Contents of Monthly Reports

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

All times are local (= UTC - 6 hours)

12/1971 (CSLP 99-71) Fumarolic activity at summit on 1-2 December

Card 1334 (22 December 1971) Fumarolic activity at summit on 1-2 December

"Beginning 3 May 1971 . . . San Cristóbal's summit crater has been the site of intense gas emissions visible from Leon and other nearby cities. From May to early July there were some reports of small explosions and minor ash or sand fallout at Chinandega (10 km SW). Sulfurous gas has been frequently reported at Hacienda Las Rojas on the volcano's western slope. Trees within the crater and on the downwind flank appear to have been killed. An expedition by Dartmouth College geologists on 30 November 1971 reached the rim of the second nested crater at the summit. Fumaroles at this location were depositing gypsum. Intense sulfurous gas emission obscured the view inside the crater. Volcanic ash probably several months old was sampled. Flight over the crater on 2 December 1971 indicated that fumarolic activity is most intense in an elongate (E-W) area in the crater, the area approximately coincident with the innermost of three pre-1971 nested craters."

"All of this is a great change from San Cristóbal's dormancy of nearly 300 years. Pre-1971 observations and air photos have shown no sign of activity. Local sources verify that the crater was the site of only weak fumarolic and mudpot activity prior to May 1971. The volcano has also been called El Viejo or Chinandega."

Information Contact: Richard E. Stoiber, Dartmouth College.

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03/1976 (SEAN 01:06) Minor seismicity and small ash eruptions

A team from Dartmouth College visited the volcano from 12 to 30 March . . . . The level of activity was very low and static. There was no regular activity, and no big changes have occurred. Every few days there was a little shudder of superficial tremor in the volcano, and perhaps a short, spasmodic earthquake, but nothing very important. There was nothing out of character with the gradual buildup of activity that has been observed over the past couple of months.

On 9-10 March an eruption of ash drifted into the town of Chinandega, and there was a small ash eruption, mainly confined to the crater area, on 16 March at 1130. The team collected some sand from the latter eruption. Because the gases were too thick and heavy the crater could not be entered. When viewed from the air, sulfur coated everything in the crater to the point where it was actually overlapping the rim on the leeward side. The team did not get the impression that any inordinate activity was imminent at San Cristóbal.

Information Contact: Dept. of Earth Sciences, Dartmouth College.

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04/1976 (SEAN 01:07) Significant increase in gas output

It is believed that the activity during April remained static. The gas output was about five times more than it was before the two small ash eruptions in March, and this involves a sulfur output of thousands of tons/day.

Information Contact: R. Hazlett, Dartmouth College.

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08/1976 (SEAN 01:11) Minor ash eruption on 29 August

On 29 August San Cristóbal erupted briefly with a minor ejection of ash -- the eruption being another signal in the rise of the volcano's thermal gradient as evidenced by earlier eruptions in May 1971 and 9-10 March 1976, both of which were minor. The volcano has been quiet since the most recent eruption.

According to Richard Stoiber, both gas emission and its SO₂ content increased from early 1971 to March 1976, but had declined by mid-July 1976. The ash ejection of 29 August has increased anticipation of major activity. Stoiber has been monitoring the volcano for the past 6 months, collecting condensed water samples from active fumaroles. Analysis of those samples has not yet been completed.

Information Contact: A. Aburto Q., Instituto de Investigaciones Sísmicas.

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12/1976 (SEAN 01:15) Fume cloud still large in December

The fume cloud . . . was still very large in December, but the SO₂ content of the gas had declined since the summer.

Information Contact: R. Stoiber, Dartmouth College.

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10/1977 (SEAN 02:10) Small ash eruption

A small ash eruption . . . began on 16 October at 0800 and lasted about 45 minutes. According to local estimates, the eruption column rose about 1,500 m above the summit. Ashfall at Chinandega, near the volcano, was very slight.

Information Contact: R. Stoiber, Dartmouth College.

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02/1980 (SEAN 05:02) Large white plume

A large white plume was observed from the air. Remote sensing of SO₂ revealed very low levels as compared to records from monitoring over the last 8 years.

Information Contacts: R. Stoiber, S. Williams, and M. Bruzga, Dartmouth College.

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07/1980 (SEAN 05:07) Moderate vapor plume

A moderate vapor plume was emitted. Remote sensing showed that the SO₂ content of the plume was low in comparison to recent years.

Information Contacts: R. Stoiber and S. Williams, Dartmouth College; M. Carr and J. Walker, Rutgers Univ.; A. Creusot, Instituto Nicaraguense de Energía.

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12/1980 (SEAN 05:12) Moderate vapor plume

A moderate-sized vapor plume rose continuously from the summit. Remote sensing of SO₂ revealed increased flux since June 1980, but SO₂ emission remained far below the levels of the mid-1970's.

Information Contacts: R. Stoiber, S. Williams, H.R. Naslund, L. Malinconico, and M. Conrad, Dartmouth College; A. Aburto Q., and D. Fajardo B., Instituto de Investigaciones Sísmicas.

Further Reference: Zapata, R., 1981, La Actividad del Volcán San Cristóbal (Nicaragua), Iniciada el 24 de Agosto de 1980; Boletín de Vulcanología (Universidad Nacional, Heredia, Costa Rica), no. 11, p. 16-18.

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01/1981 (SEAN 06:01) Gas plume becomes intermittent; elevated seismicity

The gas plume released essentially continuously since 1971 has become intermittent. Periods of energetic gas release of less than 1 hour duration were separated by periods (measured in hours) of only low fumarolic release. Shallow seismic activity continued at levels above background.

Information Contacts: R. Stoiber and S. Williams, Dartmouth College; D. de Jerez, Instituto Nicaraguense de Recursos Naturales (IRENA); D. Fajardo B., Instituto de Investigaciones Sísmicas.

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03/1981 (SEAN 06:03) Emissions of SO₂ increase by an order of magnitude; high seismicity; incandescence

"A trend of decreasing SO₂ emissions had been evident since the small ash eruptions of March 1976. However, San Cristóbal has suddenly reversed this trend, after being in a heightened state of seismic activity since August 1980. In late February, SO₂ output increased by approximately an order of magnitude to the several thousand t/d level of the mid-1970's. Flights over the crater in mid-February and mid-March showed evidence of considerable recent slumping in the crater formed by the 1976 eruptions, especially on the N and NW walls. Fumarolic activity was evident all over the crater but was most concentrated in the S and SE margins of the floor and in the lower parts of the walls. No new fumaroles or fissures were observed outside the 1976 crater. Night observation revealed extensive incandescence over much of the crater, even more than that observed in December at Momotombo. High gas concentrations and unstable footing prevented measurement of any fumarole temperatures. Seismic activity continued at high levels, with almost continuous harmonic tremor and at least one earthquake with magnitude greater than 2 (which occurred one week before the elevated SO₂ emission was detected)."

Information Contacts: R. Stoiber and S. Williams, Dartmouth College; D. de Jerez, IRENA; D. Fajardo B., Instituto de Investigaciones Sísmicas.

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07/1981 (SEAN 06:07) Medium- to high-temperature gases collected

Between 14 June and 11 July, personnel from PIRPSEV, CNRS, and the volcano observation section of IPG sampled gases from five central American volcanoes. Medium- to high-temperature gases from San Cristóbal were analyzed (table 1). Maximum gas temperatures measured were 525°C.

Table 1. Analytical mean of three mid-1981 gas samples (dry, HCl excluded) from San Cristóbal.

    Component    Mean Value

    SO2            65.40%
    CO2            33.93%
    H2              0.31%
    H2S             0.33%
    N2              4.35% in air
    CO              0.02%
    CH4            65 ppm
    He             12 ppm
    COS            27 ppm

Information Contacts: H. Delorme, Univ. de Paris; J. L. Cheminée, IPG, Paris.

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12/1981 (SEAN 06:12) Small irregular white gas plume; incandescence seen

"The volcano continued to emit a small white irregular gas plume. SO₂ emission rates were measured at very low levels, similar to those reported for November 1980 and January 1981. Some caving had occurred in the summit crater since March 1981, and incandescence was again observed."

