eruptive output remained. Limited visibility often prevailed, but it was noted that Tungurahua's behavior alternated between days of tranquility and those with small to moderate explosions. Few earthquakes occurred.

On 1 May an explosion sent ash to 2 km above the summit; incandescent material fell onto the flanks up to 0.8 km from the crater. Based on information from IG, the Washington VAAC reported that a small 6 May explosion yielded a cloud composed mainly of gas, with some ash. The cloud drifted W and seismic activity decreased after the explosion.

Activity during June-July 2003. Volcanic activity increased in early June. On 6 June, strong Strombolian activity hurled incandescent volcanic blocks ~500 m from the summit; plumes of mainly steam rose to around 2 km above the volcano and drifted W. Ash fell in the settlements of Pillate (8 km W of the summit), San Juan (~40 km WSW of the summit), and Riobamba (32 km SW of the summit), with accumulations of less than 1 mm. There were reports of airborne ash interfering with main flight routes across Ecuador. Emissions on 9 June reached 3-6 km above the volcano. On 10 June vibrations from an explosion were felt in Baños, explosions could be heard in towns near the volcano, and ash fell in several villages.

On 15 June incandescent blocks were hurled to ~150 m above the crater and rolled ~1 km down the N flank. During the evening of 17 June, Strombolian activity was visible at the summit, and an explosion on 18 June deposited ash on the settlements of Cusúa (~8 km NW of the summit), Juive (7 km NNW), and Pillate. Gas emissions with small amounts of ash occurred regularly, and on 19 June observers saw ash rise to 3 km above the summit.

During the last week of June, several explosions produced ash clouds; on 25 June ash fell in Pillate and in the town of Mocha (25 km W). Ash was visible on satellite imagery, with the highest-rising ash cloud reaching ~9.4 km altitude on 27 June. Emissions on 29 June deposited ash in Pillate, and in the towns of Cotaló (8 km NW of summit) and Cevallos.

On 1 and 2 July ash plumes rose to ~2 km above the volcano and ash fell in several towns near the volcano. Strombolian activity also occurred, and ash from the eruptions damaged crops and livestock near the volcano. A state of emergency was declared on 3 July, and food rations were distributed to residents of the town of Chimborazo. After 2 July, eruptive vigor remained relatively low through the rest of the month and into August. Reports noted mainly steam and gas emissions and low plumes.

Activity during August 2003. Tungurahua entered a new phase of activity on 20 August. The new phase was characterized by a short sequence of long-period earthquakes followed by gas-and-ash emissions that reached a maximum height of 3 km above the volcano.

A small amount of ash fell in Cusúa on 20 August. During the evening the volcano hurled incandescent blocks ~300 m above the summit and some traveled ~1 km downslope. On 21 August emissions of mostly steam and small amounts of ash rose ~1 km above the volcano and drifted W; ash fell in the Riobamba, Ambato (~33 km NW), and Santa Fé de Galán areas. On 23 August plumes rose to 0.5-2.5 km above the volcano, and ash fell in the town of Guaranda. On 24 August an explosion, heard in the town of Baños, ejected blocks that traveled ~1 km down the volcano's flanks. An emission on 27 August deposited ash in Ambato and caused flight restrictions to and from the airport there. During this week, volcanic block-and-ash emissions continued, with ash plumes rising to heights of ~4 km above the volcano. These drifted primarily W and SW and deposited ash in several towns.

Activity during September-October 2003. Moderate ash emissions and ashfall continued during September and October, accompanied in mid-September by tremor related to gas discharge. Seismicity ranged from moderate levels in September to a series of long-period earthquakes and explosions in early October.

Incandescence was observed in the crater on the evening of 7 September. On 15 September two emissions produced gas-and-ash plumes that reached a maximum height of 2 km above the volcano; ash fell predominately W of the volcano. On 22 September ash clouds reached a height of 3 km above the volcano and drifted W. On 24 September ash emissions produced plumes that drifted NW, depositing small amounts of ash in the towns of Quero (~20 km WNW of the summit), Puela (~8 km SW), Juive, and Cusúa. Volcanic blocks emitted during the eruption rolled ~1 km down the NW flank.

On 1 October gas-and-ash emissions reached a height of ~4 km and drifted NE and NW, depositing ash in San Juan (~40 km WSW), Pillate, and Valle del Patate. On 9 October ash fell on northerly sectors near the volcano, including Runtún (~6 km NNE of the summit), Juive, and Baños. Strombolian activity was seen during the evening of 12 October. Associated gas-and-ash plumes up to 2 km high drifted NNE and ash fell in Ambato. On the night of 18 October incandescent blocks rolled down the crater's W side. Incandescence and Strombolian activity were observed the following night. Activity decreased slightly on 20 October with fewer explosions and no major gas-and-ash eruptions recorded. Ash plumes were frequently visible on satellite imagery during 15-20 October.

Activity during November-December 2003. Tungurahua maintained generally low activity in early November, increasing towards month's end. Following a week of small-to-moderate eruptions of gas and ash, an eruption on 2 November produced a plume that rose to ~3 km above the volcano and drifted W. Over the next few days, occasional ash-poor plumes rose to less than 1 km above the summit; a few ash-bearing emissions did occur, including ashfalls of low intensity on 5, 6, and 7 November to the E. Also on 6 November seismic stations recorded two larger-than-average explosions, one associated with an ash column rising to 2 km. Seismicity returned to low levels, with relatively few earthquakes, but tremor continued.

During 12-18 November, small-to-moderate eruptions of steam, gas, and some ash continued; plumes rose to ~2.5 km above the volcano, but there were no reports of ashfall in nearby towns. Strombolian activity was visible at the crater and avalanches of incandescent volcanic material rolled ~1 km down the volcano's flanks. Activity increased to high levels beginning 19 November; numerous moderate explosions produced plumes that were frequently visible on satellite imagery and rose up to 2 km above the crater. Ash was dispersed SSW and SW on 19 and 20 November and WNW and NW on 23 and 24 November, respectively. Throughout the week Strombolian activity was visible at night.

During 22 November to 1 December, a large number of emissions of gas, steam, and ash occurred, depositing ash to the SW, W, and NW. Plumes were visible on satellite imagery at a maximum of ~7 km altitude.

During 11-16 December, volcanic activity remained relatively high with several explosions producing ash-and-gas plumes to a maximum of 9 km altitude. There were also many long-period earthquakes, occurring with nearly constant gas-and-ash emissions. Explosions on 11 December deposited ash in the towns of Quero, Santa Fe de Galán, and lesser amounts in Bilbao. Ash-and-gas plumes were visible on satellite imagery several times during the week.

Information Contact: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); El Comercio, Quito, Ecuador (URL: http://www.elcomercio. com/); Agence France-Presse.

01/2004 (BGVN 29:01) Frequent ash plumes, prompting occasional ash falls through January 2004

A comprehensive summary of activity at Tungurahua (figure 22) over the period January-16 December 2003 was reported in BGVN 28:11. During 17-23 December, volcanic activity continued at relatively high levels, with 5-19 moderate explosions and 62-83 long-period earthquakes each day. A signal from a lahar was recorded in the Cusúa sector NW of the volcano during the afternoon of 18 December. According to the Washington VAAC, during late 2003-early 2004, plumes from Tungurahua were visible on satellite imagery to a maximum altitude of ~ 7.5 km.

Figure 22. A map of Tungurahua emphasizing local roads, settlements, and drainages (quebradas in Spanish). Some of the latter are abbreviated as follows (counterclockwise from N): QA = Quebrada Achupashal, QC = Quebrada Confesionario, QM = Quebrada Mandur, QMT = Quebrada Motilones, QLP = Quebrada La Piramide, QPU = Quebrada Palma Urcu, QR = Quebrada Rea. The map is a composite of various published maps, particularly those by Hall and others in a previously referenced publication. The one contour shown, at 4,300 m elevation on the upper flanks, was the approximate limit of glacial ice when the 1:25,000 Tungurahua topographic map sheet was made for the Instituto Geográfico Militar in 1984. At the time of this report Pete Hall noted that remaining glacial ice was only apparent in restricted regions near the summit crater.

During 24-30 December the volcano emitted gas, and ash accompanied by low seismicity. On 28 December an emission sent a plume to 1.5 km above the summit, drifting E and NE. Ash fell in the Runtún sector E of the volcano. Emissions on 28 December deposited ash in Runtún and the city of Baños. On 30 December aircraft personnel reported seeing an ash cloud ~ 800 m above the volcano. According to the Washington VAAC, ash was visible on satellite imagery to a maximum altitude of ~ 8 km.

Emissions of gas, and ash, with low-level seismicity (12-14 long-period earthquakes per day), continued over the period 31 December to 5 January. Plumes rose to a maximum height of 3 km above the crater on 31 December and an emission on 4 January caused minor ash fall in the Puela sector (SW). Similar conditions applied in the week 7-13 January. On 8 January, emissions reached ~ 1 km above the volcano, traveling W and SW, and emissions on 12 January deposited ash in the Bilbao, Cusúa, Pillate, Ulba, Pondoa, Baños, Juive, Ambato, and Patate sectors (figure 3 and table 8). Gas, and ash emissions continued over 14-19 January, with emissions to ~ 1 km above the crater containing variable amounts of ash drifting to the N and NE, accompanied by low seismicity. Similar conditions applied over the week 21-26 January. On 22 January at 0626 an explosion sent a plume to ~ 2 km above Tungurahua. On the evening of 24 January ash fell in the areas of Puela and Penipe (~ 8 km SW); and during 24-25 January ash fell in Riobamba (~ 30 km SW). Patricia Mothes of Ecuador's Geophysical Institute reported on 30 January that the volcano was very quiet with no explosions since the previous Saturday, and that a country-wide drought caused extensive fires.

Table 8. A list showing settlements and their approximate distances and bearing from Tungurahua's summit. Many of these have been referred to in previous Bulletin reports with their locations undisclosed. Information sources included various maps, previously cited references, and Patricia Mothes of the Geophysical Institute.

    Town or Location    Distance & Direction

    Ambato                   31 km NW
    Banos                     8 km N
    Bilbao                    8 km W
    Cevallos                 23 km NW
    Cotalo                    8 km NW
    Cusua                     8 km NW
    Guaranda                 64 km SW
    Juive                     7 km NNW
    Latacungo                63 km NNW
    Matus                    11 km SSW
    Macas                   100 km SSE
    Mocha/Yanayacu           25 km W
    Overo                    20 km WNW
    Pelileo                   8 km N
    Penipe                  ~15 km SW
    Pillate                   8 km W
    Puela                     8 km SW
    Puyo                     50 km E
    Quero                    22 km NW
    Riobamba                 30 km S
    Rio Negro                26 km ENE
    Runtun                    6 km NNE
    San Juan                 40 km WSW
    San Juan de Pillate       9 km W
    Santa Fe de Galan        15 km WSW
    Valle del Patate         15 km NW

Bulletin editors have compiled a sketch map focused on Tungurahua's geography and a table showing other commonly used place names mentioned in this and previous reports (figure 22 and table 1). Tungurahua, and adjacent Holocene volcanoes El Alfair and Sangay, all lie to the S within Sangay National Park. On maps, roads are absent from Tungurahua's S and E flanks but pass around the W and N flanks. With regard to glacial ice, figure 3 shows the extent of ice depicted in 1984. Regarding the extent of glacial ice at the present time, Pete Hall noted that ice had been retreating from all summits in Ecuador (as in many other places, presumably due to global warming of the atmosphere). In discussing Tungurahua's alpine glacier, Hall made this comment: "The ice cap that was only on the tip top summit of the cone prior to [Tungurahua] entering into activity is still there, but buried under meters of black ash. We can still spot it when explosions in the crater eat away at it from the crater side. Occasionally we see ice on the outer flank, but only at the summit. Thus the ice cap is exceedingly small . . . ~ 300 m in diameter . . .."

Figure 23 shows the vulnerable N-flank town of Baños. It is the largest settlement in close proximity to Tungurahua. A host of gravity-driven processes could bury portions of the town in minutes. Figure 24 shows a satellite image of the E-drifting plume from Tungurahua on 14 January 2004.

Figure 23. Oblique view looking at the town of Baños, 8 km N of Tungurahua's summit on the Pastaza River, June 2003. Copyrighted photo used with permission of Tim Travis.
Figure 24. Satellite image showing ash plume from Tungurahua on 14 January 2004 (at 1530 UTC). The image came from the Moderate Resolution Imaging Spectroradiometer (MODIS), an instrument aboard the Terra satellite. Photo courtesy NOAA/NASA.

Information Contact: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Tim Travis [contact information removed by request] (URL: http://www.DownTheRoad.org/).

06/2005 (BGVN 30:06) Ash plumes and LP earthquakes still common in 2004-2005

The eruption of Tungurahua that began at the end of December 2003 (BGVN 28:11) continued through January 2004 (BGVN 29:01). Figure 25 shows an ash plume emitted on January 2004 in a Moderate Resolution Imaging Spectroradiometer (MODIS) image.

Figure 25. A NASA MODIS image showing an ash plume from Tungurahua acquired 14 January 2004. N is up; the plume's height and length were undisclosed. Arrow points to Tungurahua and is along the approximate trend of the densest portion of the plume. The plume blew NE across the Andes and remained visible well over the thickly vegetated lowlands farther E. (Visible Earth v1 ID 26233.) Courtesy of NASA. Inset map showing major active Ecuadorian volcanoes courtesy of the USGS.

On 5 February 2004 there was a slight increase in seismic activity at Tungurahua; steam emissions rose to low levels, and small lahars traveled down the volcano's W flank via the Achupashal and Chontapamba gorges. On 9 February emissions of steam, gas, and moderate amounts of ash occurred, deposited to the W in the sectors of Pillate and San Juan. During mid February, several avalanches of incandescent volcanic blocks traveled ~ 1 km down the volcano's flank. During late February through mid April 2004, degassing continued at Tungurahua with occasional explosions of steam, gas, and ash, producing plumes to ~ 500 m above the volcano.

On 2, 11, and 15 March lahars traveled through the Pampas sector. During the night of 28-29 March incandescent material was observed avalanching on the upper slopes. From 30 March to 3 April, volcanic activity was at relatively low levels, but emissions of steam and ash occurred, and incandescence was visible in the crater. On 4 April at 1902 an explosion produced a plume containing a moderate amount of ash that rose to 800 m above the crater, and on the evenings of 10 and 11 April, incandescence was visible in the crater.

