Asosan

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  • 32.884°N
  • 131.104°E

  • 1592 m
    5222 ft

  • 282110
  • Latitude
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10 September-16 September 2014

JMA reported that Alert Level 2 at Asosan continued during 8-16 September. A persistent white plume was observed 1,000 m above the crater.

Preliminary counts for volcanic earthquakes (394-564 per day) and tremor (80-174 per day) were reported during 8-15 September. Field surveys conducted on 9 and 12 September yielded elevated temperatures from fumaroles and the surface of the S crater wall.

Source: Japan Meteorological Agency (JMA)

Index of Weekly Reports


2014: January | February | August | September
2011: May | June
2005: April
2004: January
2003: July
2002: August

Weekly Reports


10 September-16 September 2014

JMA reported that Alert Level 2 at Asosan continued during 8-16 September. A persistent white plume was observed 1,000 m above the crater.

Preliminary counts for volcanic earthquakes (394-564 per day) and tremor (80-174 per day) were reported during 8-15 September. Field surveys conducted on 9 and 12 September yielded elevated temperatures from fumaroles and the surface of the S crater wall.

Source: Japan Meteorological Agency (JMA)


3 September-9 September 2014

JMA reported that Alert Level 2 at Asosan continued during 1-8 September. A small eruption occurred on 1 September from Nakadake crater, generating an off-white plume that suggested a small amount of ash; the plume rose 1,200 m above the crater. Incandescence from the crater was detected with a camera on 2 September. Volcanic earthquakes (48-92 per day) and tremor (429-500 per day) was detected during 1-4 September.

On 6 September a small eruption occurred from Nakadake crater that generated a plume 600 m above the rim. Elevated SO2 (1,200 tons/day) was detected during a field survey (the previous measurement on 21 August was 1,000 tons/day). Volcano-tectonic earthquakes (55-129 per day) and tremor (401-463 per day) was detected during 5-7 September.

Tokyo VAAC issued advisories based on JMA reports of eruptions on 1 and 6 September, though no volcanic ash was visible in satellite images.

Sources: Japan Meteorological Agency (JMA); Tokyo Volcanic Ash Advisory Center (VAAC)


27 August-2 September 2014

On 27-29 August JMA reported volcanic earthquakes and elevated lake temperatures at Asosan's Nakadake Crater. On 30 August an eruption occurred with a gray-white plume of indeterminate height due to clouds prompted raising the Alert level to 2. On 31 August- 2 September was a small eruption and a gray white plume rose 800-1200 m above the crater rim. During 30 August-1 September the Tokyo VAAC reported ash plumes that rose 1.5-2.1 km (5,000-7,000 ft) a.s.l. and drifted N and NE. The Alert Level was raised to 2 (on a scale of 1-5).

Source: Japan Meteorological Agency (JMA)


19 February-25 February 2014

JMA reported that a very small explosion from Asosan's Nakadake Crater occurred on 19 February. An off-white plume rose 200 m above the crater rim and drifted SW. During fieldwork on 21 February volcanologists noted that sulfur dioxide emissions remained high. The Alert Level remained at 2 (on a scale of 1-5).

Source: Japan Meteorological Agency (JMA)


12 February-18 February 2014

JMA reported that a very small explosion from Asosan's Nakadake Crater occurred on 16 February. An off-white plume rose 300 m above the crater rim and drifted S. The Alert Level remained at 2 (on a scale of 1-5).

Source: Japan Meteorological Agency (JMA)


5 February-11 February 2014

According to a JMA report, volcanologists conducting a field survey of Asosan's Nakadake Crater on 5 February detected decreased sulfur dioxide emissions and fewer volcanic earthquakes. The Alert Level remained at 2 (on a scale of 1-5).

Source: Japan Meteorological Agency (JMA)


29 January-4 February 2014

Based on pilot observations, the Tokyo VAAC reported that on 29 January an ash plume from Asosan rose to an altitude of 2.7 km (9,000 ft) a.s.l. and drifted NW. Later that day a plume rose to an altitude of 1.5 km (5,000 ft) a.s.l. and drifted N. JMA reported that a very small explosion from Naka-daka Crater occurred on 31 January. An off-white plume rose 100 m above the crater rim and drifted S. The Alert Level remained at 2 (on a scale of 1-5).

Sources: Japan Meteorological Agency (JMA); Tokyo Volcanic Ash Advisory Center (VAAC)


22 January-28 January 2014

JMA reported that seismicity at Asosan increased from 21 to 23 January, and then decreased on 24 January. On 23 January a volcanologist observed ash plumes rising from the central vent on the crater floor. The Alert Level remained at 2 (on a scale of 1-5).

Source: Japan Meteorological Agency (JMA)


8 January-14 January 2014

On 27 December 2013 JMA raised the Alert Level for Aso to 2 (on a scale of 1-5) because volcanic tremor amplitude had been increasing since 20 December. However, on 2 January 2014 the amplitude rapidly decreased. Sulfur dioxide emissions were 1,200 tons per day during 2-9 January and 1,500 tons on 10 January. Volcanic tremor amplitude increased between 0800 and 1900 on 12 January. At 1215 on 13 January a very small eruption from Naka-dake Crater generated a grayish white plume that rose 600 m and drifted S, producing ashfall downwind.

Source: Japan Meteorological Agency (JMA)


8 June-14 June 2011

Based on notices from JMA, the Tokyo VAAC reported that during 7-9 June plumes from Aso rose to altitudes of 1.5-1.8 km (5,000-6,000 ft) a.s.l. and drifted NW, N, NE, and E.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)


1 June-7 June 2011

Based on notices from JMA, the Tokyo VAAC reported that during 1-7 June plumes from Aso rose to altitudes of 1.5-2.1 km (5,000-7,000 ft) a.s.l. and drifted NW, N, NE, E, and S.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)


25 May-31 May 2011

Based on notices from JMA, the Tokyo VAAC reported that during 25, 27-28, and 31 May ash plumes from Aso rose to altitudes of 1.5-1.8 km (5,000-6,000 ft) a.s.l. and drifted NW, N, E, and S.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)


18 May-24 May 2011

Based on notices from JMA, the Tokyo VAAC reported that during 18-22 May ash plumes from Aso rose to altitudes of 1.5-2.1 km (5,000-7,000 ft) a.s.l. and drifted N, NE, and SE. A pilot noted on 18 May that an ash plume rose to an altitude of 3 km (10,000 ft) a.s.l.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)


11 May-17 May 2011

Based on pilot observations, the Tokyo VAAC reported that on 15 May an ash plume from Aso rose to an altitude of 2.1 km (7,000 ft) a.s.l. and drifted NE. JMA reported that the next day plumes rose to altitudes of 1.8-2.1 km (6,000-7,000 ft) a.s.l. A pilot noted that an ash plume rose to an altitude of 2.4 km (8,000 ft) a.s.l. and drifted N that same day. During 17-18 May the JMA reported that ash plumes rose to an altitude of 1.8 km (6,000 ft) a.s.l. and drifted E and SE.