Information Contacts: R. Stoiber, S. Williams, R. Naslund, J.B. Gemmell, and D. Sussman, Dartmouth College; D. Fajardo B., Instituto de Investigaciones Sísmicas.

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01/1982 (SEAN 07:01) Very little SO₂ in the plume; low seismicity

"COSPEC measurements indicated that very little SO₂ was being emitted in the vapor plume. Levels were lower than any measured since San Cristóbal began emitting gases in 1971. Seismicity beneath the volcano has diminished to normal levels, but incandescence could still be observed in the crater."

Information Contacts: R. Stoiber and S. Williams, Dartmouth College; D. Fajardo B., INETER.

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03/1982 (SEAN 07:03) Small white plume almost all water vapor

"During a crater visit 9 March we found that the small white vapor plume was almost entirely made up of water vapor, with little acid gas content. We were unable to reach the fumaroles, but Bruce Gemmell of Dartmouth College measured fumarole temperatures as high as 590°C in December 1981.

Information Contact: S. Williams and R. Stoiber, Dartmouth College, I. Menyailov and V. Shaper, IVP, Kamchatka; D Fajardo, INETER.

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08/1982 (SEAN 07:08) Continuous gas column; incandescent areas

A gas column, usually drifting SW, was continuously present over the volcano. The strongest activity was concentrated in the E part of the summit crater, where several incandescent areas were observed. A temperature of 565°C was measured in July. Landslides within the crater enlarged its E portion. Since the strong seismic activity of March 1981, only occasional small earthquakes have been recorded, normal at San Cristóbal.

Information Contacts: D. Fajardo B., INETER; R. Parnell, Jr., Dartmouth College.

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04/1983 (SEAN 08:04) Slow fumarole temperature increase; gas column

Temperatures continued to increase slowly, to 620°C on 2 February. A strong gas column was blown W. New N-S-trending fissures were observed. During February, 13-14 seismic events were recorded per day, but could only be detected by very sensitive instruments.

Information Contact: D. Fajardo B., INETER.

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08/1985 (SEAN 10:08) Plume sighted from space shuttle

Space shuttle astronauts photographed plumes from two volcanoes on 2 September. A whitish, rather wispy plume about 70 km long was sighted over San Cristóbal.

Information Contact: C. Wood, NASA.

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01/1988 (SEAN 13:01) Gas puffs from the main crater

Geologists from the Open Univ and INETER collected data from several volcanoes in December 1987-January 1988. Temperatures were taken with an infrared radiometer measuring surface brightness temperature in the 8-14 µm range with emissivity set at 1.

On 16 January, gas was being emitted from the main crater in discrete puffs of varying strength that sometimes cleared the crater rim. A small ash eruption may have occurred in November 1987 at about the same time as at Telica.

Information Contacts: B. van Wyk de Vries, H. Rymer, and G.C. Brown, Open Univ; P. Hradecky and H. Taleno, INETER.

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02/1989 (SEAN 14:02) Fumarole temperature measurements

Geologists from INETER and Open Univ measured fumarole temperatures at several volcanoes between 17 February and 10 March. Brightness temperatures were obtained with Minolta Cyclops 33 (bandpass 8-14 µm, 1° field of view) and Cyclops 52 (bandpass 0.8-1.1 µm, 0.33° field of view) infrared thermometers. Temperatures are not corrected for emissivity and atmospheric absorption effects.

Geologists climbed the volcano 9 March. Most fumaroles were on the steep inner wall beneath the perched crater terrace. On the edge of the terrace, one 40-cm-wide crack yielded a brightness temperature of 329°C (Cyclops 33; looking down to 2 m depth from 1 m distance). A group of vigorously fuming vents at the base of the wall registered a brightness temperature of 315° (Cyclops 33) from about 40 m distance.

Information Contacts: C.M.M. Oppenheimer and D.A. Rothery, Open Univ; B. van Vyk de Vries, O. Castellon, and L. Urbina, INETER.

06/1989 (SEAN 14:06) New fumaroles along fissure on SE spur of Casita

Previously unobserved fumarolic activity on the SE spur of Casita (at sites 150 m and 0.5-1 km below the communications complex on the summit) was noticed on 8 June. Area residents report that the activity has been present for some time. Emissions appear to originate from a N-S fissure (figure 1). Casita was last reported active in the l6th century.

Information Contacts: B. van Wyk de Vries and O. Castellón, INETER, Managua.

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02/1991 (BGVN 16:02) Strong gas plume from San Cristóbal; fumarolic activity at Casita

"The gas plume [from San Cristóbal] was continuously present and had not diminished since 1989. The plume could usually be traced up to 60 km from the volcano. Geologists visiting the crater [of Casita] reported fumarole temperatures of up to 98°C. Fumaroles in the S and E parts of the crater were H₂O-rich and below 50°C."

Information Contacts: B. van Wyk de Vries, O. Castellón, A. Murales, and V. Tenorio, INETER.

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04/1992 (BGVN 17:04) Vigorous gas emission

A moderately large steam plume, with the same general appearance as in 1982, was observed many days during 23-29 April fieldwork at nearby Cerro Negro. Geologists were unable to measure the SO₂ flux, but the plume was estimated to represent several hundred t/d.

Information Contact: S.N. Williams, Arizona State Univ.

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03/1993 (BGVN 18:03) Gas plume; little evidence of acid rain

"San Cristóbal was producing a gas plume in January 1993 at levels comparable to recent years. On 6 and 7 January a plume extended over Chinandega (~15 km SW) at about 1,000 m above sea level. No gas was smelled at farms at the base of the volcano. There was patchy evidence of acid rain on the leaves of a few coffee bushes and other plant species, but effects were mild and the coffee crops were not reported to be affected. Analyses of rainwater collected for drinking purposes at one farm revealed cation and anion concentrations expected of normal rainwater, as did a sample from the spring of a neighboring farm. The analyses were undertaken by the British Geological Survey."

Information Contacts: Andrea Borgia, Instituto Nazionale di Geofisica, via di Vigna Murata 605, 00143 Roma, Italy; B. van Wyk de Vries, Open Univ; and Peter J. Baxter, Dept of Community Medicine, Fenner's, Gresham Road, Cambridge, England.

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09/1993 (BGVN 18:09) Large steam plume

A large steam plume was observed at the volcano each day during the first week of September. The plume was clearly visible at distances tens of kilometers from the vent, and apparently filled the entire crater.

Information Contacts: Michael Conway and Andrew Macfarlane, Florida International Univ; Charles Connor, Southwest Research Institute; Oscar Leonel Urbina and Cristian Lugo, INETER.

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09/1994 (BGVN 19:09) Increased seismicity during March-July declines in August

The number of high-frequency seismic events increased from 46 in March to 897 in July. The number decreased again in August and September, but there were large tremors. For an unspecified time interval prior to 21 August the gas plume extended several kilometers from the volcano.

Information Contacts: H. Taleno, L. Urbina, C. Lugo, and O. Canales, INETER.

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12/1996 (BGVN 21:12) December vapor plumes appear smaller than previous ones

Since the end of the last rainy season, the vapor plume continuously present over the volcano has reduced in size compared to recent years. Late in 1996, fumaroles outside the crater decreased significantly. Fumaroles inside the crater concentrated in the lower part of the S and SE walls and had temperatures estimated to be in the 650-700°C range.

During the night and early morning on 24 December several incandescent areas were observed in the crater. These areas were similar to those reported over the past 15 years. Incandescence was visible from the top of the volcano at 500 m distance, and from the rim of the 1976 crater, a feature 400 m in diameter and 100-150 m deep. During 1996 small local earthquakes were detected monthly.

Information Contacts: Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua.

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05/1997 (BGVN 22:05) Eruptions cause ashfall on the slopes; plumes to 2,500 m

According to press reports quoting an official from the Instituto Nicaraguense de Estudios Territoriales (INETER), an eruptive phase began at San Cristóbal during the night of 19-20 May. Because of the possibility of ashfall, authorities declared a state of alert in the city of Chinandega, one of the largest cities in Nicaragua, situated 20 km WSW, the main downwind direction. A report about precursory seismicity was provided by INETER on 18 May. Observations after the start of eruptive activity were sent by Benjamin van Wyk de Vries (Open University), and include information from INETER scientists.