Sulfur-dioxide flux measurements taken on 11 April were the highest measured for several weeks; 1,600-1,700 metric tons per day. Heavy rain during the afternoon and night of 13 April triggered a lahar that cut the La Pampa section of the Baños-Pelileo road.

Volcanic activity at Tungurahua at the end of April 2004 was at moderate levels. On 21 April, a column of steam, gas, and ash rose to a height of ~ 1 km above the volcano and drifted NW. Ash fell in Bilbao, Cusúa, San Juan, Cotaló, Pillate, and Juive sectors. A plume reached ~ 0.5 km on 22 April and deposited ash in the towns of Ambato (to the NW) and Baños (to the N). During the evening of 24 April, incandescence was visible in the crater, and incandescent blocks rolled a few meters down the volcano's NW flank.

Volcano-tectonic earthquakes on 27 and 28 April preceded a slight increase in the number of sudden explosions at Tungurahua on 30 April. According to news reports, ash fell in the towns of Cotaló, and San Juan (W of the volcano) on 1 and 2 May. The level of seismicity at Tungurahua decreased on 4 May. On 12 May, an explosion produced an ash cloud to ~ 3 km above the volcano that drifted SW, and on 13 May seismicity increased moderately, related to the increased numbers of emissions. Incandescence was visible at the lava dome during some nights.

From mid May through June, small-to-moderate emissions of gas, steam, and ash continued at Tungurahua. The highest rising plume reached ~ 2.5 km above the volcano on 23 May. On the morning of 19 May a mudflow occurred in the Pampas sector, but it did not affect the highway. Strombolian activity was visible in the crater on the evening of 23 May. During 2-8 June, activity remained moderate with several weak to moderate explosions recorded per day. Sporadically observed gas-and-ash and gas-and-steam plumes rose up to 1 km above the summit. A strong explosion on 5 June produced a gas-and-ash plume that rose 2 km above the summit. All plumes drifted W. Seismicity remained at moderate levels. On 3 June, possible lahars were noted on the N and NW flanks.

Several explosions occurred on 10 June, with the largest rising ~ 3 km above Tungurahua's summit and drifting W. A small amount of ash fell in the Pillate area, and a lahar destroyed a bridge in the Bibao zone. During mid to late June, there was a slight increase in volcanic activity at Tungurahua in comparison to the previous weeks. There were several emissions of steam, gas, and moderate amounts of ash, and 5-10 explosions occurred daily. Seismicity was characterized by long-period earthquakes.

From July through December 2004 the level of volcanic and seismic activity diminished at Tungurahua, with sporadic moderate explosions of ash and gas. The highest rising plume reached ~ 1.5 km above the volcano. Seismicity was at relatively low levels. Incandescence in the crater was observed at night on several occasions. Some explosions on 20 September generated plumes with ash, causing ashfall in Bilbao and Pondoa, and on the evening of 21 September, Strombolian activity was seen, with volcanic blocks thrown as high as 200 m above the volcano. On 27 October an explosion produced an ash column to a height of ~ 3.5 km above the volcano. During the evening, ash fell in the towns of Baños, Runtón, and El Salado. Explosions on 31 October also deposited small amounts of ash in Bilbao and Motilone, and on 15 November, incandescence was observed in the crater of the volcano and explosions generated steam columns with moderate ash content that rose ~ 2 km above the crater and drifted S. During 22-27 December, activity at Tungurahua consisted of small-to-moderate explosions, several long- period earthquakes, and episodes of tremor. Emissions of steam, gas, and small amounts of ash rose a maximum of 1.5 km on 22 December.

Increased seismicity and volcanic tremor registered at Tungurahua during early January 2005. There were eleven signals suggesting volcanic emissions and one small explosion. Seismicity then returned to a low level. On 11 January, steam plumes rose ~ 300 m above the volcano and extended WNW, and incandescence was observed emanating from the crater during 12-13 January. On 14 January, a white column of steam-and-gas was observed that reached a height of 500 m above the crater and extended to the NW. A steam- and-gas plume reached a height of 200-300 m above the crater on 16 January, and extended SE.

The Washington Volcanic Ash Advisory Center (VAAC) reported 18 January that an ash plume reached ~ 5.5 km altitude and extended to the E of Tungurahua's summit for ~ 15 km. During 19-24 January 2005, there were several emissions from Tungurahua of steam, gas, and ash. The plumes that were produced rose to a maximum height of ~ 1 km above the volcano and drifted in multiple directions, small amounts of ash falling in the sectors of Agoyán, San Francisco, Runtón, Pondoa, and Baños. Seismicity was at relatively low levels. Ash emission from Tungurahua on the evening of 25 January 2005 deposited a small amount of ash in the sector of Puela; ash was deposited on the volcano's N and W flanks on 26 January. The character of the eruption changed on 30 January to low-energy emissions of predominately steam. This type of activity continued through 31 January.

Volcanic and seismic activity was at low levels at Tungurahua during the period of February-mid July 2005. Low- energy plumes were emitted, and long-period earthquakes were recorded. Ashfall was reported in towns near the volcano, including Puela (SW of the volcano), San Juan de Pillate, Cusúaua, and Quern. On 23 February the daily sulfur-dioxide flux was 1,200 tons/day. On 27 and 28 February, rains generated lahars in the W zone of the volcano into the gorges of Cusúa and Bilbao. A moderate explosion occurred 18 April at 2057 that sent incandescent volcanic blocks rolling down the volcano's flanks. Ash fell S of the city of Ambato. On 20 and 21 April rain generated lahars that traveled down the volcano's W flank near the settlement of Bilbao (8 km W). An emission on 19 May around 1200 produced an ash-and- steam plume to ~ 500 m altitude that drifted N. On 7 June fine ash fell in the Puela sector, ~ 8 km SW of the volcano. On 24 June a narrow plume was identified in multispectal satellite imagery about an hour after an ash eruption was observed by the Instituto Geofisica. The ash plume was at an altitude of ~ 5.5 km and extended 35-45 km W from the summit. On 4 July 2005, low-energy plumes were emitted that rose to a maximum of ~ 5.8 km altitude.

Table 9 gives examples of some seismic statistics for several months during the reporting period from the Instituto Geofísico-Escuela Politécnica Nacional (IG).

Table 9. Summary of available seismicity (number of events) at Tungurahua during January 2004-March 2005 as published in IG monthly reports of March 2004, October 2004, and April 2005. Courtesy of the Instituto Geofísico-Escuela Politécnica Nacional (IG).

    Month/Year    Long-period    Volcano-tectonic    Emission    Explosions    Hybrid

    Jan 2004          365               6               217          28           0
    Feb 2004          255               8               147          16           0
    Mar 2004          123               7               123           2           0
    Aug 2004          620               5               142          22           0
    Sep 2004          674               9               119          43           0
    Oct 2004          390              14               168          53           0
    Jan 2005          138               8                92           6           0
    Feb 2005          113              20                29           0           0
    Mar 2005           54              20                 1           0           0

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC, 8800 Greenbelt Road, Greenbelt, MD 20771, USA (URL: http://earthobservatory.nasa.gov/NaturalHazards/; http://rapidfire.sci.gsfc.nasa.gov/).

07/2006 (BGVN 31:07) 14-15 July 2006 eruption, the strongest since onset of eruption in 1999

This report discusses Tungurahua's behavior during August 2005 through the end of July 2006. Material presented here was chiefly gleaned from a series of special reports issued in Spanish by the Instituto Geofísico of the Escuela Politécnica Nacional (IGEPN, hereafter IG). Daily reports for mid-2005 through early 2006 were dominated by descriptions of small plumes and minor ashfall; the reports also noted occasional small rain-generated lahars. For the most part 2005 was the quietest year since eruptions began in 1999, leading residents and volcanologists to ponder if emissions were terminating. This report omits much discussion of evacuations and hazard-status postings. Large eruptions with a Volcanic Explosivity Index (VEI) of 3 that continued into at least late August 2006 will be the subject of the next Bulletin report.

During late December 2005 seismometers detected sudden clusters of tremor and earthquakes. Intervals of quiet were broken by the arrival of signals with energy over a broad frequency range (figures 26 and 27). These signals and later manifestations at the surface in late March-early April were thought to be related to a new injection of magma. As a consequence, IG began to produce a series of special reports (table 10). Beginning in February 2006 and particularly during May-June 2006, the volcano was the scene of particularly significant events, including the largest detonations heard and seen since eruptions renewed in 1999. Other observations included a shift in eruptive style, and generation of some pyroclastic flows during the 14 July (VEI 2) eruption. Notable also were constant "roars" and vibrations of such strength and duration that they keep residents awake at night and caused some to voluntarily evacuate.

Figure 26. Plots showing daily tallies of Tungurahua's seismicity-volcano-tectonic, long-period, emission, explosion, total number of earthquakes, and total energy release-from 1 January 2003 to end of July 2006. Courtesy of IG.
Figure 27. (Top) Summary of seismicity recorded at Tungurahua's station RETU during 1 January and into August 2006 (slightly different end points for two plots). Numbers of events appears on left-hand scale; RSAM (line), in appropriate units, on right-hand scale (peak value is ~ 9 x 1019). (Bottom) Total energy liberated from volcanic tremor and explosions during January 2003 to 1 August 2006. The left-hand scale applies to tremor; the right-hand scale, explosions (reduced displacement). The sharp ascents formed by the "failed eruption" in mid May and the 14 July event are the largest increases since the activity's onset in 1999. Note the pronounced rise in reduced displacement from explosions in months 5-8 (May to August) 2006. Courtesy of IG.

Table 10. A summary of special reports on Tungurahua issued by the IG during 2006 (reports numbered 1-8; See IG web page-Informes Especiales-Volcanicos).

    Number    Date           Key observation(s)

    1         18 Feb 2006    Moderate explosions and tephra falls in Puela (SW), Paillate (W),
                               Ambato (NW) and Banos (N).
    2         07 Apr 2006    Episodes of strong volcanic tremor and increase in number of
                               long-period seismic events, indicating new magma injection.
    3         25 Apr 2006    Notable banded tremor, the inferred product of new injected magma
                               interacting with the hydrothermal system.
    4         12 May 2006    10 May-start of major increase in number of explosions, long-period
                               seismic events and tremor episodes. Very strong detonations (12
                               per hour). Peak of energy release on 14 May, then decline. Aborted
                               eruption.
    5         30 May 2006    Starting 16 May, significant decrease in activity and superficial
                               manifestations. Explosions occur 2-3 times per day; columns of
                               gases (water vapor mainly) with light ash content predominate.
    6         14 Jul 2006    Advisory of intensified eruptive activity; notable increase in
                               emissions and strong detonations (at 2210 & 2250 UTC).
                               Incandescent lava flung from crater. Strong ground movements
                               reported on W flank.
    7         14 Jul 2006    Very strong detonations, period of calm, then ascent of 15-km-high,
                               dark, ash-laden column. First pyroclastic flow (at 2250 UTC) and
                               others to descend six valleys (quebradas) on the W-NW-N flanks.
                               Intense lava fountaining; moderate ash and scoria fall to the W.
    8         15 Jul 2006    Unusually large discharges with a detonation (at 0559 UTC), leading
                               to the largest registered since 1999. Plume rose to ~ 15 km
                               altitude. The eruptive style later shifted to periodic detonations
                               with intervals of calm.

A map and table of commonly referred-to locations appeared in a previous issue (BGVN 29:01). Our last report on Tungurahua covered February 2004 to July 2005 (BGVN 30:06), during which time volcanic and seismic activity varied, but included some intervals with comparatively low activity and seismicity such as February to mid-July 2005.

Activity during June to mid-December 2005. From June 2005 through mid-December 2005, volcanic and seismic activity at Tungurahua was at relatively low levels. Low-energy plumes composed of gas, steam, and occasionally small amounts of ash were emitted frequently. Some noteworthy events during this interval follow.

On 7 June 2005, fine ash fell in the Puela sector, ~ 8 km SW. On 24 June, about an hour after an ash eruption, a narrow plume was identified in multispectral satellite imagery. The ash plume was at an altitude of ~ 5.5 km and extended 35-45 km W from the summit.

Ash plumes rose to an altitude of 5.8 km on 4 July. On 21 and 22 August, ash fell in the town of Bilbao, 8 km W of the volcano. On 25 August, ash fell NW of the volcano in the towns of Bilbao and Cusúa. On 1 September, ash fell ~ 8 km SW of the summit in the Puela sector.

On 10 September, a lahar affected an area near the new Baños-Penipe highway. On 14 September, a steam column with little ash reached ~ 300 m above the crater and drifted W; small amounts of ash fell in Puela. A small amount of ash fell in the towns of Cusúa and Bilbao during the morning of 21 September. Fumaroles on the outer edge of the crater were visible from Runtún (6 km NNE of the summit) after not being seen for 6 months. Steam-and-gas plumes rose ~ 1 km and drifted W. A pilot reported an ash plume on 29 September at an altitude of ~ 6.1 km.

During October, and November heavy rain caused lahars to travel down some of the gorges on the volcano's flanks. On 3 and 13 November lahars caused the temporary closure of the Baños-Riobamba highway, and a highway in Pampas. On 15 November ash plumes rose to ~ 9.1 km; on 23 November plumes rose to ~ 6.7 km.

On 13 December, lahars were generated at Tungurahua that traveled down the Juive (NNW) and Achupashal (W) gorges. On 14 December a steam-and-ash cloud rose ~ 1 km above the volcano. On 17 December, lahars were generated in the NW and W zone of the volcano. There were reports of lahars to the W in the Chontapamba sector that blocked the Baños-Penipe highway, in the Salado sector where the volume of water in the Vazcún increased by 70 percent, and in the NW (La Pampa) sector.

Return, incidence, and significance of broadband seismicity. An important variation in behavior was noted during late December 2005, with the appearance of long-period-earthquake swarms. The swarms preceded emissions and explosions. Such swarms were associated with mid-February 2006 ash-bearing explosions discussed below. After 21 March 2006, the swarms became yet more common and stronger. They were joined by low-frequency harmonic tremor.

Interpreted as related to the motion of magma, the tremor and swarms also seemed closely associated with lava fountains seen in the crater on 25 March 2006. Along with long-period earthquakes there were two episodes of high-amplitude tremor during 4-5 April 2006. Such seismicity had been absent for about a year. Small lava fountains witnessed on the night of 17 April 2006 were again preceded by long-period earthquakes and banded tremor.