Source: Tokyo Volcanic Ash Advisory Center (VAAC)


13 April-19 April 2005

According to a news article, a small explosion at Aso on 14 April emitted a plume of "white smoke" to ~200 m above the crater (~5,900 ft a.s.l.) and deposited ash around the crater. The explosion occurred after hundreds of small earthquakes were recorded by JMA during the previous 2 weeks.

Source: Associated Press


14 January-20 January 2004

According to the Japanese Meteorological Agency, a "mud eruption" occurred at Aso's Crater 1 on 14 January at 1541. The eruption was accompanied by volcanic tremor and ash emissions that rose to low levels above the crater. Small amounts of very fine ash fell in Takamori Town about 10 km ESE of the crater. The level of thermal activity at Aso had risen during the previous year, with the last "mud eruption" occurring in July 2003. The Alert Level at Aso was raised from 2 to 3, and no tourists were permitted entrance within 1 km of the crater.

Sources: Volcano Research Center-Earthquake Research Institute (University of Tokyo); Reuters


23 July-29 July 2003

During 12-14 July, JMA recorded seismic signals at Aso that were associated with five small phreatic eruptions. Around 1400 on 27 July continuous volcanic tremor started. Such activity had not been recorded at Aso since November 1995. As of 28 July ~10 earthquakes occurred per day, and around 100 isolated tremor events had occurred since 23 July. On the 28th the crater lake in Crater 1 was gray, 76 °C, and bubbling in the center.

Source: Japan Meteorological Agency (JMA)


9 July-15 July 2003

A tremor event with a moderate amplitude was recorded at Aso on 10 July at 1718. Aso weather station personnel inspected the area around Nakadake crater and found a small amount of tephra newly deposited at Hakoishi-Toge about 6 km ENE of the crater. Dr. Yasuaki Sudo of Aso Volcanological Laboratory, Kyoto University, inspected the crater area and determined that a phreatic eruption had occurred. Mud emitted during the eruption reached as far as 10 km from the crater. The color of the crater lake surface changed to dark gray from green, its color on 8 July.

Source: Japan Meteorological Agency (JMA)


21 August-27 August 2002

By 21 August isolated tremor events that began at Aso's Crater 1 on 5 August decreased in number after peaking on 15 August. On the 21st the temperature of the crater's southern inner rim was still high (314 ºC), as it had been the previous week.

Source: Japan Meteorological Agency (JMA) via the Volcano Research Center


14 August-20 August 2002

During 5 August to at least 15 August, isolated volcanic-tremor events occurred at Naka-dake, Aso's historically-active intra-caldera cone. The maximum number of events (335) was recorded on 12 August. Scientists found that the temperature of the southern crater wall remained high (307 ºC on 14 August) as it has since April 2002. There were no changes in water-pool temperature in the crater, nor had changes occurred in water level, sediment content, or fumarolic activity in the crater. The last time over 300 isolated volcanic-tremor events per day had been recorded at Aso was during 19 June-2 July 1992.

Source: Japan Meteorological Agency (JMA) via the Volcano Research Center


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.

04/1970 (CSLP 39-70) Ash eruptions from a new vent at Crater 1 of the Naka-dake cone

05/1977 (SEAN 02:05) Gas emissions increase; minor tephra ejections

06/1977 (SEAN 02:06) Minor ash emission in mid-May

07/1977 (SEAN 02:07) Gas and tephra emissions; several people injured

08/1977 (SEAN 02:08) Activity declines after 22 July

12/1977 (SEAN 02:12) Ash eruptions continue through November with some substantial ashfall

02/1978 (SEAN 03:02) Weak steam and ash ejection with seismic activity

06/1978 (SEAN 03:06) Phreatic activity continues

09/1978 (SEAN 03:09) Weak steam and ash emission in May and June

06/1979 (SEAN 04:06) Steam and ash eruption

07/1979 (SEAN 04:07) Moderate tephra eruption

08/1979 (SEAN 04:08) Mid-June eruption successfully predicted, but activity continues, killing two in September

09/1979 (SEAN 04:09) Large explosion kills three, then activity declines

10/1979 (SEAN 04:10) Occasional weak ash emission

11/1979 (SEAN 04:11) Frequent ash emission; explosion successfully predicted

01/1980 (SEAN 05:01) Short explosive eruption; 1979 activity reviewed

04/1980 (SEAN 05:04) Small ash emission

05/1980 (SEAN 05:05) Activity declines

09/1980 (SEAN 05:09) Small ash emission

06/1981 (SEAN 06:06) 30-minute ash and block ejection

10/1984 (SEAN 09:10) Block and ash ejection from fumarole

12/1984 (SEAN 09:12) Emission of ash containing fresh magma

01/1985 (SEAN 10:01) Moderate ash emissions continue; volcanic flame on two nights

03/1985 (SEAN 10:03) Moderate ash emission; volcanic flame

04/1985 (SEAN 10:04) Moderate tephra emission; new vent

06/1985 (SEAN 10:06) Ejection of ash and incandescent tephra

07/1985 (SEAN 10:07) Ash emision ends

05/1988 (SEAN 13:05) Tremor stronger and more frequent; mud ejection

10/1988 (SEAN 13:10) Glow on crater floor; crater closed to tourists

12/1988 (SEAN 13:12) Minor ash emission

03/1989 (SEAN 14:03) Ash ejection; tremor increase; crater closed to tourists

04/1989 (SEAN 14:04) Brief ash emission

06/1989 (SEAN 14:06) Ash ejections continue; new vent on crater floor

07/1989 (SEAN 14:07) First strong eruption since 1985 ejects ash to 2,500 m

08/1989 (SEAN 14:08) Stronger ash emission

09/1989 (SEAN 14:09) Continued tephra emission

10/1989 (SEAN 14:10) More explosions; stronger tremor; new vent

11/1989 (SEAN 14:11) Frequent tephra ejection continues

12/1989 (SEAN 14:12) Ash emission and seismicity decline

02/1990 (BGVN 15:02) Block and ash ejections increase in late January; daily ash emission in February

05/1990 (BGVN 15:05) Strong block and ash ejection after 2 months of quiet, then continuous ash emissions

06/1990 (BGVN 15:06) Ash and block ejection; gradual increase then abrupt decrease in tremor amplitude

07/1990 (BGVN 15:07) Sporadic eruptive activity stops; variable tremor amplitude

08/1990 (BGVN 15:08) Mud, water, and steam ejected from lake in active vent

09/1990 (BGVN 15:09) Scoria ejected from new vent

10/1990 (BGVN 15:10) Weak ash emission and glow; increased tremor

11/1990 (BGVN 15:11) Tephra ejection resumes

12/1990 (BGVN 15:12) Periodic tephra emissions from new vent

01/1991 (BGVN 16:01) Frequent ash emission, but tremor declines

02/1991 (BGVN 16:02) Tremor amplitude and steam emission decline

05/1992 (BGVN 17:05) Mud/water ejections from heating crater lake; tremor episodes

06/1992 (BGVN 17:06) Explosions follow increased seismicity and heating of crater lake

07/1992 (BGVN 17:07) Phreatic activity and seismicity decline after block ejection