Precursory activity as of 18 May. Strong seismic activity was detected starting in May, by far the strongest seismicity observed since the new seismic station (CRIN, ~3 km NW of the crater at the base of the volcano), was installed at the end of 1992. As of 18 May CRIN was recording 500 volcano-seismic events/day, with frequencies of 1-6 Hz and durations of ~1 minute or more. Sometimes roughly mono-frequency wavetrains of ~3 Hz were observed. RSAM values climbed from <10 at the end of April to ~70 by 15 May, when the level stabilized. Degasification seemed to be higher than before and local residents reported a strong SO₂ odor. INETER volcanologists visited the volcano on 17 May, but could not climb to the crater because of high gas concentrations.

On 18 May INETER was preparing an observation point at Casita volcano, ~4 km SE of San Cristóbal. At the seismic station, CRIN, additional channels with low amplification were switched to be transmitted to the Managua data center. The installation of additional seismic stations is planned. INETER volcanologists took gas and water samples for analysis, and local residents were being interviewed to obtain information about recent changes at the volcano. INETER informed the Nicaraguan Government and the Civil Defense Organization about the situation.

Activity during 21-26 May. B. van Wyk de Vries visited the volcano during 21-23 May, setting up the base for a deformation network, and checking the general state of the cone and crater. RSAM levels stayed fairly constant during this period, but began a slow decline at the end of May. Eruptions through 26 May produced light gray clouds of ash that normally rose 50-200 m above the crater. There were a few notable large eruptions, including one at about 0245 on 22 May that reached 800 m. The height differences were partly due to how quickly the plume was pulled over by the wind. One pulse made a noise that was heard 3 km away at the Casita observation post. Pedro Perez (contracted to The Open University), who was at the crater edge on the morning of the 22nd, noted dull noises before each ejection of ash. Sounds were not heard the next day, when the ejections were less powerful. There were periods of up to a few hours with very weak gas release, then equally long periods of eruptions every 5-10 minutes, or a constant plume. If the wind was strong the plume was dragged down the cone to ~900 m, after which it rose to 1,000-1,200 m; during calmer periods the plume rose to 2,500 m.

Summit visit on 23 May. There was no activity when the crater was reached at 1200 on 23 May. Volcanologists stayed for an hour installing a GPS station on the edge of the outer crater. They made a very quick descent into the crater and saw a new vent. Before the start of activity in early May, San Cristóbal was generally the same as when van Wyk de Vries first saw it in 1986. The main change has been the slow progressive growth of the inner pit crater (figure 2). There is an old outer crater ledge on the S side, then the main crater (10-50 m deep); a small inner cone has been gradually hollowed out by a pit crater. The cone has now mostly fallen in, and only a small part to the S and W still exists. The pit is ~80 m deep and Pedro Perez reported that in early May it had a flat bottom. At that time there was little degassing: a few fumaroles on the SW side and a few fumarole mounds on the SE part of the main crater floor. There had been a vigorous fumarole on the edge of the pit crater, with a temperature of ~600°C, but this part had fallen in. On 23 May there was a 4-m-wide vent in the SW part of the pit crater floor; looking in at 45° no bottom could be seen, and in shadow it was not glowing. On the walls of the pit were dark patches of wet rock, where water seepage or fumarolic emanations had discolored the new ash. The whole of the crater was covered by ash. On the crater floor it was 10 cm deep and around the edges 1-2 cm. The upwind crater rim (SE-N) had a slight dusting. On the downwind rim ash had been stripped off the top of rocks but was accumulating in fine layers on the vertical upwind surfaces. The ash was being continually blown around and formed some beautiful ripples.

Ash covered the W side of the mountain, giving it a light-gray appearance down to ~800 m. Although vegetation hid the ash layer from distant viewers, it continued down to ~600 m where a 1-3 mm thick layer coated the ground and dusted the leaves. Gas masks were worn during the descent from the summit because the ash billowed up. Down at the farm of Las Rojas (500 m elevation) there was a slight dusting of ash; other farms reported heavier falls. Vegetation damage was not observed, possibly because everything is so dry in the area. Very light ashfall was occurring 10-15 km downwind.

No tremor was felt at the crater, though there were ~600 events/day being recorded. Campesinos living on the pass between San Cristóbal and Casita felt shocks on the night of 21-22 May. The only significant deformation features noted on 23 May were several fissures around the edge of the inner pit crater, which indicate that it is still enlarging. The fractures on the main crater floor were covered in ash, which was not fractured, indicating that there was no movement over the previous few days. Pedro Perez noted some rockfalls during explosive activity on the morning of the 22nd, from the N side of the pit crater.

Deformation network. The new GPS network consists of two triangles. The outer triangle is centered on a point on the pass between Casita and San Cristóbal, with one position on Casita, one 2 km SE of Chinandega, and one 10 km NE of Chinandega. The lines are ~10 km long. The center point should form one corner of an inner triangle, the other points being at Las Rojas farm to the W and one as yet undecided on the N flank. These lines are <5 km long. Within the inner triangle, points were placed at ~1,500 m elevation on the SE flank and on the W side of the crater edge at ~1,700 m; both are 1-2 km from the center point. Simultaneous measurements at all four of the outer triangle points were taken with the Open University GPS and the INETER GPS. Two points on the volcano were fixed, and will be re-occupied by INETER personnel. Although the whole of the network is not in place, at least three points on the volcano have been fixed and will indicate any major movement.

Volcanic history. San Cristóbal is a stratovolcano 100 km NW of Managua that has erupted about nine times since the Spanish conquest. Its previous most recent confirmed eruption was a 45-minute ash emission in October 1977 (SEAN 02:10), but a small ash emission may have occurred in November 1987 (SEAN 13:01). The San Cristóbal complex comprises San Cristóbal cone, El Chonco cone, Cerro Montoso, Casita volcano, and La Pelona Caldera (Hazlet, 1987; Van Wyk de Vries and Borgia, 1996). San Cristóbal proper is the youngest feature of the complex. Casita was probably active in the 16th century, and has several active fumarole fields. La Pelona looks as though it was once a large stratocone that underwent a caldera-forming eruption in the Quaternary. El Chonco is an 800-m-high andesite-dacite cone. Cerro Montoso is a 600-m-high andesitic scoria cone cut by large faults.

The complex as a whole has a tendency to produce significant amounts of dacitic magma. Examples include the Chonco cone and Loma La Teta (a dacite dome associated with El Chonco), recent pyroclastic-flow and tephra deposits on San Cristóbal and Casita, and the La Pelona Caldera ignimbrite. This type of magma contrasts with the predominant basalt-basaltic andesite of the other nearby volcanoes, such as Telica, Rota, and El Hoyo/Cerro Negro. Martha Navarro (INETER), who has done geological and hazard mapping at San Cristóbal, noted that the dacitic pyroclastic-flow deposits of San Cristóbal are similar in composition to a thick tephra-fall deposit that the forms subsoil over much of the west side of the cone. On the cone itself, these deposits are covered by more recent andesitic scoriae and bombs, some or all of which come from the historical eruptions in the 17th century and the 1970's. There have not been any historical lava flows, but several lava flows are still only partially vegetated.

References: Hazlett, R.W., 1987, Geology of the San Cristóbal volcanic complex, Nicaragua, in Williams, S.N. and Carr, M.J. (eds.), Richard E. Stoiber 75th Birthday Volume: J. Volcanol. Geotherm. Res., v. 33, p. 223-230.

Van Wyk de Vries, B., and Borgia, A., 1996, The role of basement in volcano deformation, in McGuire, W.J., Jones, A.P., and Neuberg, J. (eds.), Volcano Instability on the Earth and Other Planets: Geological Society Special Publication 110, London, p. 95-110.

Information Contacts: Wilfried Strauch and Pedro Perez, Department of Geophysics, Instituto Nicaraguense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: wil@ibw.com.ni); Benjamin van Wyk de Vries, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom (Email: b.van-wyk-de-vries@open.ac.uk); Reuters.