As a result, IG distributed two special reports (#2 & 3). The latter contained a spectrogram for late April 2006, illustrating intervals of relative quiet (up to ~ 5 hours long) punctuated by broad-band signals (i.e. coincident earthquakes and tremor) sometimes in tight clusters lasting ~ 90 minutes.

January-May 2006. At the beginning of January 2006, explosions generated moderate amounts of ash, but seismicity remained low. Though clouds obscured the volcano during much of 18-24 January 2006, steam clouds with minor ash content were seen on 20 and 22 January. A discharge of muddy, sediment-laden water along W-flank valleys on 23-24 January blocked the highway. On 25 January light rain caused lahars to flow in the NW sector. The lahars descended a NNW-flank gorge from the village of Juive, causing the closure of the Baños-Penipe highway. Around 28 January, ash fell in the village of Puela. On 31 January, a steam-and-ash plume rose ~ 1 km above the volcano and drifted W. A small lahar closed a road in Pampas for 2 hours.

On 5 February at 0600, a moderate explosion sent a steam plume, with a small amount of ash, to ~ 1 km above the volcano; the plume drifted SW. Light rainfall on 7 February generated a lahar in the La Pampa area NW of the volcano.

During 6-14 February, several moderate-sized emissions of gas and ash occurred at Tungurahua, with plumes rising to ~ 500 m above the volcano. Long-period earthquakes increased in number on the 6th. An explosion around midnight on 12 February expelled incandescent volcanic material that traveled down the N flank of the volcano. A small amount of ash fell in the town of Puela, SW of the volcano.

IG issued a report (#1; Boletín Especial Volcán Tungurahua) on 18 February 2006 noting slight increases in activity that week. Explosions were moderate; however, ashfall occurred in some settlements bordering the volcano. IG summarized the week with a table similar to one below, with multiple cases of ash fall on local towns (table 11).

Table 11. A summary of Tungurahua's ash falls during an active interval, 13-18 February 2006, and the settlements affected. OVT stands for the Observatorio Volcán Tungurahua, a facility 13 km NW of the summit, down valley from the town of Patate. The report was issued at 1330 on the 18th, explaining why the entries only applied to the first half of that day. Courtesy of IG (special report #1).

    Date              Number       Location and comment
                  of explosions

    13 Feb 2006         1          Puela (~ 8 km SW of the summit), ashfall during the day.
    14 Feb 2006         4          Puela, ashfall during the afternoon.
    15 Feb 2006         4          No ashfall reported.
    16 Feb 2006         9          Runtun, ashfall at dawn; Observatory (OVT), Pelileo, Banos,
                                     and Garcia Moreno subjected to light ashfall in the morning;
                                     Bilbao, Cusua, Puela, Humbalo, Bolivar, and Pillate
                                     subjected to strong ashfall in the morning.
    17 Feb 2006         3          In Chacauco, Cusua, and Juive ash fell at dawn and part of the
                                     morning. In Pillate, Cotalo, Cusua, and Huambalo, ash in the
                                     morning. In Bilbao, ashfall all day; and Chogluntus, small
                                     ashfall during the afternoon.
    18  Feb 2006        1          In Banos, OVT, Salasaca, Pondoa, Bilbao, San Juan, and
    (until about                     Pelileo, ashfall at dawn. In Banos and Ambato, ashfall in
     1200)                           both the morning and afternoon.

Activity at Tungurahua during 28 February to 6 March consisted of low-level seismicity and emissions of steam and gas, with low ash content. An explosion on the 28th produced a plume composed of steam, gas, and some ash that reached ~ 3 km high.

In addition to the moderate explosions during 8-10 March, light drizzle produced muddy water in the gorges on the volcano's W flank. As a result the Baños-Penipe highway was closed for several hours. On 9 March, ash fell in the zone of Juive on the volcano's NW flank. On 10 March, ash fell in the towns of Pillate, Pondoa, Runtún, and Cusúa (on the W to NW to NNE flanks).

During 16-20 March, small-to-moderate explosions occurred at Tungurahua that consisted of gas, steam, and small amounts of ash. Plumes rose to ~ 3 km above the volcano. During 22-27 March, similar explosions consisted of gas, steam, and small amounts of ash. Plumes rose as high as ~ 1 km above the volcano on several days. An explosion on 26 March was accompanied by incandescent blocks that rolled down the volcano's NW flank.

On 18 February, small amounts of ashfall were reported at the observatory, Cotaló, Cusúa, and other settlements (table 11). On 19 February, rainfall generated a small mudflow SW of the volcano in the Quebrada Rea sector of Puela.

Table 12 summarizes observations associated with plumes and seismicity during 15 February to 8 May 2006. Many observations in that interval noted small-to-moderate explosions or other emissions. Ash plumes to 1-3 km above the volcano (6-8 km altitude) were typical.

Table 12. A compilation of some daily and weekly observations from Tungurahua during 15 February to 8 May 2006. Courtesy of IG.

    Date range       Description of activity (plume heights in kilometers above the summit).
     (2006)

    15 Feb-19 Feb    Ash plume as high as 3 km.
    26 Feb-27 Feb    Steam and gas with low ash content; on the 26th, 1 plume to ~ 3 km; on the
                       27th, to 1 km. Both plumes drifted NW.
    28 Feb-06 Mar    Steam and gas with low ash content; on the 28th, 1 plume to ~ 3 km. Low
                       seismicity.
    08 Mar-10 Mar    Several explosions with low ash content resulting in plumes under 2 km.
    16 Mar-20 Mar    Small-to-moderate explosions consisting of gas, steam, and small amounts of
                       ash. Plumes rose to ~ 3 km.
    22 Mar-27 Mar    Small-to-moderate explosions consisting of gas, steam, and small amounts of
                       ash. Plumes rose as high as ~ 1 km above the volcano on several days.
    29 Mar-02 Apr    Small-to-moderate explosions consisting of gas, steam, and small amounts of
                       ash.
    04 Apr-10 Apr    Small-to-moderate explosions at Tungurahua consisting of gas, steam, and
                       small amounts of ash. On the 9th, plumes rose to ~ 3 km.
    11 Apr-17 Apr    Small-to-moderate explosions produced gas, steam, and small amounts of ash.
                       On the 13th, plumes rose to ~ 2 km. High seismicity.
    19 Apr-23 Apr    Small-to-moderate explosions produced gas, steam, and small amounts of ash.
                       On the 19th, plumes rose to ~ 3 km. High seismicity.
    28 Apr-01 May    Small-to-moderate explosions; gas, steam, and small amounts of ash. On the
                       28th, a plume rose to a maximum height of ~ 2 km. High seismicity.
    04 May-08 May    Small-to-moderate explosions; gas, steam, and small amounts of ash. High
                       seismicity, dominated by explosions and long-period earthquakes.

During this 15 February to 8 May time interval ash affected localities as follows. During 29 March to 2 April, ash fell in the Bilbao, Choglontus, Puela, and Manzano sectors, and incandescent blocks rolled down the volcano's NW flank. Around 9 March, ash fell in the Baños, Guadalupe, Chogluntus, Bilbao, and Manzano sectors. Around 1500 on 9 March, several lahars traveled down W-flank gorges, disrupting traffic along the Baños-Penipe highway. An explosion on 26 March was accompanied by incandescent blocks that rolled down the NW flank. During 11-17 April, a small amount of ash fell in the Pondoa sector N of the volcano.

Increased activity starting 10 May 2006. Seismicity for mid-April 2006 to mid-August 2006 appears in figure 28. The figure shows the time sequence of hypocenters with various signal types given separate symbols. Between April and May there was a shallowing of event locations (indicated by the arrow on the left) from -4 km to +2 km. At that point, explosion signals suddenly began to dominate. Those explosion signals came from depths in the range from 0 to over +4 km depth. The 14 May seismic crisis seemingly ended without a large eruption. Explosion signals continued; however, they ceased dominating until around the time of the 14 July eruption when they again became the chief signal (circled area) just prior to the eruption breaking out at the surface.

Figure 28. Temporal evolution of depth for various kinds of hypocenters recorded at Tungurahua between April and August 2006. Left-hand scale, depth, is fixed to sea level (i.e. 0 is at mean sea level.). The legend shows the symbols for the various signal types shown: VT (volcano-tectonic earthquakes), LP (long-period earthquakes), EXP (explosion signals), and EMI (emission signals). Courtesy of IG.

IG put out special report #4 with a cautionary tone. In the 48 hours starting around 10 May, there was a very important increase in activity. IG judged the anomalous, high-activity conditions as severe as previous ones during this crisis (specifically, equivalent to those of October-December 1999, August 2001, September 2002, and October 2003). The summary that follows largely omits the discussion of plausible scenarios aimed at public safety; however, the IG noted that if rapid escalation were to occur during the current unstable situation, they might not have time to issue alerts. They also noted that the eruption might calm.

During the roughly two-day interval, seismometers registered over 130 explosion signals, averaging about three explosions per hour, but with a maximum of 12 per hour. The general tendency was towards yet more increases in the number of explosion signals. The activity was accompanied by continuous signals described as harmonic tremor and emission-related tremor, and after 10 May these tremor signals were also more intense and frequent. In spite of the increase in explosion and tremor signals, emissions of magmatic gases (SO2) and ash stayed at relatively low levels.

First-hand observations during 10-12 May described extraordinarily loud explosions heard from 30-40 km away in Pillaro and from~ 31 km NW in Ambato, but absent 30 km SW in Riobamba. In settlements near the volcano, including Cusúa on the volcano's W foot, glass windows shattered. In some areas, roars were sufficiently intense that vibrations in windows and houses kept inhabitants awake at night. The intensities of eruptions from 10 May were reminiscent of the eruption's onset in 1999.

From the observatory in the Guadelupe sector (13 km NW of the cone) night observers saw the ejection and rolling descent of large glowing blocks of lava, and the crater gave off a permanent glow. However, ash emissions were considerably reduced; the chief component venting was steam with few other gases. The resulting outbursts were not continuous and they were too weak to form mushroom clouds. This was in contrast to other periods of high activity (e.g. August 2001, September 2002, and October 2003), when sustained ash-bearing eruption columns and ash falls were common.

IG special report #4 noted that the tremor signals during a 48-hour interval after 10 May were the strongest recorded since the eruptions renewed in 1999. The number of explosions and their seismic energy were the highest recorded since the end of 2003, but was less than registered during November 1999 and mid-2000.

On 30 May IG issued its next special report (#5), which noted elevated eruptive activity during 8-14 May, but a clear decrease thereafter. During 10-21 May, the following instruments detected the stated numbers of explosions: seismometers, 801; and infrasonic recorders, 682. The peak in these explosions occurred on 14 May, a day when the instrument counts were as follows: seismometers, 221; infrasonic, 204. As in the previous report, inhabitants close to the volcano heard loud roars, and in some cases were sleepless due to vibrations heard or felt in their homes at night. These conditions convinced residents in Cusúa to move during the night. But starting the 16th, the number and intensity of explosions per day decreased drastically, with only 17 explosions recorded on the 16th, dropping in later days to 2 or 3 daily explosions. According to a local mayor, given the lack of noises and relative calm, evacuees from Cusúa returned home.

The lull in explosions coincided with ongoing fluctuations in seismicity. The IG interpreted this as a sign of continued instability linked to the motion of fluids at depth. The lull in explosion signals accompanied increased gas emissions, which gradually came to contain more and more ash. Small, local ash fall again began to occur. Starting 17 May it became common to see ash columns extending to 4 km above the summit, frequently blown NW.

Reports for the week following 17 May by the Washington VAAC also discussed the increasing ash plumes. On 18 May, an ash plume reached an altitude of 5.2 km above the crater and extended NW. The Washington VAAC also noted that on 19 May, the Instituto Geofísico observed an ash plume that reached an altitude of 12 km. On satellite imagery, ash plumes were visible on 20 and 23 May and extended SW. Hotspots were visible on satellite imagery 19, 20 and 23 May. The ash plume and incandescence on 23 May were also observed on the scene by Instituto Geofísico staff. On 25 May a significant meteorological advisory (SIGMET) indicated an ash plume to an altitude of 5 km. On 27 and 30 May, the VAAC reported that the Instituto Geofísico observed ash plumes at altitudes of 7.9 km and 5 km respectively. IG noted that behavior during the last few weeks of May seemed consistent with a gradual decrease from the state of elevated activity seen in mid-May.

Although satellite thermal data produced alerts during 8-14 May, these ceased later in the month. The reduced thermal flux was taken to suggest reduced manifestations in the crater during mid to late May. Coincident with that, deformation data suggested relative stability, particularly compared to the significant variations seen earlier in May.

During 28 June-4 July, small-to-moderate explosions at Tungurahua produced plumes composed of gas, steam, and small amounts of ash that reached 1.5 km above the summit. Light ashfall was reported in nearby localities during 29 June-2 July. On 29 June, reports of ground movement coincided with an explosive eruption that sent blocks of incandescent material as far as 1 km down the W flank.

During 5-11 July, seismic activity indicating explosions increased at Tungurahua. Incandescent blocks were ejected from the crater during 5 to 8 July, when blocks rolled approximately 1 km down the NW flank. Ash-and-steam plumes with moderate to no ash content were observed to reach maximum heights of 2.5 km above the summit and drifted to the W and NW.

Eruptive style changes after powerful discharges of mid-July 2006. On 14 and 15 July, IG issued its next special reports (#6, 7, and 8) documenting events surrounding the strongest eruption yet seen during the entire 1999-2006 eruptive process. The basis for the size assesment was made from the seismic record based on reduced displacement, sometimes called normalized or root-mean-square amplitude (a means to correct seismic data to a common reference point; McNutt, 2000) The largest discharge occurred at 0559 on 15 July.

On 14 July, seismicity was elevated above that seen in the previous several days. IG noted that at 1710 several large explosions were recorded on instruments, as well as heard by people. An eruption column formed, bearing moderate ash. It initially rose several kilometers but later was estimated to have attained ~ 15 km altitude. This was followed by 20 minutes of quiet. At 1733 a huge explosion presumably opened the conduit. Immediately local authorities were contacted and they evacuated people living on the lower NW-W flanks of the cone. Pyroclastic flows and explosion signals are notable in the seismic record (figure 29).

Figure 29. Consecutive records for 14-15 July 2006 (upper and lower panels, respectively) observed from the broadband seismic station Mson located on Tungurahua's SW flank at 3.2 km elevation. Time marks on the y-axis show hours (0 to 24) of the day, x-axis marks show minutes (0 to 60). Note the relative quiet on 14 July prior to eruption's onset at 1733. The latter was preceded by ~ 20 minutes of tremor. Courtesy of IG.