08/1992 (BGVN 17:08) Explosion ejects blocks

09/1992 (BGVN 17:09) Blocks ejected by explosive episode

10/1992 (BGVN 17:10) Explosion produces 2500-m plume and wet ashfall; weak ejections from crater lake

11/1992 (BGVN 17:11) Renewed block ejection; gas plume

12/1992 (BGVN 17:12) Mud/water ejection from crater lake; steam plumes; new vent

01/1993 (BGVN 18:01) Block ejection and steam emission; seismicity remains high

02/1993 (BGVN 18:02) Scoria eruption; seismicity declines

03/1993 (BGVN 18:03) Activity decreases; crater lake forms

05/1994 (BGVN 19:05) Mud ejected; tremor amplitude increases

06/1994 (BGVN 19:06) Volcanic tremor; water ejections from pond in crater floor

07/1994 (BGVN 19:07) Crater 1 at Nakadake still restless

08/1994 (BGVN 19:08) Mud and stone ejections from crater floor

09/1994 (BGVN 19:09) Explosions eject mud and blocks

10/1994 (BGVN 19:10) Continued mud ejections and ash plumes from Nakadake crater 1

11/1994 (BGVN 19:11) Minor phreatic activity from crater lake

12/1994 (BGVN 19:12) Intermittent mud ejection and large-amplitude tremor

01/1995 (BGVN 20:01) More mud ejections, tremor, and a white plume

03/1995 (BGVN 20:03) Mud ejection beyond the crater and an ash cloud to 1 km

05/1995 (BGVN 20:05) Mud and water ejections from crater lake; tremor

07/1995 (BGVN 20:07) Water rises and covers the crater floor, minor water and mud ejections

08/1995 (BGVN 20:08) Continued mud and water ejections; increasing tremor episodes

09/1995 (BGVN 20:09) Continued mud and water ejections and many isolated tremors

10/1995 (BGVN 20:10) Isolated tremor; ejections of mud and water

12/1995 (BGVN 20:11/12) Numerous isolated tremors

02/1996 (BGVN 21:02) Continuous tremor; crater floor still covered with water

05/1996 (BGVN 21:05) Crater glow

07/1996 (BGVN 21:07) Crater glows; water and mud ejected

11/1997 (BGVN 22:11) Two tourists killed by volcanic gas on 23 November

10/2003 (BGVN 28:10) Phreatic eruptions during 10-14 July cause ashfall 14 km away

01/2004 (BGVN 29:01) June 2003 phreatic outbursts and a January 2004 mud eruption

09/2011 (BGVN 36:09) Small ash-bearing eruptions during May and to lesser extent in June 2011

08/2012 (BGVN 37:08) Minor mud ejections resumed in 2011, the first since 2008


Contents of Monthly Reports

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

All times are local (= UTC - 9 hours)

04/1970 (CSLP 39-70) Ash eruptions from a new vent at Crater 1 of the Naka-dake cone

Card 0925 (30 April 1970) Ash eruptions from a new vent at Crater 1 of the Naka-dake cone

The Aso Volcanic Observatory reported that Mt. Aso has become active for the first time in four years. Volcanic ash and gases erupted more than 150 m into the air from a new opening inside of the No. 1 crater on the Nakadake volcanic cone of Mt. Aso. At first, a small amount of ash erupted from a little gap located in the NNE corner of the crater's bottom on 21 April. On 22 April a depression about 20 m long and 10 m wide was observed in the same place which formed a new crater. It spewed gray ash into the air reaching the brim of the outer crater 150 m above it. An eruption of this size is the first of its kind since May 1966.

Information Contact: Aso Volcanic Observatory, Kumamoto, Kyodo, Japan.

05/1977 (SEAN 02:05) Gas emissions increase; minor tephra ejections

Normal fumarolic activity in [Crater 1] began to increase on 31 March. Slight rumbling on 4 April was succeeded on 11 April by heavy rumbling, further increase in gas emission, and ejection of some grayish ash. On 12 April, the grayish [plume] contained many fist-sized cinders, which fell on the floor of Naka-dake's crater. Glow was seen 21 April but the [plume] had turned white and contained only a little ash. There was slight [ash] emission from the crater bottom, which had contained a small hot water pool.

Information Contact: JMA.

06/1977 (SEAN 02:06) Minor ash emission in mid-May

Minor ash emission, similar to the April activity occurred in mid-May. By late May, the ash emission had abated.

Information Contacts: JMA; D. Shackelford, CA.

07/1977 (SEAN 02:07) Gas and tephra emissions; several people injured

Emission of gas and tephra from the [Crater 1] of Naka-dake continued through June. Much ash and sand, and occasional fist- to hand-size cinders were ejected. Activity declined in early July, but an explosion on 20 July at 1321 sent a column of ash . . . about [1,500] m above the crater and [ejecta lightly] injured [three] persons nearby.

Information Contacts: JMA; T. Tiba, Nationl Science Museum; UPI.

08/1977 (SEAN 02:08) Activity declines after 22 July

The increased activity on 31 March was accompanied by continuous large amplitude tremor. A small vent formed on 8 May and emitted ash. During the night, a 2-10 m "flame" was observed. In late May, more fist-sized ejecta fell on the crater floor. An earthquake swarm occurred 3 June, and large-amplitude tremor, some of which could be felt near the crater, was recorded 18 June. Small scale eruptions deposited ash in early June. These eruptions increased in strength 20-23 June, depositing fist-sized ejecta inside the crater.

Activity then declined until 20 July, when explosions at 1321, accompanied by airshocks, projected black and gray ash clouds 1,500 m above the crater. Ash fell up to 500 m from the crater, reaching a maximum depth of 30 cm, and blocks up to 80 cm across fell on the rim. Similar explosions occurred at 1341 on 22 July, producing a 1,300 m cloud and depositing considerable ash around the crater. Activity then declined. No fresh magma was ejected at any time during the eruption.

Information Contacts: JMA; D. Shackelford, CA.

12/1977 (SEAN 02:12) Ash eruptions continue through November with some substantial ashfall

A steam and ash cloud rose more than 300 m from Naka-dake at 0704 on 8 November. Ash ejections continued through the end of November, producing substantial ashfalls in and near the crater on 14 and 18 November.

Information Contact: JMA.

02/1978 (SEAN 03:02) Weak steam and ash ejection with seismic activity

Steam and ash emission from Naka-dake continued in December and January, producing occasional slight ashfalls nearby. Frequent earthquakes and volcanic tremor were recorded.

Information Contacts: JMA; D. Shackelford, CA.

06/1978 (SEAN 03:06) Phreatic activity continues

Increased activity from [Crater 1] of Naka-dake continued through April. The vapor cloud contained little or no ash during March, but on 4 April the emissions increased in volume and a small ashfall was observed. Continuous emission of a grayish-white cloud began 7 April and lasted through the end of the month. Small quantities of mud and fine particles were ejected from vents in Naka-dake crater, but none of this material rose more than 20 m above the crater bottom. None of the ejecta contained evidence of fresh magma. Short-period volcanic tremor recorded by seismographs near the crater continued through April.

Information Contacts: JMA; D. Shackelford, CA.