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06/1997 (BGVN 22:06) Seismicity, thin tephra falls, crop damage, and evacuation plans

Additional information has come to light regarding the eruption that began on the night of 19-20 May 1997 (BGVN 22:05). On the morning of 20 May, fine ash was found on cars in the town of Chinandega (18 km W of the volcano) and the same day people from Hacienda Las Rojas (~3 km SW of the crater) reported 1 mm of ashfall.

The 20 May explosions sometimes occurred nearly continuously for intervals of a few minutes, and at other times stopped for minutes or even hours. Yet, around that time the amount of ash expelled remained very low. The plumes typically moved to the SW, though on some occasions shifting winds carried them N or NE.

INETER volcanologists who visited Hacienda Las Rojas on 8 May prior to the eruption, noticed that the vegetation had been strongly affected by volcanic gases. As time passed, nearby people feared that continued eruptions would threaten local coffee plantations. Moreover, by an undisclosed date thirteen banana growers in Chinandega estimated damage at 40,000 boxes, a loss with an estimated value of US $160,000.

Small eruptions recurred, though in June the rainy season brought decreased visibility. As a result, both observation points at Casita volcano and San Cristóbal crater were frequently cloud-covered. Although during June ash only fell in small amounts and very near the volcano, starting on 3 July ash accumulated at a village 2 km W of the volcano (Las Rojas) and by an undisclosed time it reached 2-mm thickness. Also, starting on 7 July residents reported new ash in Chinandega, this time in greater amounts and composed of coarser particles than the May ash. The concern at INETER was that these coarser and thicker ashes could indicate the ascent of magma to shallower levels.

To prepare for outbursts of increased intensity the local administration together with the Civil Defense Organization took some preventive measures. Immediately after the 14 May alert information submitted by INETER, they organized meetings with local residents to explain evacuation plans. In Chinandega a meeting was organized that included high-ranking government officials and INETER volcanologists who explained the volcanic risk. Key roads in the populated area near the volcano were repaired to facilitate rapid evacuation if necessary.

Seismic data. The RSAM (Real-time Seismic Amplitude Measurement) index of seismicity was computed from the 10-minute mean of seismic amplitude at station CRIN (figure 3). This station, which lies near Hacienda Las Rojas (3 km SW of the crater) contains an L-4 seismometer and a 60-dB amplifier. The available time series contained occasional artifacts of telemetry or system interruptions. Some RSAM interruptions resulted from low batteries due to clouded skies and ash on solar panels. These artifacts were removed and then the data were filtered with an ~3-hour-long gliding mean that effectively removed the short time-interval maxima. Prior to this processing some maxima had reached 140 RSAM units.

The RSAM index reached a maximum on 14-15 May (figure 2) and remained stable until 23 May. It then declined to a minimum on 26 May. RSAM again reached ~60 at the beginning of June where it remained until early July, when it climbed to a maximum near 95 units.

Information Contacts: Wilfried Strauch, Department of Geophysics, and Marta Navarro C., Department of Volcanoes, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: wil@ibw.com.ni).

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10/1998 (BGVN 23:10) Heavy rains from hurricane Mitch result in deadly avalanche and lahar from Casita

On 30 October 1998 a disastrous event (called a "mudflow" in newspapers) occurred on the S flank of Casita volcano. According to official reports, the incident killed between 1,560 and 1,680 people, displaced hundreds more, destroyed several towns and settlements, and disrupted the Pan American Highway at numerous bridges. On 11 and 12 November the first scientific team visited the volcano to investigate the disaster. The team examined the summit area on the first day and made a complete traverse of the devastated zone as far S as the Pan American Highway on the second day. This report presents the team's conclusions and provides some recommendations regarding future risks.

Background. Casita is within the Cordillera Maribios, a 70-km-long volcanic chain that extends from the N shore of Lake Managua to the vicinity of Chinandega. Casita is part of the San Cristóbal volcanic complex, which consists of five principal volcanic edifices. The largest volcano in Nicaragua, San Cristóbal lies 4 km WNW of Casita and has exhibited frequent episodes of historical activity; at present it is emitting a vigorous fumarolic plume. For these reasons San Cristóbal has been studied in greater detail.

Casita is a composite volcano with deeply dissected morphology. The top of the volcano consists of a cluster of dacite domes. At its summit is a 1-km diameter crater that could be reached by a road - now impassable - to service telecommunication towers. A set of prominent NE-trending normal faults cut the summit area bounding each side of the crater. Explosion craters on the southern plain are aligned along a conjugate set of fractures trending NW-SE. No historical volcanic activity has been reported at Casita; however, the domes of the summit area are autobrecciated and exhibit strong hydrothermal alteration, which is consistent with low-temperature fumarolic activity.

Meteorological conditions. Hurricane Mitch was a major factor in the disaster. Abnormal rainfall related to Mitch began on 25 October. By 27 October the precipitation reached 100 mm/day and increased continuously to a maximum of ~500 mm/day on 30 October, the day of the avalanche. The total rainfall in October was 1,984 mm. Within three days, precipitation dropped to normal levels. For comparison, the average rainfall for October is 328 mm; thus the rainfall associated with the disaster was more than 6 times the average.

Source zone. The main source of the avalanche was 200 m SW of the volcano summit, and 60 to 80 m below the telecommunication towers. A secondary source was located at the same elevation but 100 m SE of the summit. The rock in this area is a hydrothermally altered and brecciated dacite dome. The principal rupture occurred along a ~500-m-long segment of a NE-trending fault that intersects the summit. A slab measuring ~20 m thick, 60 m high, and 150 m long detached slid down the fault plane that was inclined about 45 degrees SE. The volume of source block for the first rockslide was ~200,000 m³.

Avalanche event. Inhabitants of the lower plains described the sound of the avalanche as similar to a helicopter. Multiple witnesses gave the time as between 1030 and 1100 on 30 October. The main slide mass immediately shattered into its original breccia blocks coated by vein precipitates. The initial SE movement of the avalanche blocks was deflected to the SW along a deep gully oriented parallel to the fault. A smaller part of the avalanche surmounted a small ridge and continued SE towards the village of Argelia.

For the first 2 km the main avalanche remained confined to a narrow valley. The top of the flow was 150 to 250 m wide; its depth, 30 to 60 m. A typical cross section of the peak flow was 7,500 to 9,000 m². The flow swashed back and forth on its downward course. Super-elevation calculations at locations of overbank flow gave a velocity of ~15 m/s in the upper reaches. Deposits high on the volcano consisted of altered dacite blocks up to meter-size. They contained essentially no matrix, with the finest particles centimeter-sized. The margin of the avalanche was sharp and flying rocks scarred the adjacent trees at 2-3 m height. A few trees were decapitated at heights of several meters.

At a prominent break in slope 2-3 km from the source, large ramps of avalanche materials formed imbricate ridges. Here the deposits, 4-6 m thick, still lacked matrix. The avalanche materials were essentially clast supported. The avalanche scoured blocks of lava from the walls, and up to 10 m deep into clay-rich soil in the base of the valley where it passed.

Lahar runout flow. Soon after the onset of the avalanche, a lahar runout flow, as defined in Scott (1988), initiated from the major accumulation zone of the primary avalanche. In other words, the source of the lahar runout flow formed in the thickest accumulation of debris at the mouth of the avalanche valley, 3 km from the summit and 3 km above the towns of El Porvenir (formerly Augusto Cesar Sandino) and Rolando Rodriguez. The populations of these two towns were respectively 600 and 1,250 according to the last census. The location of the sites of El Porvenir and Rolando Rodriguez could only be found by GPS data; there remained almost no evidence of former human habitation.

Apparently the lahar runout flow resulted from rapid dewatering of the saturated avalanche. The flood surge moved as a hyperconcentrated flow, depositing a thin (~40 cm thick) layer of gravel with some clay matrix on the overbank zones, and transporting meter-size blocks within the incised channels. The peak height of the flood surge was 3 m as it entered El Porvenir, as evidenced by stripped bark from the few standing trees. Nearly all vegetation and soil was removed by the leading edge of the wave. However, a few islands of vegetation were spared on some hills. The width of the flood surge in its upper reaches was ~1,500 m. Assuming an average peak depth of about 3 m, this yields a cross sectional area of flood surge at 4,500 m².