At 0050 on 14 July a pyroclastic flow poured down the NW flank (the Juive Grande drainage). An associated fine ashfall was noted 8 km SW in the town of Puela. Intense Strombolian activity ensued, including glowing blocks tossed 500 m above the crater that bounced downslope for considerable distances. Associated noises were particularly loud and heard widely, including in Ambato (30 km NW). Lookouts described these sounds as distinctive ("bramidos doble golpe;" roughly translated as 'double roars'), a new sound in the suite of those heard since 1999. In the Cusúa area, and up to 13 km NW in the sector of the Observatory of Guadelupe, residents felt intense ground movements.

At 1930 that day pumice fell on the W flank (the sector of Pillate) reaching a thickness of ~ 1 cm. About 10 minutes after the pumice fall, the IG issued the second special report (#7) on the 14 July events. It cautioned residents to remain away from the volcano's W side. The next special report (#8) noted that variations in activity prevailed through the end of 14 July, and that much of the first hour of 15 July brought decreased activity. Tremor continued on 15 July, often in episodes with durations of 4 to 5 minutes, separated by intervening calm intervals of similar duration.

After 0500 on 15 July the eruptive process changed, with the new regime characterized by sequences of abundant large explosions followed by intervals of calm lasting 30-40 minutes. A critical detonation occurred at 0559 on 15 July. On the basis of reduced displacement, it ranked as the largest since the eruption began in 1999. Other detonations with similar character followed the initial one. During 0500-0555 there were 20 large detonations. In assessing the 14-15 July eruptions, satellite analysis by both the Washington Volcanic Ash Advisory Center and the U.S. Air Force Weather Agency confirmed the highest ash-plume tops to altitudes of 15-16 km.

At sunup on 15 July observers found signs that a pyroclastic flow had descended a W-flank drainage (Achupashal valley, between Cusúa and Bilbao). The deposits filled the valley (to 5- to 10-m thickness). Small fires had ignited in the vegetation. A rockfall was also seen in the Bilbao area. Ash falls were reported, containing both ash and scoria fragments, affecting the cities of Penipe, Quero, Cevallo, Mocha, Riobamba, and Guaranda.

Additional fieldwork revealed that pyroclastic flows had traveled down at least six quebradas around the volcano, including Achupasal, Cusúa, Mandur, Hacienda, Juive Grande, and Vascún valleys (the latter, upslope from the western part of the touristic city of Baños).

Figures 30-33 depict the distribution of fresh deposits as well as some photos taken during the 14-15 July eruptions. Tilt and SO2 monitored at Tungurahua appear on figures 34 and 35. Satellite photos from 25 June and 18 July appeared on the NASA Earth Observatory website.

Figure 30. Paths where pyroclastic flows descended during Tungurahua's eruption of 14-15 July 2006. The associated ashfall deposits are identified at points W of the volcano's summit (thicknesses in mm). For scale, adjacent E-W grid lines are 4.44 km apart (and Cotalo, on the NW flank is ~8.5 km from the summit). Grid lines are latitude and longitude in degrees (heavy type) and decimal degrees (light type); lines are separated by 0.04 degrees N-S, and 0.05 degrees E-W. Courtesy of IG.
Figure 31. Pyroclastic flow routes and deposits on Tungurahua's lower W flank (near Cusúa). Photographed 14 July 2006 Courtesy of IG.
Figure 32. Three photos depicting the onset of strong pyroclastic flows on Tungurahua at about 1814 on 14 July 2006. This particular pyroclastic flow descended the Juive Grande river valley. Photo taken from Loma Grande, located about 9 km NNW of the crater. Photographed by L. Gomezjurado; courtesy of IG.
Figure 33. At Tungurahua, a pyroclastic flow descending the NW-trending Mandur valley at 0653 on 16 July 2006. Photo by P. Mothes, IG.
Figure 34. Plot showing radial tilt (at station RETU located at 4 km elevation on the N flank), 13 April-11 August 2006. During mid-May to mid-June 2006, tilt at the instrument had been in an inflationary trend. Around 22 June the tilt shifted to deflation, which became strong for a few day just prior to the eruption. The eruption occurred after several hours of sudden inflation. After the eruption, the broad deflationary trend continued until around the beginning of August.Courtesy of IG.
Figure 35. SO2 flux at Tungurahua as measured by DOAS, July 2004-July 2006. Courtesy of IG.

Reference. McNutt, S., 2000, Seismic monitoring, in Encyclopedia of Volcanoes: Academic Press (editor-in-chief, Haraldur Sigurdsson), p. 1095-1119, ISBN 0-12-643140-X.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

12/2006 (BGVN 31:12) Intense ongoing activity in 2006; new bulge on the N flank

According to the Instituto Geofísico-Escuela Politécnica Nacional (IG), Tungurahua, located 180 km S of the capital Quito, erupted on 14 July 2006 (BGVN 31:07), and again with great intensity in mid-August, resulting in at least five deaths. The 14 July event marked the beginning of a new energetic phase of activity different from that seen since October 1999. This report was taken from the IG's Special Reports and Bulletins (Numbers 7-15) discussing events from mid-July 2006 through early January 2007.

The new phase that began in July 2006 was characterized by highly explosive activity. It was associated with the arrival of a large volume of magma and the expulsion of hot, sometimes incandescent, pyroclastic flows. These flows traveled downslope with speeds of ~ 40 km/hour from the W, NW, and N flanks of the cone and ended at the Chambo river. The primary route of the flows was via the Achupashal, Cusua, La Hacienda, Juive Grande, Mandur, and Vascún gorges (see maps and figures in BGVN 31:07). Some of these flows were sufficiently large and mobile as to reach the Baños-Penipe road. The settlements of Cusúa, Bilbao, and to a lesser extent Juive Grande, were affected by these flows, which devastated pastures, fields, livestock, and basic infrastructure.

Additionally, small-volume pyroclastic flows descended the Vascún valley (upslope from the western part of Baños) but ended 2.5 km upstream from the hot springs of El Salado. The emission of hot incandescent pyroclastic flows ended one week after the explosive eruptions of 14-15 July.

Volcanic activity decreased significantly several weeks after 14 July, as shown by the seismic-based indicator developed for Tungurahua by the IG (figure 36). The daily indicator value decreased after mid-July and remained consistent until a slight increase during 8-13 August.

Figure 36. Index of daily seismic values (top) and index expressed in percentages (bottom) for Tungurahua from 1 January 2006 to 2 January 2007. Arrows mark the dates of the eruptions mentioned in this report. Courtesy of IG.

On 1 August, a pyroclastic flow traveled W and SW down the flanks and reached the Rea gorge, where it left deposits an estimated 50 m thick consisting largely of blocks and ash. On 2 August, a small lahar traveled NW and blocked a highway. Strombolian activity was observed at night on 3 August. Small explosions were registered during 3-7 August. On 6 August, light ashfall was reported ~ 8 km SW in the town of Manzano.

During 9-15 August, small-to-moderate explosions produced plumes composed of gas, steam, and small amounts of ash that reached heights of ~ 1 km above the summit. Light ashfall was reported in nearby localities during 9-10 August. On 9 and 13 August, explosions expelled blocks of incandescent material that rolled 100 m down the W flank.

Bulge detected and intense explosive eruptions on 16 August. Clinometer measurements on 16 August indicated a bulge on the N flank as compared to 11 August. Seismic activity increased, as ash-and-gas plumes reached heights of 3 km above the summit and drifted W and NW. About 3,200 people were evacuated from "at-risk" areas. At 1900 on 16 August, a new, intense explosive eruption began that continued throughout the day. A large pyroclastic flow did not reach the road.

The eruption continued through 0145 on 17 August at high levels of intensity, ejecting incandescent rocks and generating pyroclastic flows. Several pyroclastic flows were reported in the Achupashal, Mandur y La Hacienda, Juive and Vascún ravines. On the 17th tephra fall (with pumice clasts ~ 3 cm in diameter) was reported from several areas in a wide zone that extended from Penipe in the SW to ~ 15 km NW. Ash plumes reached estimated heights of 10 km above the summit and covered the central part of Ecuador, forming a cloud ~ 742 km long and ~ 185 km wide trending NNW and SSE.

As of mid-August, about 20 pyroclastic flows had descended the above-mentioned ravines, and possibly others on the SW flank. At 2125 the largest pyroclastic flow of the sequence descended the Achupashal ravine and reached and dammed the Chambo river.

At 0033 on the 18th activity intensified, generating several pyroclastic flows and ashfall in several sectors from Penipe in the SW to the N; hot ashfall was reported in the villages of Pelileo, Cevallos, San Juan, and Cotaló. The large quantity of accumulated ash on roofs in the village of Pillate caused their collapse. The city of Baños lost electrical service. During this period, one of the pyroclastic flows descended near Juive Grande and crossed the Ambato-Baños. A sustained eruption column appeared incandescent and glowing at the base, with an associated ash cloud to 7 km above the crater that was blown W and SW. Continuous sprays and jets of lava rose hundreds of meters above the crater.

The explosive eruption on 16-17 August culminated around 0200 (figure 37), after which time the activity gradually decreased. The IG report stated that on 17 August, although the eruptive activity had ceased, longer lasting events including movement of magma and continuous deformation on the N flank indicated that residual magmatic fluid in the volcanic system was putting pressure on the structure. Additionally, emission of SO2 continued to be detected in moderate amounts. This report (Special Report #14, 17 August 2006) also reminded authorities that the events of 1918 included five explosive eruptions interspersed with periods of reduced activity and that the entire W flank remained at risk of collapse, which could release a large volume of lava and produce much larger pyroclastic flows. Communities at most risk would be on the NW flank (Juive Grande, Los Pájaros, Cusua, Bilbao), but more distant zones at the bottom of the Chambo and Pastaza valleys (Chacauco, and eventually Puñapí) would also be affected.

Figure 37. An interpreted satellite image of a tall Tungurahua eruption column. The satellite was NOAA-18 (Channel 4-5) (1.5 nautical mile resolution) at 0209 (0709 UTC) on 17 August 2006. Courtesy of the Air Force Weather Agency.

Dramatic developments. On 18 August, incandescent blocks ejected from the summit descended ~ 1.7 km down the flank. Also that day, based on seismic interpretation, one of the blockages damming part of the Chambo River had been breached.

During 18-19 August, the N flank continued to inflate. During 20-21 August, steam emissions were observed during breaks in the cloud cover and the N flank exhibited deflation. On 23 August, two slow-moving lava flows were identified on the NW slope.

According to news reports, falling ash and debris caused fires and severe damage to five villages. An estimated 20,000 hectares of crops were destroyed. At least five people were dead or missing, and several more were injured. An estimated 4,000 people relocated to shelters.

Relative quiet, late August-October. During 23-27 August, visual observations of Tungurahua were impaired due to inclement weather. Based on seismic interpretation, lava continued to slowly flow NW towards Cusúa and La Hacienda. Seismicity was low and dominated by long-period earthquakes. Inclinometer measurements indicated no additional inflation on the flanks.

During September, seismicity remained low. On 1 September, lava flows on the NW flank were confirmed to have ceased. On 7 September lahars descended the NW gorges of Chontapamba and Mandur. During the month, there were several steam-and-gas plumes with little or no ash content. The emission heights ranged from ~ 0.1 to 2 km above the summit, and the primary wind drift was to the NW and W. Incandescence at the summit was observed at night. On the afternoon of 21 and on 22 September, moderate ash emissions occurred. Three more emissions on 23 September caused ashfall in Penipe; one plume rose 3 km and another 4 km above the summit. Ash plumes were seen again on 25 September.

No ash emissions were reported between 27 September and 2 October; however, on 2 October a slow-moving lava flow was seen descending the NNW flank and some fumarolic activity from the crater was observed. On 3 October an explosion resulted in ash falling in nearby communities to the W. According to the IG and aviation sources, the plume rose to at least 5 km above the summit.

During 4-5 October, Tungurahua's N flank fumaroles were active and steam emissions with minor ash content rose to 1 km above the summit and drifted W. Additional steam plumes possibly originated from the recent lava-flow's front. The IG again reported an increase in emissions and seismicity on 11 and 12 October, when steam plumes with slight to moderate amounts of ash reached 9-12 km altitude. Light ash fell in areas to the NW and W. During 13-17 October, seismicity decreased and plumes reached 7-8 km altitude.

On 16 October a small lava flow spalled off incandescent blocks; gas plumes were observed. Lahars traveled N toward Baños and down the Vazcún and Ulba gorges.

During 18-19 October, ash emissions increased in intensity and seismic tremor was continuous. During the night, lava fountains reached heights of 1 km above the crater rim and blocks rolled 800 m down the flanks. According to the Washington VAAC, around this time a pilot reported an ash plume to an altitude of 8.5 km. Ash plumes drifted NE and E and generated ashfall about 50 km E, in Puyo. According to news articles, about 300 villagers were evacuated.

Emissions continued during 20-24 October, producing plumes to 7-8 km. Ashfall was reported from towns on the N, NW, W, SW, and E flanks. On 28 October, incandescent blocks were expelled from the summit and rolled about 500 m down the W and E flanks. The next day, a lahar traveled NNW down the Mandur drainage and muddy water swelled in the Vazcún drainage. Incandescence from the crater was seen during most of October.

Ash plumes of steam and gas, and moderate ashfalls, were reported from several downwind towns on 5 and 6 November, including Bilbao (8 km W), Cotaló (13 km NW), and Manzano (8 km SW). On 2 November incandescent blocks were expelled from the summit and rolled 700 m down the W and E flanks. Nighttime incandescence was observed during 2-4 November.

On 7 November, a voluminous lahar traveled down gorges to the W and reached as far as the Chambo river, ~ 7 km from the summit. On 8 November, blocks expelled from the summit rolled down the flanks and ashfall was reported from areas including Casúa (7 km NW) and Baños (8 km NE). On 10, 11, and 13 November, ash fall was reported from areas including Penipe (8 km SW). During 12-13 November, lahars traveled down W and NW drainages and the Vazcún river swelled with muddy water.