09/1978 (SEAN 03:09) Weak steam and ash emission in May and June

Steam and ash emission from [Crater 1] of Naka-dake occurred 29 May-1 June, producing an ashfall on the N side of the crater. Rumbling accompanying the activity could be heard from [JMA's Asosan Weather Station] 1 km from [the crater of] Naka-dake. Mud and [block] spattering was observed in mid-June, but the ejecta rose only 40-50 m and remained within the crater. No further activity has been reported.

Information Contacts: JMA; D. Shackelford, CA.

06/1979 (SEAN 04:06) Steam and ash eruption

An explosive eruption from Naka-dake crater began at 1510 on 13 June. Activity lasted more than 1 hour, producing a 1,500-2,000-m-high steam and ash column, and thundering sounds. [Blocks] larger than a man's head were thrown 400 m above the crater rim. Kyodo radio reported that the eruption reintensified during the night of 15-16 June. Hot tephra was ejected to 200 m above the crater rim, accompanied by a roaring noise. Doors and windows rattled in nearby houses and some residents fled the area, according to police eports. . . .

Information Contacts: T. Tiba, National Science Museum; Kyodo Radio.

07/1979 (SEAN 04:07) Moderate tephra eruption

A small explosion occurred at the beginning of June from Crater 1 of Naka-dake (figure 1). Activity then declined for 10 days, although an arthquake was felt on [9] June. Loud rumbling and frequent small explosions egan on 12 June and incandescent block ejection was seen that night. At 1510 on the 13th, a larger explosion produced a 2-km-high ash column and the ejection of numerous blocks. Lightning could be seen in the ash column. Ash emission as observed until nightfall and incandescent blocks continued to be ejected every few minutes through the night of 13-14 June. A field investigation on 14 June revealed more than 10 cm of ash and many scoria bombs and blocks (up to 70 cm in diameter) in the summit area.

Figure 1. Map of the summit area of Naka-dake at Aso. Naka-dake's crater includes smaller craters 1-7 (hachured lines). Vents 741 (active since 1974) and 791 (formed during the June activity) are shown in crater 1. The dashed line delineates the area where tephra larger than 3 cm in diameter fell during June, 1979. Courtesy of JMA.

Frequent periods of activity, consisting of weak and continuous ash emission or explosive block ejection, occurred daily until 25 June. Incandescent coria was seen at night on the 14th, 15th, and 16th. A new vent (791 pit) was formed during the June eruption at the bottom of Crater 1 near 741 pit, which had been active since 1974 (figure 1). Since the end of June, activity has declined to occasional weak ash emission. Seismicity remained low in June, in contrast to the strongly increased seismicity that accompanied the eruptions of 1977 (figure 2).

Figure 2. Number of recorded seismic events per 10-day period at Aso, January 1976-March 1980. Arrows represent eruptions. [Originally from 05:04.] Courtesy of JMA.

Information Contact: JMA.

08/1979 (SEAN 04:08) Mid-June eruption successfully predicted, but activity continues, killing two in September

"The eruption of Aso continued through July (table 1). Seismicity remained low. It is empirically known for Aso volcano that the amplitude of continuous tremor becomes large before an eruption and remains large throughout the eruptive period (figure 3).

Table 1. Activity at Aso, late May-29 September 1979. Ash thicknesses, at about 1 km from the source, were estimated by weighing the small amount of material accumulated in a measured area. Courtesy of JMA.

    Date (1979)     Activity

    late May-       Intermittent phreatic eruptions; wet ash and blocks fell on crater floor,
      early Jun       common activity at Aso.
    [09] Jun        Felt earthquake [at 0904].
    11 Jun          Continuous tremor amplitude increased during the morning; [zone 1 km from
                      crater closed] at 1310.
    12 Jun          Further tremor amplitude increase in the morning but sudden decrease in
                      amplitude at 1821; ash eruption began in the evening; a few incandescent
                      blocks seen at night.
    13 Jun          Further tremor amplitude decrease at 0336, then steep increase at 1457; large
                      explosion at 1510 produced a 2-km-high ash cloud; blocks fell 350 m from
                      the vent.
    14 Jun-15 Jun   Intermittent ash eruptions; incandescent blocks; very loud rumbling ([people
                      said] the strongest in 50 years) began at 2340 on 15 June and lasted until
                      the next morning, rattling doors an windows.
    16 Jun          Activity decreased briefly in the morning, but loud rumbling resumed at 1110
                      and explosions started again, ejecting incandescent scoria.
    17 Jun-26 Jun   Ash ejection every day; 25 cm deposited in the summit area 12-23 June;
                      rumbling declined during the morning of the 18th, then resumed at 0950 on
                      the 20th, continuing through 27 June; block ejection was observed on 3
                      days, and reflected glow could be seen on 22 and 23 June; lightning was
                      seen on 19 June.
    27 Jun-30 Jun   Poor weather prevented observations; estimated volume of ejecta in June, 1.4
                      million tons.
    01 Jul-05 Jul   Ejection of ash and incandescent blocks, accompanied by rumbling; largest
                      scoria bombs measured at 86 cm in diameter.
    06 Jul-19 Jul   Ash ejection and weak rumbling; no blocks observed.
    20 Jul-31 Jul   Ash and incandescent blocks ejected; lightning and reflected glow seen;
                      continuous weak rumbling, punctuated by occasional louder periods; July
                      ejecta volume estimated at 1.64 million tons.
    01 Aug          Ejection of ash and incandescent blocks; reflected glow; lightning.
    02 Aug-03 Aug   Ejection of ash and incandescent blocks; lightning.
    04 Aug          Ash ejection; lightning.
    05 Aug-06 Aug   Ejection of ash and incandescent blocks.
    07 Aug          Bad weather -- no observations.
    08 Aug-09 Aug   Ejection of ash and incandescent blocks; lightning.
    10 Aug          Ejection of ash and incandescent blocks stopped about 1300, then loud
                      rumbling began.
    11 Aug-26 Aug   Loud rumbling; white steam emission, a few ash ejections.
    27 Aug          161 mm of rainfall; rumbling stopped; steep decline in continuous tremor
                      amplitude at 0900.
    28 Aug-05 Sep   No eruptive activity [but slight ashfall 5 September]; tremor remained weak.
    06 Sep          Large explosion at 1306, killing three persons; tremor amplitude increased
                      sharply at 1350 then decreased gradually over the next 14 hours.
    07 Sep-12 Sep   White steam emission; weak tremor.
    13 Sep-22 Sep   Quiet; no ashfalls observed.
    23 Sep          Ash eruption, lasting about 10 minutes.
    24 Sep          10-minute ash eruption in the morning; continuous ashfall 1450 until night;
                      accumulation less than 0.1 mm.
    25 Sep-26 Sep   No ashfalls observed.
    27 Sep          Continuous ashfall from 0540 to 1730, accumulating about 0.2 mm.
    28 Sep          No ashfalls observed.
    29 Sep          About 0.1 mm of continuous ashfall between 1510 and 1645.
Figure 3. Aso seismicity since 1973. Monthly number of isolated tremor events (top) plus 10-day means of continuous tremor anplitudes during the same period (bottom). Arrows show months in which Aso was erupting. Courtesy of JMA.