Casualties and damage. Based on observations in the field, the towns of El Porvenir and Rolando Rodriguez were destroyed beyond recognition. It is unknow how many people survived. Visible cadavers and dead livestock on the overbank had been burned for sanitary reasons. Many other small hamlets, residences, and farms were destroyed.

Future hazard potential. The disaster of 30 October, was produced by the coincidence of two discrete events: extraordinarily heavy rains and an avalanche. Neither of these alone would have produced such extensive damage to the surrounding area. In this respect note that the towns of El Porvenir and Rolando Rodriguez were established only a few decades ago in this area of high geologic risk. To reduce threats for new settlements, comprehensive geologic hazard studies can help identify regions with elevated risk.

In the absence of another episode of heavy rainfall, the new deposits seem to be stable. In fact, there is little mud or silt within the deposits at higher elevations to facilitate remobilization. However, the conditions near the summit that favored the rockslide avalanche still exist. Altered and fractured dacite occurs on steep slopes at a high elevation. Destabilizing events, such as an earthquake or torrential rains, could produce another avalanche in an adjacent area. The probability of such an extreme avalanche seems remote. However, an assessment of the associated hazards and risks should be undertaken.

Reference: Scott, Kevin M., 1988, Origins, behavior, and sedimentology of lahars and lahar-runout flows in the Toutle-Cowlitz River system: U.S.G.S. Professional Paper 1447-A, 74 p.

Information Contacts: Michael F. Sheridan, SUNY, Buffalo, New York (Email:mfs@acsu.buffalo.edu); Claus Siebe, UNAM, Mexico (Email: csiebe@tonatiuh.igeofcu.unam.mx); Christophe Bonnard, EPFL Lausanne, Switzerland (Email: christophe.bonnard@epfl.ch); Wilfried Strauch (Email: wil@ibw.com.ni), Martha Navarro (Email: ineter@ibw.com.ni), Jorge Cruz Calero (Email: ineter@ibw.com.ni), and Nelson Buitrago Trujillo (Email: ineter@ibw.com.ni), INETER, Nicaragua.

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02/2000 (BGVN 25:02) Continued frequent eruptions and ashfall through December 1999

A new eruption began at about 2200 on 20 November 1999 (figure 4) with a series of explosions that caused ashfall near the volcano. More than 100 people were evacuated from the Hacienda Las Rojas. A commercial airline pilot reported that the ash plume reached about 2.4-3 km altitude. As of the start of this increased activity, the Nicaraguan Institute of Territorial Studies (INETER) reported that volcanic tremor and sporadic minor ash emissions had occurred for more than a year.

The last previous significant eruptive activity at San Cristóbal began on the night of 19-20 May 1997 and spread fine ash on Chinandega, 18 km W (BGVN 22:05 and 22:06). On 30 October 1998 earth movements on the S flank of Casita volcano (4 km ESE of San Cristóbal) resulted from heavy hurricane rains, killing an estimated 1,560-1,680 people, with hundreds more displaced and several towns and settlements destroyed (BGVN 23:10). The following is based on INETER reports from November through the end of February 2000, with additional information based on crater visits in January and February and satellite imagery of a large eruption plume on 21 February.

Seismic tremor increased during the night of 19-20 November and quickly reached a maximum that was not matched at least through the end of December (figure 5). Tremor amplitude declined throughout 20 November, and then remained at levels of ~25-60% of the 20 November peak until a volcano-tectonic earthquake on 27 November.

Small explosions preceded emissions on 22 November. Activity decreased after the initial eruptions, but a lack of winds caused the ash and gases to remain near the volcano. Concentrations of CO₂ and SO₂ measured on 22 November at three different sites exceeded the permissible limits established by the World Health Organization (WHO). INETER inspections showed that expelled gas and ash have been accumulating in local communities. However, on 23 November a slight increase in the wind speed facilitated the transport of volcanic material as far as the city of Chinandega 10 km to the SW. That day an observation post was established near the summit of Casita, where INETER staff could make visual observations and measurements of eruption column heights.

Ash emanations during 24-27 November varied in frequency, and the observer at Casita saw ash columns to heights of 100-300 m above the crater during the same period. At 0920 on 27 November a small earthquake (M 2.3) occurred ~2 km underneath the volcano. Volcanic gas monitoring on 24 November showed a significant increase in the concentrations of SO₂ in the San Rafael and Las Rojas areas. However, the concentration of CO₂ showed a diminution compared to 23 November. Concentrations of SO₂ and CO₂ decreased again on the 25th, although in some communities the level of SO₂ remained unchanged. A correlation spectrometer (COSPEC) for measuring SO₂ flux was brought from the INSIVUMEH of Guatemala on 26 November, with the support of the Coordination Center for the Prevention of Natural Disasters in Central America (CEPREDENAC).

Seismic tremor amplitude remained fairly stable, around 30-50% of the 20 November peak, during 26 November through 13 December. Seismicity fluctuated, with periods of higher and lower amplitude; a corresponding fluctuation in the ash-and-gas emissions was noted. The amounts of gases emitted by the volcano also showed this correlation, with SO₂-flux values that oscillated between 100 and 1,000 tons/day.

By 2 December ash emissions had decreased considerably, although low-level gas emissions remained continuous. There was a slight increase in emissions on 4 December, but activity remained low through the 13th. Higher wind speeds during this period helped keep gas concentrations low in local towns. Seismic tremor began to rise the night of 12-13 December to a high of 70% of the previous peak. On 14 December small amounts of volcanic ash fell in Chinandega. Tremor amplitude returned to ~40% of the peak on 15 December, then decreased to 20-30% of the peak by dawn on the 16th.

Lahars on 16 December 1999. The Civil defense and local residents reported that a mass-flow on the NW side of the volcano on 16 December stopped ~2 km from populated areas. On 21 December, two specialists visited the affected Rancherías region NW of San Cristóbal in the Municipality of Chinandega (Dpto. Chinandega) to investigate the event. According to the meteorological station of Chinandega, 7.9 mm of rain fell from 1824 until 2012; in the first hour strong rains were reported, and from 1920 there were light to moderate rains. Previous strong rains have produced erosion gullies on the slopes of the volcano.

Due to the recent eruptive activity, large amounts of ash and lapilli had accumulated in these gullies; these were mobilized by the rains into lahars. Several sources were identified between 1,400 and 1,500 m elevation. The mobilized material formed a debris flow containing ash and lapilli that carried blocks varying from centimeters to meters in size. The flow quickly cemented into an extremely hard deposit. The material eroded from the highest slopes of San Cristóbal moved along one main gully, leaving a very deep channel and, below 500 m elevation, an extended lobate deposit that reached within ~2 km of the community of Ranchería. The main flow had a length of ~7 km and variable widths between 10 and 150 m; deposit thickness varied from less than 10 cm up to 2 m at the terminus.

Similar events happened on the S slope of the volcano that same day. At least five lahars were visible from the Leon-Chinandega highway.

Minor ash emissions continue. Activity remained consistently low and unchanged until noon on 28 December when earthquakes began. These were centered SE of Casita with magnitudes between 2 and 3.5 and depths of a few kilometers. The occurrence of earthquakes near San Cristóbal is a new phenomenon in the current eruption. Ash emission also increased and small amounts fell in Chinandega.

Seismic tremor on 29 December increased to as high as 40% of the 20 November peak. Small amounts of ash fell in Chinandega and El Viejo. No significant changes in activity were noted on 30 December. Moderate ashfalls were reported near the volcano and in Chinandega. Depending on the wind direction, ashfall sporadically reached the communities of Higueral, 10.5 km NE, and Pelona, 9 km E.

Activity during January-February 2000. Small explosions continued during January with ash and gas emissions (figure 6) and 4,444 registered volcanic earthquakes. Seismicity was higher in the first 17 days of January, but the seismic tremor (RSAM) stayed constant. Between 17 and 23 February activity increased, causing significant ashfall in Chinandega. The number of registered volcanic earthquakes in February was of 1,784.