On 17 November, an ash plume reached an altitude greater than 10 km and drifted NW and NE. During 26-27 November, Strombolian activity propelled incandescent material up to 600 m above the summit. Blocks rolled 2 km down the flanks. Lightning was visible in an ash plume that reached 7 km altitude and ashfall was reported from areas 8 km WSW. On 27 November, an ash plume rose to 9 km and drifted W. These conditions continued on into early December. On 6 December, plumes reached an altitude of 10 km. Ashfall was reported in areas including Cotaló, about 13 km NW, Pillate, about 7 km to the W, and Puela, about 8 km SW. On 9 December, ashfall up to 1 mm thick was reported about 12 km N in Baños.

Around this time seismicity was minimal in both intensity and duration. The plumes drifted in multiple directions. On 14 December, a lahar traveled SW down the Mapayacu gorge. On 21 December, lahars from Tungurahua traveled NW down the Mandur gorge resulting in a road closing and W down the Bilbao gorge. Gas-and-steam emissions produced small plumes on 22, 23, and 25 December. During 27 December-2 January, seismicity at Tungurahua remained moderate to low. On 27 and 28 December, lahars traveled down drainages including Bilbao to the W, Mandur to the NNW, and Mapayacu to the SW. During 3-9 January, seismicity at Tungurahua remained low to moderate.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); International Federation of Red Cross And Red Crescent Societies (IFRC) (URL: http://www.reliefweb.int/); Agence France-Presse (URL: http://www.afp.com/); Associated Press (URL: http://www.ap.org/).

04/2007 (BGVN 32:04) Post-eruptive quiet spurs return of residents, but activity increases again in 2007

This report covers the time interval early January to 2 March 2007, based on Special Reports of the Ecuadorian Geophysical Institute (IG). This reporting interval was mainly one of relative quiet. In contrast, our previous report (BGVN 32:12), covered IG reports describing energetic eruptions of July and August 2006. Those IG reports also mentioned eruption-related fatalities and the discovery of a new growing bulge on the volcano's N flank. A map and geographic background were tabulated in BGVN 29:01.

Relative quiet prevails and some residents return. As touched on in BGVN 32:12, after August 2006, the volcanic vigor at Tungurahua was minimal and of low energy. The decrease in activity was gradual through mid-December 2006. The vigor remained low until mid-January 2007. Ash emissions did occur but were consistently minor.

IG reports noted that the relative tranquility at Tungurahua could reflect a pattern similar to that seen there in 1918. That was a case when various months of volcanic quiet occurred, only to be followed by explosive eruptions of large size. The latter generated pyroclastic flows.

During the quiet that followed the July and August 2006 eruptions, residents who had evacuated from the margins of the volcano returned to their properties. The IG noted that, unfortunately, these returning residents became more vulnerable to volcanic hazards and made emergency response more difficult.

Vigor increases. Between 20 January and 5 February 2007 internal seismic activity resumed, behavior consisting of a few earthquakes inferred as associated with fractures (volcano-tectonic earthquakes, VTs). On 13 February the volcano emitted an eruptive column with moderate ash content. After 19 February there was a reoccurrence of seismic VTs. These were of shorter duration but higher intensity than those that occurred during the previous period.

During 23-24 February 2007, volcanic tremors and seismic LP's were registered at the Volcanic Observatory of Tungurahua (VOT). At 0310 on 24 February, VOT staff and local observers reported continuous roars of moderate intensity, and discharge of incandescent material that both rose to ~ 800 m above the summit and descended ~ 1000 m down the volcano's flanks.

The emission column headed NW. Fine tephra fell, followed by a thick ashfall that was black in color. It left a deposit 3 mm thick in the towns of Pillate and San Juan. Reports received from Cotaló, Bilbao, Manzano, and Choglontús that indicate a thick, dark ashfall in those spots left a deposit 2 mm thick. Ashfall was also reported in the area of Quero.

Seismic activity decreased on 24 February as well as the intensity and frequency of the roars. As of 2 March, sporadic explosions of ash and incandescent material had been observed. Around this time some bad weather prevented clear views of the upper volcano; however, some reporters noted minor ashfall along the SW portion of the crater. Additionally, the SO2 flux increased to ~ 2,000 metric tons a day for the first time since the beginning of the year. The IG's "Seismic Activity Index" indicated an increase of the volcano's internal activity.

Two scenarios envisioned. Given the available data, the IG concluded that the volcano had received a new influx of magma. They proposed two potential scenarios: (1) the current levels of activity will continue and constant emissions of ash, (potentially more intense) will be generated. Ash clouds will be blown by winds that at this time of the year are predominantly westerly, with occasional S and NW variations. These ash clouds could generate heavy ashfall in the towns downwind from the volcano; or (2) the volume and speed of ascent of the magmatic gases originating from the new magma will increase dramatically, in which case, new explosive eruptions of pyroclastic flows similar to those on 14 July and 16 August could occur.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

08/2007 (BGVN 32:08) During March-July 2007, many lahars; variable eruptive behavior

Ecuador's Instituto Geofisico (IG) wrote that significant though variable eruptions and lahars occurred at Tungurahua during mid-2007. Our previous report (BGVN 32:04) focused on early January to 2 March 2007, noting some variations in the pace of eruptive activity then. This report summarizes IG reports for March-July 2007. The substantial eruptions of July and August 2006 left abundant pyroclastic-flow deposits on the mountains slopes, potential source materials for new lahars (mudflows). The abundant seismicity during that interval punctuated a longer-term variable pattern (table 13 and figure 38).

Table 13. Summary of Tungurahua seismicity recorded during July 2006 through Mar 2007. Courtesy of IG.

    Time                     Total       Long-     Volcano-    Hybrid    Emission    Explosion
    interval              earthquakes    period    tectonic              signals     signals

    Total for Jul 2006       3482         3475         5          2        1185        6442
    Daily avg Jul 2006        112          112       0.16       0.06         38         208

    Total for Aug 2006       2546         2518        19          9         467        1643
    Daily avg Aug 2006       82.1         81.2       0.61       0.29       15.1        53.0

    Total for Sep 2006       2189         2149        35          5         111           0
    Daily avg Sep 2006       73.0         71.6       1.16       0.16        3.7           0

    Total for Oct 2006       3159         3131        20          8        1023           4
    Daily avg Oct 2006        102          101       0.64       0.25       33.0        0.12

    Total for Nov 2006       1849         1846         3          0        1049           1
    Daily avg Nov 2006       61.6         61.5       0.1          0        35.0        0.03

    Total for Dec 2006       2172         2168         5          0         648           0
    Daily avg Dec 2006       70.1         69.9       0.16         0        22.8           0

    Total for Jan 2007        829          817        12          0          10           0
    Daily avg Jan 2007       26.7         26.4       0.38         0        0.32           0

    Total for Feb 2007        983          966        15          2         312          54
    Daily avg Feb 2007       35.1         34.5       0.53       0.07       11.1         1.9

    Total for Mar 2007       1126         1125         1          0        1215         334
    Daily avg Mar 2007       36.3         36.3       0.03         0        39.2        10.7

    26 Feb-04 Mar             427          427         0          0         364          51
    05 Mar-11 Mar             235          235         0          0         269          87
    12 Mar-18 Mar             134          133         1          0         203         112
    19 Mar-25 Mar             241          241         0          0         356          86
    26 Mar-01 Apr             465          465         0          0         300          47
Figure 38. Tungurahua seismicity during September 1999 to March 2007 plotting the number of both explosion (EXP) and long-period (LP) earthquakes. Other kinds of earthquakes also took place but after 2001 were rarely seen. Courtesy of IG.

The IG report for March stated that a relatively energetic eruptive phase began on 24 February 2007 and continued throughout the month. That phase included abundant, ash emissions, sometimes discharging incandescent material, numerous, sometimes large explosions, and frequent noteworthy ashfall. The ash emissions and ashfalls were sometimes sustained. Blocks ejected in Strombolian outbursts fell up to 1 km below the crater rim.

During March, there were rises in both tremor amplitude and the number of long-period (LP) earthquakes (the later during March averaging 36 per day). SO2 gas fluxes averaged ~ 1,050 metric tons/day (t/d). Flank deformation was minimal. March ash falls came from frequent sustained ash plumes 2-6 km over the summit (figure 39). Seismically detected eruptions took place 29 times per day, including some of large size. Tremor nominally took place around 1 Hz, but its frequency remained irregular, non-harmonic, and pulsating. Intervals of pulsing emissions in mid-March had cycle times of ~ 10 minutes.

Figure 39. Tungurahua emitting an ash plume on 9 March 2007. Sustained plumes were seen during much of the month. Photo taken from Pondoa, on the N flank by Patty Mothes (IG).

An explosion on 27 March caused an "overflow" of incandescent material that traveled 800 m down from the head of the Mandur drainage. Other similar eruptions may have occurred but cloudy conditions forestalled clear observations. Hot lahars, however, traveled down the Mandur and Chontapampa drainages. Ash falls were common on the cone's N and NW sectors, and in addition, observers noted a small pyroclastic flow.

During the first weeks of April 2007 the IG noted continuous, strong emissions with a very high ash content. These emissions accompanied conspicuous lava fountains, visible at night, and strong roars that made windows vibrate. Ash columns reached 6 km above the crater (~ 11 km altitude). Activity decreased notably during the last 10 days of April (but were even lower in late May). Seismometers recorded an average of ~ 10 daily low-amplitude LP earthquakes. A differential optical absorption spectroscopy (DOAS) instrument measured SO2 fluxes of 3,600 and 3,700 t/d during the last 10 days of April.

During May, seismicity was low (table 13), with the average number of registered earthquakes each day averaging about 20. The Seismic Activity Index at the beginning of the month indicated a level 5 (moderate-high activity), which later on fell to a level 3 (moderate-low activity). This was the lowest Seismic Activity Index registered since February 2007. Ash emissions were low to moderate with a westerly direction. The SO2 levels were approximately 800 t/d. With the exception of the frequent formation of lahars, the level of volcanic activity was low in May.

The vigor of June 2007 eruptions from Tungurahua remained at moderate to low levels. Seismicity at the start of the month was low, chiefly LP earthquakes. Eruptions columns were modest and charged with moderate to low amounts of ash. June SO2 fluxes were comparatively high, ~ 2,900 t/d; observers heard light roaring noises similar to a turbine engine. Seismicity increased slightly towards the end of the month.

June brought prolonged intervals of low intensity rain, but heavy rains also occurred. The result was lahars (mud flows) that were numerous and in some cases large (table 14). The 21st of June was particularly noteworthy (table 14). Figure 40 shows one such lahar, which was partly eroded resulting in extension of lahars farther downslope. The lahars sometimes closed the route along the N side of the volcano between Baños and Pelileo and also the route from Baños around the volcano's W flank to Penipe (~ 15 km SW of the summit). No fatalities were reported.

Table 14. List of Tungurahua's main lahars during June 2007. A map and table of Tungurahua drainages (quebradas) appeared previously (BGVN 29:01). Courtesy of IG.

    Date (2007)    Drainage         Relative size and comments

    01 June        Bilbao           Small
    06 June        Bilbao           Small
    07 June        Vazcun           Small
                   La Pampa         Small; caused road closure
                   Bilbao           Small
                   Motilones        Small
                   Pingullo         Small
                   Rea              Small
                   Viejo Minero     Muddy water
    11 June        Mandur           Muddy water
                   La Pampa         Small
    12 June        La Pampa         Muddy water
                   Viejo Minero     Muddy water
    13 June        La Pampa (2)     Large and medium; a truck remained stuck
                   Viejo Minero     Muddy water
    14 June        Mandur           Small
                   La Pampa         Small
                   Bilbao           Small
                   Pingullo         Small
                   Motilones        Small
    15 June        Mandur           Small
                   Mapayacu         Small
                   Motilones        Small
                   Pingullo         Small
                   La Pampa         Small
                   Rea              Small
                   Choglontus       Small
                   Cusua            Small
                   Vazcun           Small
                   Viejo Minero     Muddy water
    16 June        La Pampa         Muddy water
    20 June        La Pampa         Medium; closing the road
                   Mandur           Small
                   Viejo Minero     Small
                   Achupashal       Small
                   Bilbao           Small
                   Motilones        Small
    21 June        La Pampa         Large, closing the road
                   Viejo Minero     Large
                   Mandur           Large
                   Vazcun           Large
                   Nueva Cusua      Large
                   Achupashal       Large
                   Motilones        Large
                   Pingullo         Large
                   Bilbao           Large
                   Rea              Large
                   Confesionario    Large
                   Ulba             Growing
    22 June        Vazcun           Growing
                   Ulba             Growing
                   La Pampa         Muddy water
                   Viejo Minero     Small
                   Mandur           Small
    24 June        La Pampa         Muddy water
Figure 40. A lahar in the La Pampa sector of Tungurahua showing an active, steep sided erosional channel down the axis of the deposit. Photographed 13 June 2007 by P. Ramón (IG). Courtesy IG.

There was a minor increase in seismicity during the month of July. Distribution of events was variable: 240-330 events per week the first and last week of the month; 50-70 events during each of the other two weeks. They were primarily LPs ~ 2 km below the summit.

The rate of SO2 emission averaged 1,071 t/d with a high of 2,050 t/d. Ashfall was semi-continuous, reaching areas W and SW of the summit, near communities like Bilbao (8 km W of the summit), Chogluntus (SSW of the summit), and El Manzano (7 km WSW). The plume headed toward Manta once the column reached 4 km above the summit.

During July, the road to Baños-Las Juntas was temporarily closed six times due to small-to-moderate lahars.

Information Contact: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

06/2008 (BGVN 33:06) Explosions up to 14 km altitude during July 2007 to February 2008

Our previous report on Tungurahua (BGVN 32:08) discussed the volcano's activity during March-July 2007. During that period, Ecuador's Instituto Geofisico (IG) reported significant, but variable eruptive behavior, along with many lahars, ash plumes that reached 4 km above the summit, and semi-continuous ashfall.

Table 15 presents a brief summary of the weekly activity at Tungurahua between 18 July 2007 and 19 February 2008. The plumes were described variously as ash, ash-and-gas, steam-and-gas, steam, or steam-and-ash. They rose up to 13 or 14 km altitude (25-26 October 2007 and 7 February 2008, respectively) but more typically, for many weeks, to below 8 km altitude. Around December 2007 IG stated a caution. They likened Tungurahua's behavior as similar to after its explosive phase of 14 July 2006. In that case, volcanic activity kept going, and this lead to the most explosive phase on 16 August 2006. That dramatic pattern was not repeated the next month, but the pace of volcanism kept up and led to the vigorous 7 February eruption.

Table 15. Summary of weekly activity at Tungurahua between 18 July 2007 and 19 February 2008. Courtesy of IG.