The local disaster control group for Aso volcano [closed the area within 1 km of the crater] at 1310 on 11 June because high-amplitude continuous tremor had begun to be recorded at JMA's Asosan Weather Station [originally referred to as Aso Observatory] during the early morning. Civil Defense personnel kept people 1 km from the crater, visited by many persons when the volcano is inactive. The eruption began during the evening of 12 June. No casualties have occurred.

"The second characteristic event of this eruption was the decrease in the amplitude of the continuous tremor just before the largest explosion, on 13 June. The extraordinary decrease in amplitude was observed for 11 and 1/2 hours, from 0336 to 1457 on the 13th. The explosion occurred at 1510, after a steep increase in tremor amplitude for 13 minutes. Many cases of a decrease in tremor amplitude before a larger explosion are known for past eruptions at Izu-Oshima and Aso. For example, a decrease lasting four days was recorded before Aso's large explosion of 31 October 1965."

Kyodo radio reported that [three] persons were killed and [11] injured by blocks ejected at about 1300 on 6 September. The area within 1 km of the active vent remained off limits.

Information Contacts: JMA; Kyodo radio.

09/1979 (SEAN 04:09) Large explosion kills three, then activity declines

"Eruptive activity continued through early September. Ash eruptions occurred almost every day from mid-July until 10 August (table 1) and ash fell on towns near the volcano; ash reached Takeda City, 30 km NE of Aso, in early August. Strong rumbling resumed on 10 August, but the volcano suddenly stopped emitting ash at about 1300 that day. The rumbling lasted until 26 August and was occasionally heard at the towns of Aso-machi and Ichino-miya-machi, 10 km from the crater. A steep decrease in the amplitude of the recorded continuous tremor took place at about 0900 on 27 August and the volcano was very quiet (no ash or block ejection, nor any rumbling) until 6 September.

"A loud explosion occurred at 1306 on 6 September. A dark ash cloud, in which lightning was seen, rose 700 m. The air shock reached 0.8 millibars and the ground shock had an amplitude of 17 µm at the JMA's Aso Observatory, 1.2 km from the crater. Three tourists were killed, two injured seriously, and nine slightly, by falling blocks 10-20 cm in diameter at a site 0.9 km from the vent (figure 4). Numerous blocks pierced the roof of a ropeway station (also 0.9 km from the vent), made of concrete as thick as 25 cm. A few of the lesser injuries occurred inside the station house. People said that the blocks that fell around the station house were hot and the cores of some of them were dimly glowing. Ash reached Oita city, 65 km NNE of Aso. The activity declined to white vapor emission 7 minutes after the explosion and no further eruption had occurred as of 12 September.

Figure 4. Summit area of Aso. Casualities from the 6 September 1979 explosion took place at the site marked X. Cinders larger than 3 cm fell in the area enclosed by a dashed line, a maximum of 1,200 m from the vent, Crater 1 of Naka-dake. Craters 1-7 are shown within the larger Naka-dake Crater, by hachured lines. Courtesy of JMA.

"The amplitude of recorded continuous tremor remained small (about 2 µm) through the explosion, became large (to 17 µm) 40 minutes after the explosion, then declined gradually to around 5 µm 14 hours after the explosion. It is not known whether the low-amplitude stage, which lasted from 27 August to just after the explosion, was an example of `amplitude decrease prior to explosion' (4:8). Seismicity remained relatively low both before and after the explosion.

"The restricted area [designated] on 11 June by the local disaster control committee for Aso volcano was still [closed] on 6 September. The committee will reexamine the size of the restricted area (within 1 km of the crater), although the casualties occurred in this area.

"The summit area of Naka-dake was surveyed by JMA personnel and by Koji Ono of the Japan Geological Survey 8-11 September. Cinders larger than 3 cm were scattered in the area enclosed by a dashed line in figure 4, reaching 1.2 km from Crater 1. A large block found 0.3 km from Crater 1 was 4.6 x 2.6 x 2.6 m and weighed about 50 metric tons. No scoria or other essential fragments were found. The explosion is considered to be a steam explosion, and may have been caused by heavy rainfalls on 27 August (161 mm) and 3-4 September (127 mm)."

Further Reference. Wada, T., Kikuchi, S., and Ono, H., 1980, The explosion of Naka-dake, volcano Aso on the 6th of September, 1979: Bulletin of the Volcanological Society of Japan, v. 25, p. 245-253.

Information Contact: JMA.

10/1979 (SEAN 04:10) Occasional weak ash emission

After the large 6 September eruption that killed three persons, Aso remained quiet through 22 September. Occasional weak ash emission took place 23-29 September (table 1). All of the eruptive activity occurred from the 6 September vent.

Information Contact: JMA.

11/1979 (SEAN 04:11) Frequent ash emission; explosion successfully predicted

Frequent ash ejections resumed on 24 September and continued through late November. During October, no blocks were seen to reach the rim of Naka-dake Crater nor were incandescent blocks observed at night. By the end of October, the concentration of ash at the JMA's [Asosan Weather Station] (1 km from the active vent) had reached more than 10 kg/m2, equal to about 1 cm of ash thickness. Although continuous tremor amplitude had correlated well with June-September eruptive activity, amplitudes remained low (about 0.5 µm) during October. The number of local earthquakes also remained low in October.

A characteristic decrease in the amplitude of continuous tremor began at about 0900 on 2 November, lasting until a large explosion at 1626. An eruption cloud rose 1.5 km above the crater during about an hour of ash ejection. Four mm of ash fell at the [Weather Station]. A survey by [Weather Station] personnel two days later found scoria up to 200 m from the vent, overlying 0.6 m of ash that had fallen in the summit area since the eruption began 12 June. The tremor amplitude decrease was the third since June that had preceded a sizeable explosion. An alert was issued from the Observatory one and a half hours before the explosion. No casualties occurred.

Ash emission in November was stronger than in October, causing heavy ashfalls near the volcano. Slight ashfalls occurred occasionally at Mt. Takachiho (110 km S), Kumamoto city (40 km W), and in Oita Prefecture (50 km E). Ejection of incandescent blocks was observed at night on 11 and 19 November, for the first time since 6 August. Tremor amplitude increased through most of November, but declined late in the month. The Strombolian activity of June, July, and early August occurred while tremor amplitude was high.

Further Reference. Tanaka, Y., Tsuchiya, Y., and Yamaura, Y., 1981, Detection of volcanic smoke and ashfall area at Aso from Landsat data: Papers in Meteorology & Geophysics, v. 32, no. 4, p 275-291.

Information Contact: JMA.

01/1980 (SEAN 05:01) Short explosive eruption; 1979 activity reviewed

Activity stopped on 28 November after strong ash emission through most of the month (table 2). Between June and November, ash caused about 1 billion yen ($4 million) in damage to crops and forests. No further ashfalls were observed until an explosion at 2107 on 26 January deposited 3 cm of ash and fist-sized scoria on the rim of Naka-dake, the source crater. A small amount of ash fell on Aso-machi town, at the base of the volcano. The explosion caused no damage, and the volcano returned to quiescence the next day.