Alain Creusot visited the crater on 10 January and observed rhythmic, phreatic explosions, which included rock ejections and ash columns from three vents. At 0600 that day, a violent explosion threw bombs high above the crater rim. Creusot visited the crater again on 4 February and observed a 50-cm-deep ash layer over the crater area and 30-cm depths over the entire summit. Rhythmic phreatic explosions continued and new bombs were observed and sampled on the E crater rim (figure 7). These bombs apparently originated from a Strombolian explosion on 30 January. A third visit on 20 February showed a 1-m-deep ash layer in the crater area and 50-cm depths elsewhere in the summit area. Rocks 50 cm in size had been ejected.

Benjamin van Wyk de Vries noted that GOES images on 21 February showed a long plume extending from San Cristóbal to ~100 km over the Pacific Ocean. He also reported that this was the strongest ash eruption at San Cristóbal since the 1997 eruption. At 1100 on 24 February, a violent explosion threw bombs over the entire summit. A similar explosion and effects occurred at 0900 on the 25th.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/; Email: wil.gf@ineter.gob.ni); Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua (URL: http://www.ine.gob.ni/); Benjamin van Wyk de Vries, Departement des Sciences de la Terre, Universite Blaise Pascal, 63038 Clermont-Ferrand, France (Email: vanwyk@opgc.univ-bpclermont.fr).

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05/2000 (BGVN 25:05) Emissions decreased in March; several lahars occurred in May

In May 2000 a series of lahars at San Cristóbal mobilized ash that had been deposited on the volcano from the 20 November 1999 eruption (BGVN 25:02) and from sporadic ash-and-gas emissions during November to May.

In March 2000 the number of ash-and-gas emissions decreased from levels in February. The number of seismic events also substantially decreased from 1,748 in February to 181 in March, but seismic tremor remained high. Two noteworthy seismic events occurred on 15 March at 1200 and 1311. One was felt by settlers who live on the volcano's flanks and by observers on Casita's summit.

INETER personnel noted that ash emissions from the volcano's secondary crater continued in March. They observed a large number of fumaroles that surrounded the internal wall of the secondary crater. The fumaroles were determined to have a maximum temperature of 120 °C using a hand-held infrared sensing instrument. On 19 and 20 March Pierre Delmelle (Université de Montréal) and Julio Alvarez (INETER - GEOFISICA) performed COSPEC measurements near the volcano that yielded SO₂ fluxes of 352-1,644 tons/day, with an average value of 873 tons/day. Activity during April was similar to March with sporadic gas-and-ash emissions.

Just after the beginning of the rainy season on 13 May, lahars traveled down the flanks of the volcano to the N, NW, and S. INETER seismologists detected the lahars in progress using two seismic stations located S and SW of the volcano. The lahars were small, traveled long distances, and were not considered dangerous because they traveled within channels and caused no destruction to populated areas. The deposited material consisted partly of volcanic ash that had accumulated since the ongoing eruption began in November 1999. The average thickness of the deposited material was 30 cm and the total volume was 275,000 m³.

After strong local rains on 17 May, lahars similar to the 13 May lahars traveled down the slopes of San Cristóbal to the NW and SW. The thickness of the deposited material was 30-35 cm, and the volume was between 25,000 and 200,000 m³. In addition, on 19 May lahars traveled to the N, SW, and S.

On 30 April Alain Creusot observed the new crater for 4 hours from the NE crater rim (figure 8). He noted that 2 to 4 million m³ of ash was deposited at the summit area. He also found the crater was 200 m deep, which was 50 m deeper than when it was observed in January. In addition, a new vent had opened in the crater to the NE. The change in the crater's depth indicated that 4 to 5 million m³ of lithic ash had been ejected. Rhythmic phreatic explosions reached an altitude of 500-600 m and a permanent jet-like noise was heard from steam-and-gas emissions. Rock fall activity was concentrated to the SE of the old crater.

During a visit on 22 June he found the 2-4 million m³ ash deposit at the summit had been washed mainly down the volcano's SW slope by rain. The new crater had been refilled to an approximate depth of 150 m. Fumaroles in the old part of the crater were measured to be 200-250 °C, and 500-600 °C in the crater's deep part. Steam-and-gas emissions continued to produce a loud noise.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Department of Geophysics, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: wil.gf@ineter.gob.ni); Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua (URL: http://www.ine.gob.ni/).

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05/2001 (BGVN 26:05) Small gas-and-ash plumes during May and June 2001 cause ashfalls

Beginning on 11 May 2001 volcanic activity increased above normal levels, with small eruptions producing gas-and-ash clouds that deposited small amounts of ash on a neighboring town. The previous report of anomalous volcanic activity at San Cristóbal was in May 2000 when a series of lahars occurred as a result of the remobilization of ash that had been deposited on the volcano from the 20 November eruption (BGVN 25:02 and 25:05).

On 22 July 2000, ten months prior to the May 2001 eruption, Alain Creusot visited the summit of the volcano. He reported that seismic activity during 18-19 July caused two lakes to empty that were observed during a previous trip. He also found that active fissures inside the crater were partially sealed, which caused the intensity of degassing to decrease.

INETER reported that on 11 May 2001 tremor began to rise at a seismic station on San Cristóbal (figure 9). The tremor reached a maximum level at noon and then slightly diminished, but stayed at relatively high levels for several days. Seismic activity during this period exceeded the maximum level of seismicity throughout the entire December 1999-February 2000 eruption (BGVN 25:02). Beginning on 11 May INETER personnel stationed near the summit of the volcano occasionally observed small plumes of volcanic gas with small amounts of ash emanating from the volcano. In contrast, on 10 May very low levels of gas were emitted from the crater. On 14 May observers noted that gas emissions with small amounts of ash continued. On 17 May the level of seismic activity significantly increased, and pulses of gas and ash rose ~100 m above the crater rim. Small amounts of ash fell in the town of Santa Barbara, 14 km SW of the volcano.

INETER noted that rain could mix with ash deposited on the flanks of the volcano and generate dangerous lahars. This occurred after the 1999-early 2000 eruption when rainfall in May 2000 mixed with ash that accumulated on the flanks of the volcano. The lahars were especially strong in the S part of the volcano.

According to news reports, on 21 June an explosion at San Cristóbal sent an ash cloud to a maximum height of 800 m. The cloud extended approximately 25 km downwind of the crater, and ash fell in the town of Chinandega, ~15 km SW of the volcano.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Department of Geophysics, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: wil.gf@ineter.gob.ni); Alain Creusot, Instituto Nicaraguense de Energía, Managua, Nicaragua (URL: http://www.ine.gob.ni/); La Noticia (URL: http://www.lanoticia.com.ni/); El Nuevo Diario (URL: http://www.elnuevodiario.com.ni/); La Prensa (URL: http://www.laprensa.com.ni/).

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11/2001 (BGVN 26:11) Minor ash eruptions during May-November 2001; elevated seismicity

Ash fell at San Cristóbal during May and June 2000. Relative calm prevailed after then until May 2001, when activity began to increase. Thousands of earthquakes per month occurred during June through at least October 2001. Explosive eruptions in mid-August produced columns that reached 400 m.

Seismic signals registered on 2, 4, and 7 May 2001 indicated that small explosions had probably occurred. At 0900 on 11 May, seismic tremor increased to a level exceeding that observed during the eruption in December 1999 and the early months of 2000 (BGVN 25:02). The volcano emitted ash and gas beginning on 12 May. A total of 2,748 seismic events were registered during the month. No dominant frequency was observed during the beginning of the month, but during the rest of the month dominant frequencies of up to 6.7 Hz were noted. Pulses of gas-and-ash emissions were seen rising up to 100 m above the crater rim, and light ash fell in the town of Santa Barbara, 14 km SW of the volcano. The volcano was relatively calm at the end of May.

During June there were three periods of increased seismicity, rapid degassing, and release of gas and ash. The total of 2,276 earthquakes during the month were mostly associated with degassing. On 7 June at 0240, seismic tremor increased, and minutes later dark clouds were observed. At 0500, gray ashfall was reported 10 km SW of the volcano. Activity decreased beginning 9 June until 16 June, when high-energy seismic activity and ash emissions increased for about five hours. The dominant frequency of the 16 June earthquakes ranged between 10 and 12 Hz. On 20 June at 1048 tremor increased again and ashfall began one hour later. A vibration was felt, and noise was heard as far as 6 km from the volcano. The activity ceased five hours later.