    Date                  Plume altitude    Activity

    18 Jul-24 Jul 2007    5.2-8 km          Roaring, noises resembling cannon shots or rolling
                                              blocks, lahars, ashfall.
    25 Jul-31 Jul 2007    up to 2-3 km      Many small ash-bearing explosions and several
                          above crater        unusually large ones, blocks fell up to 0.5 km
                                              below crater's rim, ashfall.
    01 Aug-07 Aug 2007    up to 5.5 km      Roaring, explosions, rolling blocks, steam emissions,
                                              ashfall.
    08 Aug-14 Aug 2007    up to 6 km        Explosions, incandescent material fell inside the
                                              crater and on the flanks, ashfalls, lahars down NW
                                              drainage disrupted road traffic between Ambato and
                                              Banos.
    15 Aug-21 Aug 2007    5.5 km            Cannon shot noises, explosions, ash emissions,
                                              ashfall.
    22 Aug-28 Aug 2007    6-9 km            Explosions, incandescent blocks down flanks, lahars
                                              in the NW drainage disrupted road traffic, ashfall.
    29 Aug-04 Sep 2007    7 km              Explosions, roaring and cannon shot noises,
                                              incandescent blocks ejected, lahars disrupted road
                                              traffic, ashfall. On 4 September incandescence and
                                              rolling blocks on the E and N flanks.
    05 Sep-11 Sep 2007    5.3-8 km          Explosions, incandescent blocks rolled down flanks,
                                              ashfall.
    12 Sep-18 Sep 2007    5.5-8 km          Strombolian eruption, explosions, incandescent
                                              material ejected above the summit and blocks rolled
                                              100 m down the flanks, roaring and cannon shot
                                              noises, ashfall.
    19 Sep-25 Sep 2007    5.3-7 km          Explosions, roaring and cannon shot noises,
                                              incandescent material ejected above the summit and
                                              blocks rolled 500 m down the flanks, ashfall.
    26 Sep-02 Oct 2007    6-7 km            Roaring and cannon shot noises, 28 September, blocks
                                              ejected above the summit and descended 500 m down
                                              the flanks, ashfall.
    03 Oct-09 Oct 2007    6-8 km            Ash plumes, roaring and cannon shot noises, noise of
                                              rolling blocks, ashfall.
    10 Oct-16 Oct 2007    6.2-8 km          Ash plumes. During 11-12 October incandescent blocks
                                              ejected and descended 300 m down the W flank;
                                              roaring noises from multiple areas on 11, 13, and
                                              14 October. Ashfall.
    17 Oct-23 Oct 2007    5.5-9 km          Ash plumes. 17 October, roaring, incandescent
                                              material erupted from the summit fell onto the
                                              flanks. Fumarolic activity on NW flank, lahars
                                              closed road on NW drainage. Ashfall SW on 21
                                              October.
    24 Oct-30 Oct 2007    up to 13 km       Ash and steam plumes. 25-26 October, incandescence at
                                              summit, roaring and cannon shot noises, blocks
                                              rolling down the flanks; ashfall.
    31 Oct-06 Nov 2007    5.5-8 km          Explosions, roaring, incandescent blocks at summit,
                                              lahars closed road, ashfall.
    07 Nov-13 Nov 2007    6-9 km            Roaring and cannon shot noises, incandescent blocks
                                              rolled a few hundred meters (1 km on 12 Nov) down
                                              the flanks, fumarolic activity, lahar, ashfall.
    14 Nov-20 Nov 2007    up to 7.3 km      Roaring and cannon shot noises, incandescent blocks
                                              rolled down flanks, thermal anomaly detected.
    21 Nov-27 Nov 2007    6-8 km            Explosions, roaring, incandescent blocks 1 km down
                                              the flanks, lahars (4-5 m high in one area) closed
                                              road, ashfall.
    28 Nov-04 Dec 2007    6-8 km            Elevated seismicity, explosions, continuous emissions
                                              of steam and ash, roaring and cannon shot noises,
                                              incandescent blocks 0.5-1 km down flanks, ashfall.
    05 Dec-11 Dec 2007    6-8 km            Explosions, roaring and cannon shot noises,
                                              incandescent material about 1 km down flanks,
                                              ashfall.
    12 Dec-18 Dec 2007    6-7 km            Explosions, roaring and cannon shot noises, almost
                                              constant emission of of ash plumes, incandescent
                                              blocks rolled down flanks, ashfall.
    19 Dec-25 Dec 2007    6-8.5 km          Roaring and cannon shot noises, incandescent blocks
                                              hundreds of meters down flanks, ashfall. News
                                              reports indicated that 1,200 people from Penipe
                                              were evacuated nightly.
    26 Dec-01 Jan 2008    6-8 km            Explosions, roaring, and cannon-shot noises,
                                              incandescent blocks 500 m down flanks, ashfall.
    02 Jan-08 Jan 2008    5.5-8 km          Explosions, roaring and cannon shot noises,
                                              continuous ash emissions, incandescent blocks 500 m
                                              down flanks, ashfall. News reports indicated that
                                              nearly 1,000 people were evacuated for the night on
                                              6 Jan.
    09 Jan-15 Jan 2008    6-9 km            Strombolian eruption, roaring and cannon shot noises,
                                              incandescent blocks 0.5-1 km down flanks. News
                                              reports indicated that residents from two provinces
                                              evacuated at night and about 20,000 health masks
                                              were distributed in Banos and Quero.
    16 Jan-22 Jan 2008    5.5-9 km          Strombolian eruption, roaring and cannon shot noises,
                                              incandescent blocks 1-2 km down flanks, small
                                              pyroclastic flow 400 m down NW side of crater,
                                              ashfall.
    23 Jan-29 Jan 2008    5.5-9 km          Roaring and cannon shot noises, incandescent blocks
                                              500-800 m down flanks, lahars blocked road to
                                              Banos, ashfall up to at least 40 km from summit.
    30 Jan-06 Feb 2008    6-9 km            Explosions (65-208 per day), roaring and cannon shot
                                              noises, incandescent blocks rolled 600 m down
                                              flanks, lahar, ashfall.
    06 Feb 2008           —                 New phase of eruptions began with a moderate
                                              explosion.
    07 Feb 2008           6-14.3 km         Tremors of variable intensity, ash columns to heights
                                              of 3 km beginning a new phase of eruptive activity;
                                              satellite images show a hot spot in the crater.
                                              Strombolian eruptions, explosions, strong roaring
                                              and cannon shot noises, incandescent material
                                              rolled 1.2 km down the flanks, tremors followed by
                                              pyroclastic flows on the NW and W flank, tephra                                              fall SW, ashfall. News articles stated several
                                              hundred to 2,000 people evacuated.
    08 Feb 2008           —                 Internal volcanic activity as well as emissions of
                                              ash, incandescent material, and explosions and
                                              roaring noises slowly diminished; current eruptive
                                              episode should not be considered as finished.
    09 Feb-12 Feb 2008    —                 Strombolian eruptions, explosions, strong roaring and
                                              cannon shot noises, incandescent material rolled
                                              1.2 km down the flanks, pyroclastic flows, tephra
                                              fall, ashfall. News articles stated several hundred
                                              to 2,000 people evacuated.
    13 Feb-19 Feb 2008    6-9 km            Roaring, noises resembling blocks rolling down
                                              flanks, lahar, ashfall.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/); Reuters (URL: http://www.reuters.com/); Associated Press (URL: http://www.ap.org/); Pan American Health Organization (PAHO), 525 23rd St. NW, Washington, DC 20037, USA (URL: http://www.paho.org/).

07/2009 (BGVN 34:07) Eruptions in 2008-2009; two fatalities in sudden flood on 22 August 2008

Our previous report on Ecuador's Tungurahua (BGVN 33:06) summarized the ongoing activity through mid-February 2008. This report covers February 2008-July 2009. The Instituto Geofísico-Escuela Politécnica Nacional (IG) has maintained a continuous watch on Tungurahua in order to provide immediate alert of significantly heightened activity to prevent as much damage to property and population as possible. Several pyroclastic flows occurred.

Steam and ash plumes have been continuous since early 2008. These plumes have generally risen to ~ 8-9 km with occasional higher plumes as a result of increased activity. Ashfalls were frequently associated with the steam, gas and ash emissions, and deposited small layers of the larger particles downwind, sometimes 8-11 km from the crater. On 1 March 2009, an explosion produced a significant plume that rose to an altitude of ~ 10 km and drifted NW. By 3 March, the ash on the volcano's W side covered at least 2.5 km2 of cropland, and additional cattle-grazing pasture.

Ashfall accumulation (figure 41) was recorded for a time interval slightly before the current reporting interval, but the available later maps were similar. Towns affected on figure 41 included Choglontús, El Manzano, Palitahua, Cahuají, Sabañag, Santa Fe de Galán, Penipe, and Bayushig. Lighter ashfall was also repeatedly noted in Riobamba and Guano.

Figure 41. Ashfall accumulated from Tungurahua eruptions during 30 January-10 February 2008. N is towards the top and the horizontal scale can be read from to the index marks on the map's margin, which are at 5-km intervals. The three isopachs shown represent thicknesses of 1, 2, and 3 mm (increasing thickness inward). Courtesy of IG.

On various occasions, incandescence and the ejection of large blocks were Strombolian in character. Roaring, explosions, and "cannon shot" noises were reported almost daily. On 4 August 2008 one explosion was heard as far away as Ambato, 31 km to the NW.

Lahars, floods, and two fatalities. Lahars or mudflows descended drainages to the NW, W, SW, and S repeatedly during the reporting period (many times per week). On 8 March 2008, lahars transported blocks up to 3 m in diameter; in many other cases the largest blocks were around 1 m in diameter. Lahars occasionally affected roads in the Pampas sector to the S and disrupted the access road to Baños.

On 21 August 2008, intense rains prompted the Volcanic Observatory of Tungurahua (OVT) to issue a warning of potential lahars in the Vascún river. A natural dam in that river had been previously identified as a potential hazard.

On 22 August, the dam ruptured and a flood descended. A bridge crossing the river on the outskirts of Baños endured the flooding but was overridden by ~ 20 cm above the railing (figure 42). The flood also destroyed two homes and the El Salado public pools, 1,700-1,800 m downstream of the dam (figure 43). Two people were reported injured and two were reported missing and presumably killed.

Figure 42. (left) A view of a bridge impacted by the Tungurahua flood and lahar of 22 August 2008. A vertical support appears damaged. (right) A smaller bridge showing high water mark about 1 m above the road. On the far bank is scouring near the base, and above that, fresh deposits, including some on the guardrail. The lahar may have caused or contributed to damage on abutment and horizontal support beam, which appears battered and deformed. Courtesy of IG.
Figure 43. Three photos of the El Salado pool facility on the N flank of Tungurahua, where an August 2008 flash flood destroyed significant portions of the buildings and the retaining wall, and gravels swept as high as the roof of some buildings. (top) An overview of the ruined pool facility. (bottom) Measurements help convey the scale of the river's high stand and aftermath. IG authors also sketched a line showing the highest water level. Inset photo was taken when the pool was in use prior to the flood; the river is at right out of view. Courtesy of IG.

Lahars on 23 October again descended the Vascún river, causing a landslide and rupturing a water pipe that serviced Baños. On 1 November, lahars descended multiple drainages, carrying blocks up to 1 m in diameter to the SW. Residents bordering the Vascún river temporarily evacuated, but returned after the rain stopped.

Pyroclastic flows and explosive activity. Explosive activity continued through the reporting period, including pyroclastic flows. Noteworthy pyroclastic flows occurred on 29 May 2008, when they descended the N and NW flanks of Tungurahua, with deposits observed the next day. In July 2008, a pyroclastic flow was associated with significant ash and tephra fall (with grains up to ~ 3 mm in diameter) reported in the towns of Cahuají, Chazo, Palestina, Santa Fe de Galán, and Guaranda.

Explosions often ejected incandescent blocks that rolled downslope; on 21 June 2009, lava fountains that rose to a height of 500 m above the crater expelled incandescent blocks that were later discovered as far as 2 km downslope. Notable pyroclastic flows, explosions, as well as some lahars and ash plumes were reported (tables 16 and 17).

Table 16. Tungurahua behavior during 19 February-30 December 2008. Only selected examples of near-daily lahars are shown. A map and table of Tungurahua's drainages (quebradas) and surrounding towns appeared previously (BGVN 29:01); locations mentioned include the Mapayacu and Choglontus drainages to the SW; the Pampas sector to the S; Cusúa, 7 km to the NW; Manzano, 8 km to the SW; and the particularly vulnerable city of Baños, 8 km to the N. Courtesy of IG.

    Date (2008)         Observations

    19, 20, 25 Feb      Lahars affected roads in Pampas sector
    06 and 08 Mar       Lahars descended W and S drainages; some carried blocks up to 3 m
                          in diameter
    25 Mar              Explosions; incandescent blocks from summit fell on flanks
    09, 12, 13 Apr      Lahars and mudflows on S and NW drainages that disrupted the access
                          road to Banos
    21 Apr              A lahar disrupted the Ambato-Banos road for a few hours
    23 Apr              Blocks rolled 600 m down the flanks
    01 May              Explosions and intense summit incandescence; windows vibrated in areas
                          6 km NE
    11 May              Blocks rolled ~ 1 km down the flanks
    12 May              Explosion; rockfalls occurred in an area 8 km to the S
    17-18 May           Explosion similar to that on 12 May; windows rattled in areas to the
                          SW and W
    19 May              Large explosion; numerous incandescent blocks rolled ~ 1.6 km down
                          the flanks
    22, 25-27 May       Windows vibrated in nearby areas, including at the observatory (OVT)
                          in Guadalupe
    23 May              Marked increase in explosions, ash plumes, and ashfall; summit
                          incandescence at night
    29 May              Pyroclastic flows descended the N and NW flanks, with deposits
                          observed the next day
    15 Jun              Lahars descended the NW and S drainages and resulted in a road closure
                          to the S
    19 Jun              Blocks ejected 500 m above the summit and rolled ~ 1 km down the flanks
    20 Jun              Mudflow to the SW towards the Puela river carried blocks up to 80 cm
                          in diameter
    31 Jul, 3-4 Aug     Blocks rolled ~ 1 km downslope; ashfall to SW and W; an explosion on
                          the 4th,
    19-22 Sep           Small mudflows in the W and NW; a lahar 50 cm thick to the S
    23 Oct              Muddy waters caused a landslide and a ruptured water pipe that serviced
                          Banos
    01 Nov              Lahars carried blocks ~ 50-70 cm in diameter in Juive, La Pampas,
                          and Bilbao
    04 Nov              Light ashfall was reported in Pallate and part of Riobamba
    15 Dec              A ash column rose to ~ 1 km
    15-24 Dec           Ash columns reached a height of ~ 6 km
    17, 21-23 Dec       Ejecta visible from Guadalupe Observatory
    23 Dec              Incandescent material rolled down flanks
    24 Dec              Small pyroclastic flow on NW flank; incandescent lava flowed down one
                          of the flanks
    25-26, 28-30 Dec    Blocks rolled 500 m downslope on 25 Dec, 1,500 m on 29 Dec, and 800 m
                          on 30 Dec; heavy black ash fell in areas to the SW

Table 17. Tungurahua behavior during 2 January-7 July 2009. Only selected examples of near-daily lahars are shown. Courtesy of IG.