Table 2. Monthly ashfall in thousands of tons, June-December 1979, as estimated by personnel from JMA's [Aso-san Weather Station].

    Month (1979)   Ashfall (tons)

    Jun             1,420,000
    Jul             1,620,000
    Aug             1,590,000
    Sep               300,000
    Oct               970,000
    Nov             3,270,000
    Dec                     0
    TOTAL           9,170,000

Information Contact: JMA.

04/1980 (SEAN 05:04) Small ash emission

A weak and brief emission of ash from Aso occurred on 8 March, producing ashfall on the S flank. Since strong ash emission stopped on 28 November 1979, eruptive activity had been confined to ejection of a small amount of ash and scoria, accompanied by a strong air and ground shock, on 26 January, and emission of white vapor at other times.

The amplitude of continuous tremor recorded at [Asosan Weather Station] declined in December 1979 and has remained low through March (figure 5). The number of local earthquakes increased somewhat around the January tephra ejection, but declined in February and did not increase substantially during the March activity.

Figure 5. Daily mean amplitude of continuous tremor (bottom) and occurrence of ash emission (top) observed from the JMA's Aso-san Weather Station, May 1979-March 1980. Larger ashfalls are represented by longer lines in the top section. The arrows in the bottom section indicate strong explosions on 13 June, 6 September (three persons killed), 2 November 1979, and 26 January 1980. Courtesy of JMA.

Information Contact: JMA.

05/1980 (SEAN 05:05) Activity declines

Activity at Aso has been confined to weak but steady emission of white vapor since the brief 8 March ash ejection. The number of seismic events per day and the amplitude of the continuous tremor recorded at [Asosan Weather Station] were both small in April and early May.

Information Contact: JMA.

09/1980 (SEAN 05:09) Small ash emission

A brief, weak explosion on 24 September ejected ash to about 800 m above Crater 1 of Naka-dake [after quiescence since the 8 March ash ejection]. The area within 1.5 km of the summit was closed immediately after the explosion but reopened 2 days later.

Information Contact: JMA.

06/1981 (SEAN 06:06) 30-minute ash and block ejection

Ash and block ejection from Crater 1 of Naka-dake, the northernmost of 7 in Naka-dake, was observed at 1230-1300 on 15 June, after 9 months of quiescence. Blocks rose to 30 m, but fell within the 100 m-diameter crater. 1 µm ground shocks were recorded at 1239 and 1244, and a 3.7 µm shock at 1251 [at the Weather Station]. Activity then subsided. The explosions caused no damage. The area within 1 km of the summit, closed immediately after activity began, was reopened 17 June. Asosan Weather Station personnel [visited the crater on 15 June and] observed that the greenish water pooled in Crater 1 since October had become gray tinted [but returned to its usual green the next day]. The [level of some points on the surface of the water] rose intermittently. Naka-dake is the historically active part of the Aso volcanic complex.

Information Contact: JMA.

10/1984 (SEAN 09:10) Block and ash ejection from fumarole

A warning of increased volcanic activity was issued by the [Asosan Weather Station] on 11 October. On the morning of 24 October, ash was ejected from a fumarole that had formed in mid-September on the lowest part of the E inner wall of [Crater 1 of] Naka-dake. A small gray plume rose to 300 m above the fumarole at 0920, but the ejection was too weak to send ash beyond the crater rim. Entry to the area within 1 km of the crater was prohibited shortly before 1000. Another plume rose to 200 m at 1030 the next day, accompanied by intermittent ejection of small blocks, as large as fist size. Activity had subsided by 1550.

The level of water in Crater 1 has gradually decreased since early April (figure 6). A part of the crater bottom could be seen in mid-September. As of 26 October, only 30% of the bottom was covered by the hot water.

Figure 6. Activity at Aso Volcano, 1965-84. Top to bottom: monthly number of isolated tremor episodes (open circles connected by dashed lines) and volcanic earthquakes (bars); monthly averaged amplitude of continuous tremor; Monthly maximum surface temperature of water in Naka-dake's Crater 1; eruptive phonomena--block ejection (A), ash ejection (B), volcanic flame (C), red-hot lava at the bottom of Crater 1 (D), and ash and mud ejection (E); quantity of water pooled in Crater 1--small amount (1), intermediate amount (2), full (3). Courtesy of JMA.

Information Contacts: JMA; A. Izumo, Yokohama Science Center; Japan Times; Kyodo News Service.

12/1984 (SEAN 09:12) Emission of ash containing fresh magma

Activity has gradually increased at Crater 1 of Naka-dake. After weak ash emission 2-3 January, an ash-laden plume has been occasionally observed since 11 January. A gray plume rose to 700 m above the crater rim on the 18th. . . . The ash included juvenile material.

Information Contact: JMA.

01/1985 (SEAN 10:01) Moderate ash emissions continue; volcanic flame on two nights

Ash was ejected a little more vigorously on 31 October, rising to the crater rim (about 100 m above the vent). On 2 November, ashfall was again confined to the crater. On 5-6 November weak rumbling was heard at the JMA's Aso Weather Station, 1.2 km SW of the crater.

Occasional weak ash emission that continued after 2 November was interrupted by water flowing into the active vent on 12 November. From then until 30 December muddy water and small rocks were ejected almost continuously, at varying intensity. During the strongest activity, on 9 December, this mixture fountained to an average height of 10 m above the crater floor, sometimes to more than 20 m.

Moderate ejections of ash to 150 m (as high as the crater rim) resumed 30 December and ended 2 January. On 30 December at 0815 an ash-laden plume rose 200 m above the rim. Ash fell on the S slope of the crater . . . . A prohibited zone within 1 km of Crater 1 was established 28 November.

On 11 January a grayish plume was observed rising 400-500 m above the crater rim, where ashfall was 1 cm thick. A little ash also fell on the S flank. Since the 11th an ash-laden plume was observed almost every day in January. Activity increased slightly on 18 January, when a gray plume rose 600 m above the rim. Volcanic flame [from a pit on the crater floor] was observed [during visits] the nights of 21 and 25 January.

Information Contacts: JMA; A. Izumo, Yokohama Science Center; Japan Times.

03/1985 (SEAN 10:03) Moderate ash emission; volcanic flame

Moderate ash-laden emissions from Naka-dake were occasionally observed in February and March. Gray plumes were seen on 8, 17-18, and 25-28 February. Ash accumulation on 26 February was 295 g/m at JMA's [Asosan Weather Station] (at the SW foot of Naka-dake [1.2 km from the crater]). On 1 March, the emission from the pit that had been active from January through February declined rapidly, although a new pit formed about 40 m N of the older one. Ash emission resumed the next day. Volcanic flame [from the pit on the crater floor was observed during visits] the nights of 6 and 25 March.

In late December, the press reported that volcanic tremors were continuously felt. However, JMA noted that average tremor amplitude was within the range of 0.1-0.3 µm through December and January. As of 1 April, tremor remained at about the same level.

Information Contact: JMA.