According to news reports, on 21 June an explosion sent an ash cloud to a height of 800 m that extended ~25 km downwind and caused ashfall in the town of Chinandega, ~15 km SW. The same day, Jorge Cruz of Indiana Carcache and Martha Navarro of INETER visited the volcano and observed abundant gas-and-ash emissions. Gas sampled on 22 June contained 2.6 mg/m³ of SO₂ and 250 ppm CO₂. The low concentrations suggest weakened gas pressure and no new magmatic material.

During July 2001, San Cristóbal displayed reduced seismic tremor, but the number of volcanic earthquakes was high. More than 6,111 seismic events were registered, including long-period (LP) earthquakes and signals of small gas explosions. LP earthquakes are common at active volcanoes, and have been observed at other Nicaraguan volcanoes just before eruptions. This was the first time this type of signal had been observed at San Cristóbal, so it was not clear if they had occurred prior to or during past explosions. According to Chouet (1996), LP earthquakes are generated by resonance in fractures closed at their ends and filled with volcanic fluids (water or magma) with a certain dissolved gas level, in which an abrupt pressure change takes place. On 22 July at 0134, an LP earthquake was registered that lasted ~17 seconds with a dominant frequency of 1.2 Hz. Three seismic stations recorded the earthquakes, the most distant located ~15 km W of the volcano. In addition to these seismic data, Vicente Perez ascended the volcano during July and heard both landslides moving down the crater's walls and several rumblings.

During August 2001 tremor remained low to moderate and 4,552 earthquakes were registered. The number of earthquakes was high (averaging ~300 events per day) during 1-4 August, but began dropping gradually on 5 August. The dominant frequency of LP events was ~1 Hz. On 8 August tremor began to increase but the number of earthquakes decreased compared to the previous days. On 10 August, 9 seismic events were registered and tremor increased. On 11 August tremor stood at 30-40 RSAM units. Most of the earthquakes had dominant frequencies of 1-7 Hz. On 12 August, tremor increased again until it reached 80 RSAM units. The increase in tremor lasted until the evening of 13 August when it lowered to 30 RSAM units. During 14-15 August tremor increased again, reaching 90 RSAM units. On 14 August incandescence was visible in the crater for the first time during the current episode. INETER stated that gas and clouds above the summit crater were illuminated from below.

On 15 August beginning at 1620 a dense cloud was formed from continuous abundant out-gassing. Rumbling, incandescence, and explosions were observed during 15-17 August. On 16 August, Vicente Perez ascended the volcano to make observations and found an increase in fumarole temperatures. During 0900 through 1030, gas explosions occurred with columns that reached 400 m. Seismic tremor gradually decreased until approximately 1400 on 17 August when strong seismic activity began again. Fumarolic activity increased and small lagoons within the crater had dried. On 18 August tremor lowered to normal levels of 20 units RSAM. The absence of earthquakes and LP events was noted during this time. The dominant frequency of most of the tremor was 1.0-6.0 Hz. Ash explosions were observed until the afternoon of 19 August.

Based on the recent activity at San Cristóbal, INETER believes that magma rose slowly in the volcano's open conduit and remained close to the crater's floor, which allowed the incandescence observed at night. This was consistent with the observed increase in fumarole temperatures.

During September, seismic activity continued, along with degassing and noise in the interior of the crater. A total of 4,695 earthquakes were registered during the month. After the eruptive activity that occurred during August, San Cristóbal maintained a low level of tremor (less than 20 RSAM units). Tremor increased on 7 September, accompanied by earthquakes with dominant frequencies of 2-6 Hz that occurred every minute for 24 hours. Few LP events were registered. On 8 September, Perez again ascended the volcano and found a slight increase in the temperatures of most of the fumaroles. Abundant degassing took place during the month and noises were heard in the interior of the crater. During 17-19 September tremor increased again, and was accompanied by earthquakes that occurred in bands of time that lasted, on average, one hour. During the last week of September, another increase in tremor took place, as well as an increase in the number of earthquakes. On this occasion, tremor lasted several days and was accompanied by earthquakes approximately every hour.

During October 2001, seismic tremor remained at 20-40 RSAM units. The dominant frequency of tremor was 4-6 Hz. A total of 7,421 earthquakes were registered during the month. Most of the earthquakes had dominant frequencies of from 5 to over 10 Hz. Few events registered dominant frequencies less than 1 Hz. Despite the increase in earthquakes since June 2001, little eruptive activity has taken place (small ash explosions and gas emanations). During the month San Cristóbal displayed emanations of gas, ash, and noise in the interior of the crater. On the night of 3 October, Perez reported ashfall on surrounding communities. On 7 October, Perez ascended the volcano and reported that a collapse had occurred in the S part of the crater.

INETER reported that during the evening of 12 November small ash emissions at San Cristóbal produced ash clouds that remained around summit level. According to the Washington VAAC, on 12 November at 1645 GOES-8 imagery showed a small area of possible ash drifting NW. Ground observers noted moderate volcanic activity until 1800. Ash had dissipated by 2100 and the next day there were no ground reports of volcanic activity.

General Reference: Chouet, B.A., 1996, Volcano long-period seismicity: its source and uses in eruption forecasting: Nature v. 380, p. 309-316.

Information Contacts: Virginia Tenorio, Department of Geophysics, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: vtenorio.gf@ineter.gob.ni, URL: http://www.ineter.gob.ni/geofisica/sis/bolsis/bolsis.html); La Noticia (URL: http://www.lanoticia.com.ni/); El Nuevo Diario (URL: http://www.elnuevodiario.com.ni/); La Prensa (URL: http://www.laprensa.com.ni/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS/E/SP23, NOAA Science Center Room 401, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa. gov/).

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04/2002 (BGVN 27:04) November ash-and-gas emissions; thousands of earthquakes through May 2002

Instituto Nicaragüense de Estudios Territoriales (INETER) reported that during December 2001 through May 2002 San Cristóbal maintained generally constant levels of seismicity and moderate tremor levels. Thousands of earthquakes per month were recorded, most with frequencies of 4.0 to over 10 Hz. Very few events registered with frequencies less than 1.0 Hz.

The third eruptive stage in 2001 was during 7-25 November, when strong ash-and-gas emissions and rumblings occurred and small amounts of ash fell in surrounding areas. After visiting the crater on 11 and 25 November, and 9 December, Vicente Perez (INETER) reported rockfalls and strong emissions of gas and ash. Fumarolic temperatures on 25 November were ~40-100°C, and were similar during December. On 15 January Pedro Perez observed only sporadic gas emanations during a crater visit.

During November through 23 February seismic tremor generally remained between 20 and 60 RSAM units, with the maximum tremor occurring during 8-14 November, when ash-and-gas emissions were strongest. Tremor frequency was 4.0-6.0 Hz.

Observations on 6 February revealed an overall lack of visible changes at the volcano with the exception of gas emanations in the new crater. On 24 February seismic tremor began to increase until it reached 40 RSAM units. While the tremor increased, the number of earthquakes diminished. Strong rumblings on 22 and 26 February, coinciding with the increase of tremor on 24 February, were accompanied by gas emissions.

Another increase in tremor began on the afternoon of 6 March. Strong seismicity occurred in 2- to 3-hour periods that were generally separated by less than 1 hour of less intense activity. INETER reported that seismic tremor reached more than 50 RSAM units on 7 March. Scientists visiting the volcano found that the amount and temperature of degassing had increased. Reportedly, incandescent material in the crater was reflected on the clouds above it. On 22 March at 2219 an earthquake was felt by most of the population near the volcano. Following this event, more than twelve earthquakes with magnitudes of 2.0-3.2 occurred. According to INETER, associated activity was not strong enough to warrant raising the Alert Level.

During April an average of 30 earthquakes occurred per hour (figure 10), most associated with degassing. Very few events were volcano-tectonic or explosion earthquakes. Seismic tremor remained between 40 and 45 RSAM units.