    Date (2009)         Observations

    02-04 Jan           Blocks rolled ~ 800 m down the flanks; Strombolian activity on 4 Jan
    07, 10 Jan          Incandescent blocks rolled down flanks
    08 Jan              Continuing gas-and-vapor emission; ash columns less than 2 km high
                          drifted W, NW, SW, and NE. Ashfall in El Manzano, Choglontus,
                          Palictahu and Cahuaji
    16 Feb              Ash emissions that generated a plume with altitude of ~ 8 km and
                          drifted W
    01 Mar              Ash plume that rose to an altitude of ~ 10 km and drifted NW
    03 Mar              Ashfall covering at least 2.5 km2 of cropland and additional
                          cattle-grazing pasture
    21 Mar              Lahars carried blocks up to 30 cm in diameter to the SW
    26 Mar              Lahar in the Mapayacu drainage carried blocks up to 2 m in diameter
    06 Apr              Fumarolic plumes rose 500-600 m; light ashfall reported 8 km SW
    14 Apr              A steam-and-gas plume containing some ash rose to an altitude of
                          ~ 7.5 km and drifted N
    22 Apr              Incandescent blocks ejected from the crater and rolled down flanks
    06 May              Ashfall reported in Banos, ~ 8 km to the N
    13, 18 May          A fine layer of ash fell in Manzano, 8 km to the SW
    24-26 May           Incandescence from the crater seen and blocks rolled 100-500 m down
                          the flanks
    27 May-02 Jun       Strombolian activity
    21 Jun              Lava fountains rising to a height of 500 m above the crater
    02, 5-7 Jul         Lahars descended SW and W drainages carrying blocks up to 40 cm
                          in diameter

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/VAAC/).

08/2009 (BGVN 34:08) Vascún River map; lahar concerns; decreased activity in mid-2009

Activity at Tungurahua began to decline in late June 2009 and continued this trend into early September. July and August 2009 were characterized by an absence of explosions, a marked decrease in ash emissions, and weak steam emissions. Although no new pyroclastic material was deposited in July or August 2009, the Instituto Geofísico-Escuela Politécnica Nacional (IG) still warned that heavy rains and the great amount of material on the upper slopes presented a danger of lahars and mudflows.

Update on 22 August flood. Last month's report (BGVN 34:07) discussed a 22 August 2008 flood of the Vascún River (figure 44) after a natural dam failed. The flood destroyed two homes in the district of Las Ilusiones as well as the El Salado swimming pool complex, leaving two people injured and two others missing.

Figure 44. Topographic map of the Vascún River on the N flank of Tungurahua, as well as the locations of the dam failure at the landslide and the districts of El Salado and Las Ilusiones, which were affected by the flood waters and associated sediment. "AFM" refers to an acoustical-flow monitor, a device to help detect processes such as mass wasting and pyroclastic flows along the river (Hadley and Lahusen, 1993). The heavy line along the river below the natural dam indicates the portion of the river through which the flood and debris traveled. Courtesy of IG.

According to a 24 August 2008 article of the newspaper El Universo, strong rains began at approximately 2000 on 22 August and an earthquake was felt by residents in El Salado around 2345. El Universo stated that after the dam's failure, flood waters containing volcaniclastic material reached the El Salado area in ~ 5 minutes. The IG estimated that the flood had traveled at a velocity of 4.7-6.7 m/s.

Two photographs showed bridges that had been visibly damaged by the event (figure 42 in BGVN 34:07). One bridge is located on the main road to Baños, adjacent to the city. The other bridge, 100 m upstream from the first, serves a secondary road. Officials plan on reviewing those bridge structures to determine whether they should be reinforced to avoid any flood-related damage in the future.

El Universo reported that the two missing persons were young children who lived in one of the destroyed houses along the river in Las Ilusiones. Searches were unsuccessful and El Universo reported that the search concluded in September 2008.

References. Hadley, K.C., and Lahusen, R.G., 1993. Technical manual for acoustic flow monitor. US Geol. Surv., Open-file Rep. 93-00, 19 pp.; Lahar-Detection System, USGS scientists create new method for detecting lahars (URL: http://volcanoes.usgs.gov/activity/methods/hydrologic/lahardetection.php).

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); El Universo (URL: http://www.eluniverso.com/).

03/2013 (BGVN 38:03) Return of explosions and earthquakes through at least October 2010

As noted in our last report (BGVN 34:08), activity at Tungurahua had started to decline in late June 2009, a trend that continued through mid-December 2009. This report summarizes heightened activity witnessed from 30 December 2009 to 4 February 2010, and again from 26 May very early August 2010. Seismicity began to increase in mid-December 2009 in prelude to the above-mentioned escalation.

This report is based on reports received from and direct correspondence with staff members of the Instituto Geofísico Politécnica Nacional Casilla ("Instituto Geofísico," IG).

Seismic overview. A plot by IG presented a synthesis of seismicity (figure 45) during the interval of 1 January 2006 to 17 October 2010, and is one way to portray the long-term pace of Tungurahua's eruption. The x-axis shows time with small tick marks every 30 days, and years indicated. The y-axis shows level of seismic activity on a scale indexed to an unstated normalized peak value. During the time period plotted, the number of recorded events assessed had a maximum peak at index value of ~74%. That peak occurred around 14 July 2006 ("2006 07 14" at left on the plot).

Figure 45. A plot of Tungurahua's seismicity versus time for the interval 1 January 2006 to 17 October 2010. The scale on the left (Index value) is in terms of percent of the IG's custom defined index of activity scale (IAS) for Tungurahua. The bar was auto-scaled with 4.6 units between horizontal lines. The X-Y intercept is slightly below zero (-0.3). The Y-axis on the right shows the IG's defined 'level of the IAS.' The horizontal red dots refer to thresholds during 1999-2005, corresponding to index values at 22.8 and 26.6% (95% and 99% confidence intervals, respectively). Modified from a plot by IG.

During 2010 seismicity rose sharply in January, peaked at index values near 27%, and then descended during February into June 2010. The intervals of low seismicity in late 2009 and early 2010 are some of the lowest index values during the ~5 years displayed on this plot. The second peak during 2010 was sudden and ascended from low index values; it reached maximum values in late May 2010, approaching index values 50%. The downward arrow at the right side of the plot calls attention to the last day on the plot, 17 October 2010, when the index value stood at 11%.

The seismicity shown in figure 45 shows strong correlation with Tungurahua's behavior summarized below.

Renewed activity (and SO2 compilations). After six months of calm that included occasional diffuse emissions of vapor that rose no higher than 100 m, more intense activity resumed on 30 December 2009. On 1 January ash plumes rose to ~6 km altitude, and on 3 and 4 January they rose as high as ~9 km. Similar plume altitudes were seen at times during the rest of January and February. On 11-12 January, Strombolian outbursts ejected material ~1 km above the crater and to distances of ~1.5 km away from the vent. Ash often blew in a broadly westerly direction (see below).

Significant SO2 fluxes were measured in January 2010. On 6 January, SO2 fluxes reached a value of 3,200 metric tons/day (t/d); on 8 January, they reached 7,500 tons/day. In contrast, some recent values during the 6 month lull were between 100 and 200 tons/day.

On the broader subject of Tungurahua SO2 output, more comprehensive assessments and compilations have been captured than ever before, but the details are beyond the scope of this report. One group, NOVAC (Network for Observation of Volcanic and Atmospheric Change) established a global network of stations to measure emissions of SO2 (and BrO) by UV absorption spectroscopy by local instruments. Trained personnel working at various volcanoes and following protocols assured standard practices (DOAS, mini DOAS, Flyspec). The instruments are based at over 21 volcanoes, including Tungurahua. SO2 is also monitored from instruments on satellites (e.g. OMI, GOME 2, and SCIAMACHY). Understanding and possibly reconciling those two set of atmospheric measurements, local (ground- and aircraft-based measurements) and satellite based, has also been the subject of recent papers. Tungurahua and adjacent degassing volcanoes have played a major roles in this effort (McCormick and others, in press, Carn and others, 2008, and Arellano and others, 2008).

Figure 46 shows Tungurahua in a Strombolian eruption on or about 10 January 2010. Incandescent material fell onto the flanks.

Figure 46. After 6 months of comparative quiet, Tungurahua renewed more vigorous eruptions during 30 December 2009 to 1 January 2010. This extended-exposure night photo was taken on or around 10 January 2010. The volcano's profile appears dark, almost black, whereas Strombolian discharges at the summit crater glow a bright red-orange. The extended exposure captured the paths of glowing projectiles arcing out of the crater, and impacting and in many cases bouncing down the upper flanks. An ash cloud in the background reflects a softer more continuous glow across a broad area including, in fainter light, portions of the plume trailing off to the upper right. Photo by Jorge Bustillos A., IG.

Strong explosions were heard on the evening of 14 January. Small amounts of ash fell nearby and as far away as the Chimborazo glaciers and Guaranda (64 km SW).

The return of ongoing ash plumes initiated a variety of civil responses. In towns close in and on Tungurahua's W to SW flanks (e.g., Choglontús), roofs were inspected and where necessary, ash was removed. This measure was undertaken to prevent ash loading and potential roof collapse. Many building roofs are nearly flat, requiring ash removal with brooms and shovels.

Figure 47 shows an elementary schematic created to address the upsurge in Tungurahua's ash falls, and to broaden understanding and raise public awareness of ash fall processes and hazards from Tungurahua. It shows the progressive shift in the range of grain sizes falling out of ash plumes as they drift, in this idealized case, W to SW with plume tops rising to altitudes approaching 10 km. More complex depositional patterns may develop owing to factors like discharges to variable height, discharges containing differing grain-size distributions, and shifts in wind velocity. Assessment of ashfall often results in maps displaying the thickness or mass of these deposits on the ground (respectively, isopach and isopleth maps). Several isopach maps appear below, describing the Tungurahua's January-February 2010 upsurge in ashfall in more quantitative and technical terms.

Figure 47. An idealization of ash plume evolution emphasizing the expected changes in grain sizes falling from Tungurahua plumes and the range of distance to some settlements. The plume shown is assumed to blow W to SW (common wind directions in this region). Graphics like this help inform the public. Courtesy of IG.

Tungurahua's ash clouds are also closely monitored out of concern for keeping aircraft out of potentially damaging plumes, some of which extended ~150 km W of the summit during the reporting interval. Ecuador maintains a Meteorological Watch Office (MWO) in the city of Guayaquil, ~180 km SSW of the summit (figure 48; and see inset map showing Guayaquil's location on the coast in figure 25 in BGVN 30:06). The Guayaquil MWO feeds critical data, including observations made by IG at Tungurahua and satellite data they interpret about Tungurahua's plumes, to the Buenos Aires Volcanic Ash Advisory Center. Tungurahua was a frequent subject of aviation reports during the reporting interval. As seen on figure 48, Guayaquil sits just S of the ends of W-directed envelopes around the January 2010 ash clouds.

Figure 48. Plan view of the overall atmospheric dispersion patterns of Tungurahua ash during January 2010. This is vital information to the aviation community. Although the key is in Spanish, the gist is this: The yellow and blue envelopes apply to ash above and below 7.6 km altitude, respectively (note scale at far left). Courtesy of IG (graphic and key composed by Jorge Bustillos A. after a graphic found at the Washington VAAC).

Figures 49 and 50 respectively show 1-10 and 11-31 January ash-fall deposits created by plumes that rose to the 5.4-9.1 km altitude range (up to 3 km above the crater). On these maps, the farthest measured isopach, 1 mm in thickness, was as far away as 19 km from the crater. For figure 49, the points where the thickness was measured (yellow) best constrained the deposits maximum extent on the N, NW, and W sides. For that map, ash volumes estimated using conventional approaches (Pyle, 1989; Fierstein and Nathenson, 1992; and Legros, 2000) yielded 313,000 m3 (stated in terms of vesicle-free rock; often abbreviated as DRE, dense-rock equivalent).

Figure 49. The isopach map IG compiled for Tungurahua ash deposited during 1-10 January 2010. Key shows the following (from top): Isopach thicknesses (3 colors), points where the thickness data were collected (yellow dots); population centers (black shaded areas), and Tungurahua (reddish shading). Courtesy of IG (composed by Jorge Bustillos A.).
Figure 50. The isopach map IG compiled for Tungurahua ash deposited during 11-31 January 2010. The thickest area was inferred to lie ~9 km WSW of the summit at Cahual (where 15 mm fell). Key at left shows isopach thicknesses (9 colors), points where the thickness data were collected (green dots); and population centers (black shaded areas). Courtesy of IG (composed by Jorge Bustillos A.).

In figure 50, the thickest ash (1.5 cm) was mapped at the town of Cahual (~9 km WSW of the summit), and the mapped pattern suggested lobes trending both WNW and WSW. Such thickening at a spot well away from the summit is unusual but not unprecedented. Future studies considering meteorological or other data may help explain this pattern, which could have public safety implications.

IG's report for 11-31 January noted near constant ash plumes during that interval. Ash had impacted banana plantations and livestock, falling over an area of ~15,000 km2.

The ash volume estimate in DRE for 11-31 January was 806,000 m3 (figure 50). Taken with the earlier (1-10 January) estimate based on figure 49, this made the January total ~1,120,000 m3 (DRE). This is best shown to two significant figures, thus 1.1 x 106 m3. In contrast, the volume for the full month of February was estimated at 1.6 x 106 m3.