04/1985 (SEAN 10:04) Moderate tephra emission; new vent

Moderate ash-laden emissions from Crater 1 of Naka-dake were observed on almost every day in April (some daily ash accumulations are listed in table 3). Volcanic flame had been observed rising 20-40 m above [the pit in] the floor of Crater 1 on 25 March, and the remainder of the water pool in the crater disappeared on 16 April. Average tremor amplitude remained at around 0.3 µm in April.

Table 3. April-May 1985 daily ash accumulation at the JMA's Aso-san Weather Station, 1.2 km SW of Crater no. 1 of Naka-dake. [Ash was measured each day at 0900 and represents the amount deposited the previous 24 hours. There was no accumulation on other days in April and May.]

    Date (1985)   Ash (g/m2)

    02 Apr          141
    06 Apr          130
    07 Apr           14
    12 Apr          338
    13 Apr           37
    12 May            2.5
    14 May         1550
    16 May          757.9
    22 May          119.7

On 6 May, a small eruption was observed at a new pit about 10 m E of the one that had formed on 1 March. Rocks several tens of centimeters in diameter rose to a height of 50 m above the floor of Crater 1 and ash-laden emission was almost continuous from the 1 March pit. It was not certain if juvenile material was included in the 6 May tephra.

Information Contact: JMA.

06/1985 (SEAN 10:06) Ejection of ash and incandescent tephra

Since October 1984, activity has gradually increased at Naka-dake, site of all of the more than 140 eruptions known from the Aso complex in historic time. Activity at Crater 1 increased in May. Ash accumulated at JMA's Asosan Weather Station on 12, 14, [16], 22, 23, 29, and 30 May (table 3). An ash-laden plume was observed almost daily (figure 7). On the morning of 6 May, incandescent blocks were ejected at a new vent that had not been present the previous day. Block ejection stopped on 8 May but resumed on 15 May, when the vent increased to 40 m diameter by collapse of a wall between it and another vent that had formed in March. Incandescent blocks, including scoria a few tens of centimeters in diameter, were ejected to 50 m above the crater floor. Ash and mud jetted to the level of the top of the wall that had separated the two vents.

Figure 7. Table showing activity at Aso in May 1985. Y = observed; N = not observed; - = not observed because of bad weather. Courtesy of JMA.

Incandescent blocks were occasionally ejected at the vent after 16 May. The most powerful May eruption occurred on the morning of the 30th; larger amounts of incandescent blocks (mainly scoria) as much as 1-2 m in diameter were ejected 120-130 m above the crater floor at 1020. The intensity of ejection then gradually declined.

Information Contact: JMA.

07/1985 (SEAN 10:07) Ash emision ends

Activity at Aso has gradually increased since October 1984. Moderate ash emissions from Crater 1 of Naka-dake were observed almost daily from the beginning of May (10:6) until 20 June. White steam vapor dominated for the last 10 days of the month. Total June ashfall at JMA's [Asosan Weather Station] was 1429 g/m22.

Activity declined in July, when neither ash plumes nor deposits were observed at the [Weather Station]. On the morning of 1 July, [Weather Station] personnel found that the vent on the floor of the crater had been covered by water from heavy late-June rains. The level of water within the crater gradually rose in early July. About 70% of the crater floor has been covered by hot (60°C) water since mid-July. There were many fumaroles along the margin of the pool, and sand and water were ejected at many points on the surface. Seismic activity remained low throughout June and July.

Information Contact: JMA.

05/1988 (SEAN 13:05) Tremor stronger and more frequent; mud ejection

The number of tremor episodes recorded by a seismograph 0.8 km from [Crater 1] increased sharply to ~5,900 in May from the typical 100-500/month (figure 8). Activity declined slightly beginning 30 May to about 100 tremor episodes/day by 6 June. Background tremor was continuous and amplitudes in May were twice usual values (figure 10). The floor of Naka-dake's crater 1 had been covered by a pool of water that gradually dried up toward the end of May. Mud ejection about 10 m high from the crater bottom was observed during a field survey 26-27 May. Public access to the area around the crater was prohibited 25 May-6 June.

Figure 8. Monthly number of tremor episodes at Aso, January 1970-May 1988 (top), and monthly earthquakes, January 1965-April 1988 (bottom). Bars mark periods in which continuous tremor masked individual tremor episodes. Courtesy of JMA.

Information Contact: JMA.

10/1988 (SEAN 13:10) Glow on crater floor; crater closed to tourists

On the night of 16 October, JMA geologists noted irregular areas of red glow in a 30 x 10 m area on Naka-dake's crater floor, the first glow seen there since June 1985. The local government prohibited access within 1 km of the crater 17-27 October. Steam emission was observed from Crater No. 1 and its small crater lake shrank. The lake temperature was 71°C on 1 October. Seismicity and tilt showed no significant changes.

Information Contact: JMA.

12/1988 (SEAN 13:12) Minor ash emission

Ash ejection from [Crater 1] was observed on 28 December at 1420, during a field survey. The ash cloud rose 30 m above the crater rim. Red glow at vents and cracks in the crater floor had often been seen since 16 October. The number of isolated tremor episodes, counted from data recorded on a seismometer 0.8 km W of the crater, had gradually increased since the end of October, although the amplitude of continuous tremor episodes remained almost the same. No ash ejection was observed during a 31 December field survey.

Information Contact: JMA.

03/1989 (SEAN 14:03) Ash ejection; tremor increase; crater closed to tourists

About 3,000 volcanic tremor episodes were recorded during March, twice the number recorded in January and February (figure 9). A significant increase was recorded 22 March on a seismometer 0.8 km W of Crater 1. The amplitude of continuous tremor was generally unchanged.

Figure 9. Monthly number of isolated volcanic tremor episodes at Aso (top), earthquakes (bars, bottom), and maximum plume heights (curve, bottom), 1966-March 1989. Arrows mark periods of explosions. Courtesy of JMA.

Early 5 April, ash was ejected to ~50 m above a vent ~100 m below the crater rim. A field survey at 1140 the next day revealed that ash emission had stopped. A 1-km area (the smallest of three designated zones) around the crater was closed to tourists by the Aso Disaster Prevention Authority, at 0920 on 5 March. Such a restriction is necessary a few times a year when activity increases.

On 12 April, the restricted zone was reopened. Ash had not been ejected since 6 April although the frequency of tremor episodes remained high at 200/day. Glow at vents and cracks on the crater floor was regularly observed during night visits from October 1988 through April 1989.

Information Contact: JMA.

04/1989 (SEAN 14:04) Brief ash emission

On 27 April, the staff of AWS visited the crater rim as they have every day for the past 20 years. A vent on the SE floor of Crater 1 was releasing yellow vapor and ash to 30 m, accompanied by larger tephra. The Aso Volcano Disaster Prevention Authority closed a 1-km area near the crater to tourists. The area was reopened 2 May, when a field survey revealed only white vapor reaching ~5-6 m above the vent.

Glow on the crater floor has been observed every night since October 1988. A maximum temperature of 232°C was measured (with a infrared radiation thermometer) at a glowing site on 18 April.

Isolated tremor remained frequent in April. The daily number of tremor episodes was 100-250, with a monthly total of ~5,760 (figure 10). Amplitude of continuous tremor remained the same.