On 23 May a strong gas column was observed at San Cristóbal. The Washington Volcanic Ash Advisory Center (VAAC) stated that a surface report had indicated strong activity near the summit. A plume was visible on satellite imagery drifting SW from the summit (figure 11). A video camera near the summit indicated that the altitude of the plume was relatively low, near ~3 km. The Washingon VAAC issued a second notice stating that according to INETER, the emissions consisted solely of gas. The VAAC noted that no plume was detected in satellite imagery later that day. INETER reported that the column was the result of rain in the crater that generated steam. No other phenomena were observed that could indicate an increase in the eruptive activity of the volcano.

Information Contacts: Virginia Tenorio, Department of Geophysics, Instituto Nicaragüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email: vtenorio.gf@ineter.gob.ni, URL: http://www.ineter.gob.ni/geofisica/sis/bolsis/bolsis.html); La Noticia (URL: http://www.lanoticia.com.ni/); El Nuevo Diario (URL: http://www.elnuevodiario.com.ni/); La Prensa (URL: http://www.laprensa.com.ni/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS/E/SP23, NOAA Science Center Room 401, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa. gov/).

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08/2002 (BGVN 27:08) Tremor increases during late May 2002, activity continues in August

Activity at San Cristóbal during November 2001-May 2002 included strong [gas] emissions on 23 May (BGVN 27:04). At the time, no other phenomena were observed that could indicate an increase in the eruptive activity of the volcano. Volcanic tremor began to increase at San Cristóbal on 28 May, reaching a peak of ~150 RSAM units around noon the next day (figure 12). The summit of the volcano was not visible, but satellite imagery from the Centro de Vigilancia de Ceniza Volcánica revealed that ash was emitted from the volcano. After the 29th, the amount of tremor began to decrease. Incandescence was visible at the crater rim on 1 June. By 3 June tremor was still relatively high.

A ground observer in Chinandega, Nicaragua indicated that San Cristóbal was active on 21 August around 1600. Ash was not visible on an INETER volcano camera at 1630 due to dense cloud cover. A possible plume was detected on satellite imagery taken at 1545 during a break in the cloud cover; it was estimated to be near summit level and drifting W. The volcano remained at Alert Level Orange.

Information Contacts: Wilfried Strauch, Director General de Geofísica, Instituto Nicaragúüense de Estudios Territoriales (INETER), P.O. Box 1761, Managua, Nicaragua (Email:wil.gf@ineter.gob.ni, URL:http://www.ineter.gob.ni/geofisica/sis/bolsis/bolsis.html); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch, NOAA/NESDIS/E/SP23, NOAA Science Center Room 401, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa. gov/); La Prensa (URL: http://www.laprensa.com.ni/).

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10/2003 (BGVN 28:10) Intermittent ash emissions between August 2002 and September 2003

This report summarizes the recorded activity at San Cristóbal during August 2002-September 2003. Reports from Instituto Nicaragüense de Estudios Territoriales (INETER) include observations from frequent visits to the volcano.

August-December 2002. During August-December 2002, abundant gas emanations were accompanied by small gas explosions. Incandescence was frequently observed. Seismicity was relatively high, but fluctuated from month to month (table 2). On 12 August, ash emissions with columns up to 800 m high were observed. More ash explosions were reported on 17 and 21 August. Gas emissions and small ash explosions continued in September, and incandescence was observed during 1-7 September. A strong explosion was reported on 6 October, and a dark gas column was observed. Throughout the month gas emissions were abundant, occasionally with columns to 600 m high. Temperatures at the South Point and El Zopilote fumaroles increased in October from the previous month. Gas emissions and ash explosions continued in November and December, with increased activity on 22 November and 16 December. Trees and fruit plants were affected by the gases in the community of Las Banderas. At the end of December, a volcano observer discovered that the path to the volcano was blocked by a deposit of sand-sized material.

Table 2. Number of earthquakes at San Cristóbal between August 2002 and June 2003. Courtesy of INETER.

    Month      Number of earthquakes

    Aug 2002          2,183
    Sep 2002          2,792
    Oct 2002          1,017
    Nov 2002   technical problems-no earthquakes recorded
    Dec 2002      200-250 per day
    Jan 2003          5,671
    Feb 2003          2,595
    Mar 2003          5,329
    Apr 2003          1,713
    May 2003          5,491
    Jun 2003          4,855

January-March 2003. Activity decreased in the first few months of 2003. No changes were observed in the crater in January, and temperatures increased only at two fumaroles; those temperatures were low again in February. On 19 February the observer heard loud sustained noises and noted that the crater walls were colored green and yellow, indicating the presence of sulfur.

April-September 2003. Between April and September, fumarole temperatures were measured on each of the monthly visits to San Cristóbal, and showed very little change. The highest temperatures were generally found at South Point and were between 91 and 95°C. At El Munecho temperatures varied between 81 and 91°C, and at El Conejo between 77 and 86°C. Temperatures remained moderate at the other fumaroles.

Gas emissions were noted in particular between 20 and 23 April, days on which there were small increases in tremor; on 10 May gas emissions were strong enough to impede a visit. Activity increased in June, with abundant ash and gas emissions noted on 17 and 21 June. On 21 June incandescence was noted, and strong rumbling was heard in the evening. On 13 July gas emissions were dense, followed during 14-23 July by a dark column. Seismicity dropped from more than 350 events per day to 69 events on 3 July. By 9 July, only eight events per day were recorded; tremor remained constant at 35 RSAM units.

Gas emissions remained constant through August and September, with reports of gas explosions during a visit on 10 August and abundant gases during the 17 September visit. The strong noises and sounds of gas pressure being released decreased over these months, and no noise was noted on the September visit. Seismicity was very low in August and September, with no earthquakes and very low tremors in August, and only six earthquakes in September.

Information Contact: Virginia Tenorio, Emilio Talavera, and Martha Navarro, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni; URL: http://www.ineter.gob.ni/geofisica/).

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09/2006 (BGVN 31:09) Multi-year update: 13 June 2004, local ash fall; early 2006, small eruptions

San Cristóbal was last reported on in BGVN 28:10, covering intermittent gas and ash emissions between August 2002 and September 2003. The Instituto Nicaragüense de Estudios Territoriales (INETER) noted that low seismicity and minor gas and ash emissions characterized the period from October 2003 to June 2004.

On 7 June 2004 a lahar flowed more than 600 m. On 13 June 2004, an eruption caused ash to fall in the communities of Las Rojas, El Chonco, and El Viejo.

On 20 July 2004 at 1430, an M 4.3 earthquake occurred to the N of the volcano at a depth of less than four km. The earthquake was felt in the regions of Carlos Fonseca, Villa 15 de Julio, La Suiza, Las Rojas, Mocorón, San Jose del Obraje, Santa Carlota, San Antonio, Ranchería, and bordering regions. Some houses were damaged and the population was alarmed. The earthquake was felt in Matagalpa and Ocotal, and San Cristóbal emitted abundant gases for the following two days. During the rest of July, 95 aftershocks were registered; residents felt two more earthquakes, which occurred on 23 and 30 July.

During August to early December 2004, minor seismicity and ash and gas emissions were the norm. Ash explosions occurred on 3, 4, and 7 December. According to local people, ash fell in Chinandega and El Viejo.

The next available report discussed 16-22 November 2005. INETER detected an increase in seismicity beginning on 19 November. Increased tremor was interpreted as being related to gas and ash emissions. Ash fell W of the volcano and near the town of Chinandega, ~ 15 km SW of the volcano. The amount of tremor decreased later.

According to an Associated Press news report, explosions on 6 March 2006 produced columns of ash and gas that rose above the volcano. The activity ceased by 8 March and there were no evacuations.

INETER noted that phreatomagmatic eruptions began at San Cristóbal on 21 April 2006. Seismic tremor increased the same day around 1300. Small explosions produced gas-and-ash plumes during 21-23 April that deposited small amounts of ash in nearby towns.

Information Contact: Virginia Tenorio, Emilio Talavera, and Martha Navarro, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (Email: ineter@ibw.com.ni; URL: http://www.ineter.gob.ni/geofisica/); Associated Press, (URL: http://www.ap.org/).

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