Figure 51, an isopach map created for the entire month of February 2010, extends farther outward than either of the two maps representing January. February's 1 mm isopach extends up to 22 km from the summit in the WNW direction. February's map also shows some thicker deposits close to the active summit crater (25 mm thickness at Cahuaji), but it reflects a longer time interval than each of the two January maps. February's elevated deposition of ash conformed with IG's Special Bulletin (No. 6), which explained that after 6 February 2010 they had seen a notable increase in the frequency and magnitude of explosions, and plumes rising 4 km above the summit. People tens of kilometers away heard blasts. Observers noted glowing blocks thrown more than 1.5 km and rolling downslope. Infrasonic microphones recorded signals suggesting four explosions of high energy during 8-9 February.

Figure 51. An isopach map showing Tungurahua's ash accumulation during February 2010 (thicknesses in millimeters). Courtesy of IG (composed by Jorge Bustillos A.).

5 February ash plume dynamics. Figure 52 shows intriguing ash plume behavior on 5 February 2010, disclosing complexities infrequently discussed in the literature. At discharge, the plume rises vertically in a thrust phase that appears to be well on its way to farther ascent in a convective phase. The continued rise of earlier emissions (perhaps less energetic or perhaps in gusts of stronger wind) were thwarted by wind having sheared and carried them off. That portion of the plume appears to the right of the crater. As these earlier components progressed downwind and above the lee side of the volcano, their path descended rather than ascended. Farther downwind and away from the volcano the plume appears much broader and its top has risen higher than the original thrust phase. The presence of weather clouds like those in the background could compromise ground-based estimates of the ultimate height for the plume's top.

Figure 52. Tungurahua discharged this vapor-dominated emission (containing low-to-medium density of ash) at 2200 on 5 February 2010. That day's eruptions were described as rising 1 km above the summit and continuing W to SW. The scene is discussed further in text. Image taken from IG's 1-7 February 2010 report (photo credit to J. Bourquin IG).

Geometries like this may also complicate reliable estimates of gas-flux, although scanning imaging infrared spectrometers compensate for these problems (e.g. Grutter and others, 2008). Pilots and meteorologists are familiar with these topographic wind-related effects, which they call lee waves. An acclaimed book by Pretor-Pinney (2007) discusses and provides a diagram to explain similar wind-driven topographic effects and lenticular clouds common near tall peaks.

11 February secondary pyroclastic flows. As IG reported (in their Special Bulletin Number 6), on 11 February an M 3.3 volcano-tectonic earthquake struck in vicinity of Tungurahua. Strong explosions ensued, and about 20 minutes later an ash plume rose ~4 km above the summit. About three hours later IG observers at the Tungurahua Volcano Observatory (OVT) in Guadalupe, (11 km N of the summit), photographed the paths and thermal images revealed still hot deposits from small pyroclastic flows. These flows started at the crater rim and descended several kilometers down the upper one-third of the volcano on the N and NNW flanks.

The pyroclastic flows were interpreted as secondary, generated by the gravitational instability of still-hot ejecta that had accumulated at the crater rim and adjacent areas downslope.

February rain brings destructive lahars. Special Bulletin No. 5 of 4 February 2010 contains a detailed set of notes compiled as strong rains fell and caused lahars to descend along gullies and ravines. The Ulba river (on the volcano's E to NE flanks) was strongly impacted. The lahars also closed some recently repaired roads.

At 1100 on 4 February 2010, a large amount of rock debris flowed down the Chinchín river, which flows from the S to the N along the outer NE flank as close as ~10 km from the summit crater. According to the Baños Fire Department, the lahars destroyed or damaged ~18 homes in Chinchín (16 km NE of the summit). Four people were injured (two children, two adults) and four people were missing. The lahars carried debris up to 5 m in diameter. Debris flowed over the brink of Bride's Veil waterfall, disrupting a well-known scenic attraction.

March to June activity. Although poor visibility hampered views in March, seismicity remained high and indicative of explosions with several ash emissions reaching altitudes of 8-9 km. Comparative quiet prevailed during April and the earlier parts of May, although visibility was often compromised. Early May reports noted lahars on the N, W, and S flanks.

Eruptive vigor rose again on 26 May 2010, when an eruption resulted in small (~1 km runout distance) pyroclastic flows and an ash plume that rose to 12 km altitude.

An eruption on 28 May produced a plume that rose to 15 km altitude, resulting in pumice-fall in some inhabited areas. Pyroclastic flows extended as far as 3 km down the NW, N, and SW flanks, distances insufficient to impact inhabited areas. According to news articles, residents from two towns about 8 km NW were evacuated, and ashfall at the airport in Guayaquil temporarily shut it down.

On 2 and 7 June pyroclastic flows traveled 1.5 km down the NW flank; this took place amid the early June seismic low on figure 47.

Clouds often prevented observations during June and July, but when visible, plumes again rose to altitudes of 5-8 and occasionally 9 km. Some intervals of comparative quiet also occurred (e.g., 23 June to 4 July). As had occurred during many intervals of high eruptive vigor, loud booming noises associated with explosions often caused structures to vibrate. For example, on 6 June large windows vibrated at OVT.

During 28 July-2 August steam emissions, often containing ash, 1-2 km above the crater and drifted NW or W. Minor ashfall was reported to the SW in the Choglontus area during 28-29 July.

According to IG's October report, a lull in activity began in late July and continued through all of October. Figure 47 shows that comparatively low seismicity prevailed through at least 17 October 2010.

Online book in Spanish on Tungurahua. Readers searching for background on this restless, dangerous, news-making volcano, will find it summarized in accessible terms in a compact (118 page) well-illustrated book (Le Pennec and others (2005), in Spanish; available free online as a PDF file). Topics include events from Tungurahua's geologic past, the onset of the current eruption in 1999, and approaches to monitoring. The book covers behavior as late as mid-2005. A few important recent eruptions after the book's publication included those in July and August 2006 (respectively, VEI 2 and 3) and in February 2008 (VEI 1-2).

Figure 53 features a map from the book. In the book's digital version (a PDF file), the search function will identify place names on this map, a welcome feature.

Figure 53. Location of Tungurahua and surroundings showing cities and districts, places confronting varying degrees of impacts from the ongoing eruption. Translation of terms in key: province capital, cantonal capital, paved road, dirt road, pipeline, province margin, and drainage network. Taken from Le Pennec and others (2005).

References. Arellano, S.R., Hall, M., Samaniego, P., Ruiz, A., Molina, I., Palacios, P., Yepes, H., OVTIGEPN staff, 2008. Degassing patterns of Tungurahua volcano (Ecuador) during the 1999-2006 eruptive period, inferred from remote spectroscopic measurements of SO2 emissions. J. of Volcanology and Geothermal Research 176, 151-162.

Carn, S.A., A.J. Krueger, N.A. Krotkov, S. Arellano, and K. Yang, 2008, Daily monitoring of Ecuadorian volcanic degassing from space, J. of Volcanology and Geothermal Research, 176(1), 141-150.

Grutter, M., Basaldud, R., Rivera, C., Harig, R., Junkerman, W., Caetano, E., and Delgado Granados, H., 2008, SO2 emissions from Popocatpetl volcano: emission rates and plume imaging using optical remote sensing techniques, Atmos. Chem. Phys., 8, 6655-6663.

Le Pennec. J-L, Samaniego, P, Eissen, J-P, Hall, MP, Molina, I, Robin, C, Mothes, P, Yepes, H, Ramón, P, Monzier, M, and Egred, J., 2005, Los peligros volcánicos asociados con el Tungurahua, 2nd edición, Serie: Los peligros volcánicos en el Ecuador. Corporación Editora Nacional, ISBN: 9978-84-402-3 (118 pp., in Spanish). [PDF online at http://horizon.documentation.ird.fr/exl doc/pleins_textes/divers11 12/010036187.pdf]

McCormick, B.T., M. Herzog, J. Yiang, M. Edmonds, T.A. Mather, S.A. Carn, S. Hidalgo, and B. Langmann (2013), An integrated study of SO2 emissions from Tungurahua volcano, Ecuador, Journal of Geophysical Research (under review).

Pretor Pinney, G., 2007, The Cloudspotter's Guide: The Science, History, and Culture of Clouds by Gavin, Penguin Group (USA).

Information Contacts: Instituto Geofísico (IG), Escuela Politécnica Nacional, Casilla 1701-2759, Quito, Ecuador; Jorge E. Bustillos A. (IG), Patricia A. Mothes (IG), and Buenos Aires and Washington Volcanic Ash Advisory Centers.

Summary of eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
2010 Nov 22 2014 Feb 27 (continuing) Confirmed 3 Historical Observations
2010 Jan 1 2010 Jul 29 Confirmed 3 Historical Observations
1999 Oct 5 2009 Jul 8 ± 7 days Confirmed 3 Historical Observations
[ 1993 May 6 ] [ 1993 May 6 ] Uncertain 1  
[ 1944 ] [ Unknown ] Uncertain 2  
1916 Mar 3 1925 Dec 1 ± 30 days Confirmed 4 Historical Observations
[ 1900 ] [ Unknown ] Uncertain 2  
1886 Jan 11 1888 ± 1 years Confirmed 4 Historical Observations
1885 Jan (?) 1885 Oct 16 Confirmed 2 Historical Observations
1857 Sep 10 (?) Unknown Confirmed 2 Historical Observations
[ 1781 ] [ Unknown ] Uncertain 2  
[ 1777 ] [ Unknown ] Uncertain 2  
1776 Jan 3 Unknown Confirmed 2 Historical Observations
1773 Feb 4 1773 Jul (?) Confirmed 3 Historical Observations P2 tephra
[ 1757 ] [ Unknown ] Uncertain 2  
1644 1646 (?) Confirmed 2 Historical Observations
1640 1641 Confirmed 3 Historical Observations
1557 Unknown Confirmed 2 Historical Observations
1350 ± 50 years Unknown Confirmed 3 Radiocarbon (corrected)
1250 ± 50 years Unknown Confirmed   Radiocarbon (corrected)
1030 ± 75 years Unknown Confirmed   Radiocarbon (uncorrected) Tephra layer F
0800 (?) Unknown Confirmed   Radiocarbon (uncorrected)
0730 ± 200 years Unknown Confirmed 4 Radiocarbon (uncorrected) P1 tephra unit
0600 (?) Unknown Confirmed 3 Radiocarbon (uncorrected)
0480 ± 75 years Unknown Confirmed   Radiocarbon (uncorrected)
0350 (?) Unknown Confirmed 3 Radiocarbon (uncorrected)
0200 (?) Unknown Confirmed   Radiocarbon (uncorrected)
0100 (?) Unknown Confirmed 3 Radiocarbon (uncorrected)
0050 BCE (?) Unknown Confirmed 3 Radiocarbon (uncorrected)
0100 BCE (?) Unknown Confirmed 3 Radiocarbon (uncorrected)
0270 BCE ± 100 years Unknown Confirmed   Radiocarbon (uncorrected)
0500 BCE (?) Unknown Confirmed 3 Radiocarbon (uncorrected)
1010 BCE ± 100 years Unknown Confirmed 5 Radiocarbon (uncorrected)
7750 BCE (?) Unknown Confirmed 4 Radiocarbon (uncorrected)

The following references are the sources used for data regarding this volcano. References are linked directly to our volcano data file. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title. Additional discussion of data sources can be found under Volcano Data Criteria.

Almeida E, Ramon P, 1991. Las erupciones historicas del Volcan Tungurahua. Bol Geol Ecuatoriano, 2: 89-138.

Arellano S R, Hall M, Samaniego P, Le Pennec J-L, Ruiz A, Molina I, Yepes H, 2008. Degassing patterns of Tungurahua Volcano (Ecuador) during the 1999-2006 eruptive period, inferred from remote spectroscopic measurements of SO2 emissions. J Volc Geotherm Res, 176: 151-162.

Fee D, Garces M, Steffke A, 2010. Infrasound from Tungurahua Volcano 2006-2008: Strombolian to Plinian eruptive activity. J Volc Geotherm Res, 193: 67-81.

Hall M L, 1977. El Volcanismo en El Ecuador. Quito: Biblioteca Ecuador, 120 p.

Hall M L, 1992. . (pers. comm.).

Hall M L, Robin C, Beate B, Mothes P, Monzier M, 1999. Tungurahua volcano, Ecuador: structure, eruptive history and hazards. J Volc Geotherm Res, 91: 1-21.

Hall M L, Vera R, 1985. La actividad volcanica del volcan Tungurahua: sus peligros y sus riesgos volcanicos. Rev Politecnica, Quito, 10: 91-144.

Hantke G, Parodi I, 1966. Colombia, Ecuador and Peru. Catalog of Active Volcanoes of the World and Solfatara Fields, Rome: IAVCEI, 19: 1-73.

IAVCEI, 1973-80. Post-Miocene Volcanoes of the World. IAVCEI Data Sheets, Rome: Internatl Assoc Volc Chemistry Earth's Interior..

Johnson J B, Aster R C, Ruiz M C, Malone S D, McChesney P J, Lees J M, Kyle P R, 2003. Interpretation and utility of infrasonic records from erupting volcanoes. J Volc Geotherm Res, 121: 15-63.

Le Pennec J-L, Hall M L, Robin C, Bartomioli E, 2006. Tungurahua volcano: late Holocene activity. Cities on Volcanoes 4, Quito, Ecuador, 23-27 Jan, 2006, Field trip A1: 1-23.

Le Pennec J-L, Jaya D, Samaniego P, Ramon P, Moreno Yanez S, Egred J, van der Plicht J, 2008. The AD 1300-1700 eruptive periods at Tungurahua volcano, Ecuador, revealed by historical narratives, stratigraphy and radiocarbon dating. J Volc Geotherm Res, 176: 70-81.

Samaniego P, Eissen J-P, Le Pennec J-L, Hall M L, Monzier M, Mothes P, Ramon P, Robin C, Egred J, Molina I, Yepes H, 2003. Los peligros volcanicos asociados con el Tungurahua. Inst Geofis Escuela Politecnica Nac, Inst Recherche Devel, 1: 1-108.

Steffke A M, Fee D, Garces M, Harris A, 2010. Eruption chronologies, plume heights and eruption styles at Tungurahua Volcano: Integrating remote sensing techniques and infrasound. J Volc Geotherm Res, 193: 143-160.

Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major volcanic edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II itself collapsed about 3000 years ago and produced a large debris-avalanche deposit and a horseshoe-shaped caldera open to the west, inside which the modern glacier-capped stratovolcano (Tungurahua III) was constructed. Historical eruptions have all originated from the summit crater. They have been accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.