Figure 10. Monthly number of isolated volcanic tremor episodes at Aso (top), earthquakes (bars, bottom), and maximum plume heights (curve, bottom), 1966-April 1989. Arrows mark periods of explosions. Courtesy of JMA.

Information Contact: JMA.

06/1989 (SEAN 14:06) Ash ejections continue; new vent on crater floor

After a small ash ejection 5 April, tephra emission continued at a relatively high rate in May and June. On 8 May at 1000, a vent (1 m in diameter) on the Naka-dake crater floor ejected ash to ~10 m. At 1132, an M 3.3 shock (3 on the JMA Intensity Scale) occurred beneath the crater and was felt at AWS. Five (felt) aftershocks were recorded on 8 May (at 1120, 1147, 1216, 1417, and 2039), and 1 (not felt) was recorded the next day (at 0057) by a seismograph 0.8 km W of the crater. A 1-km area around the crater was closed to tourists by the Aso Disaster Authority. During a field survey at 1910, no ash ejection was observed.

On 16 May, ash rose ~100 m above the crater rim at 0810, and ~200 m at 1030. About 20% of the crater floor was covered by a rainwater pool, from which mud and water were continuously ejected to 3 m. During a field survey on 20 May at 1150, a strong rumbling noise was audible, but no ash ejection was seen.

Ash rose ~200 m above the crater rim on 22 May from 0740 to 0800, and 20 m above the crater floor at 0820. Activity declined, stopping by 1000. Two days later at 1000, ash was ejected to 200 m above the crater rim, and 5 g/m2 of ash was deposited at AWS. Ash had not fallen there since 28 June 1985. Red glow at the vent and in cracks on the crater floor was observed at night through May. During the night of 27 May, red glow emanated from 40-50% of the crater floor. On 28 May, ash rose about 50 m from the N portion of the vent.

In June, a vent on the NW floor of Crater 1 emitted an ash-laden steam plume a few hundred meters above the crater rim. During a 6 June field survey, the vent had enlarged and was emitting a 300-m ash plume. Flames from burning volcanic gases were occasionally observed rising 3-4 m above the crater floor during night visits. Ash accumulation at AWS was 9 g/m2 on the 7th, and 2 g/m2 on the 8th. The Crater 1 vent was buried by ash during rainfall 8-9 June. A new vent (named "891") about 18 m in diameter opened in the center of the crater floor on 10 June, and was the largest new vent since "853" formed 6 May 1985. The highest plumes of the month reached 1,000 m above the crater rim on 7 and 20 June.

Isolated volcanic tremor remained high (200-400 events/day) in May and June (figure 11) with a total of 5,760 events in May and 6,752 in June (compared to 5,821 in April). The amplitude of continuous tremor was generally unchanged in May but increased slightly in June.

Figure 11. Daily number of isolated tremor episodes at Aso, January-June 1989. Courtesy of JMA.

Information Contact: JMA.

07/1989 (SEAN 14:07) First strong eruption since 1985 ejects ash to 2,500 m

On 16 July, Aso erupted vigorously for the first time since May-June 1985, with three ash explosions from Crater 1 (at 1354, 1603, and 1625). The eruption resumed on 24 July, ejecting ash to 2,000 m above the crater rim.

Activity at Crater 1 has been gradually increasing since the end of 1988 (figure 12). Visits to the volcano October 1988-5 August 1989 revealed red glow at vents and cracks on the crater floor. After a small ash ejection on 5 April, tephra emission continued at a relatively high rate in May and June. A new vent (891) on the crater floor, first noticed during a field survey 11 June, emitted an ash-laden plume almost every day in July. Ash often fell near the vent, and daily accumulations of 7 g/m2 on 6 July, 11 g/m2 on 7 July, and 6 g/m2 on 8 July were measured at AWS.

Figure 12. Monthly number of isolated volcanic tremor episodes, average tremor amplitude, number of volcanic earthquakes, and maximum plume heights at Aso, January 1965-July 1989. Arrows represent eruptions. Courtesy of JMA.

On 14 July, the largest daily number of isolated volcanic tremor episodes (744) in 1989 was recorded by a seismometer 0.8 km W of Crater 1 (figure 13). The same day, at 1535, the amplitude of continuous tremor increased (figure 14), and white vapor and ash from Vent 891 was ejected to 1,200 m. A 1-km area around the crater was again closed to tourists by the Aso Disaster Prevention Authority. Two days later, at 1155, tremor amplitude began to increase, decreased sharply at 1303, but increased again at 1344, 10 minutes before the onset of ash emission. Ash reached about 2,500 m height at 1355. After a sharp decrease in tremor amplitude, two ash explosions followed at 1603 and 1625, ejecting a plume to ~1,000 m.

Figure 13. Daily number of isolated tremor episodes at Aso, January 1988-July 1989. Arrows represent eruptions. Courtesy of JMA. Courtesy of JMA.
Figure 14. Daily maximum tremor amplitude (top) and average amplitude of continuous tremor (bottom) at Aso, January 1988-July 1989. Arrows represent eruptions. Courtesy of JMA.

On 18-19 July, small amounts of ash were repeatedly ejected from Vent 891 after tremor amplitude sharply decreased. During a field survey at 1920 on 22 July, a maximum brightness temperature of 506°C was measured (by an infrared radiation thermometer) at a vent on the crater floor. The amplitude of continuous tremor began to increase 21 July, and significantly increased on the night of 23 July. Ash emission resumed at 1620 on 24 July and continued for about 1 hour, attaining a maximum height of 2,000 m above the crater rim. A small ash ejection was observed during a field survey on 29 July. Brightness temperatures near the vent on the crater floor were 504°C on the 29th and 498°C on the 30th. The average amplitude of continuous tremor remained high.

Information Contact: JMA.

08/1989 (SEAN 14:08) Stronger ash emission

An ash ejection on 14 August at 1050 was the fifth since a series of explosions began on 16 July (14:07). Ash emission continued for about 90 minutes, sending a blackish-gray plume to ~1,000 m above the crater rim. Amplitude of continuous tremor during the eruption was about 0.6-0.7 µm (on a seismograph 0.8 km W of the crater), compared to an average amplitude of 0.3-0.4 µm. Intermittent ash ejection continued almost daily through August, with ashfall often observed around Crater 1. During the month, a total of 787 g/m2 of ash accumulated at AWS about 1 km SW of the crater. Ash emitted 20-24 August reached villages ~40 km NW of the crater, the most distant ash deposition since 1979. On 4 September, the amplitude of continuous tremor decreased for a few minutes, followed by ash ejections at 0905 (1,000-m plume), 1300 (700 m), 1540 (2500 m) and 1725 (2,500 m). A 1-km area around the crater has been closed to tourists by the Aso Disaster Authority since 9 August.

Burning gas from the crater floor's 891 vent was observed almost daily during August. The flame was 30 m high on the night of the 23rd, the highest since burning gases were first seen in June, and a 15-m flame was seen the next day.

Information Contact: JMA.

09/1989 (SEAN 14:09) Continued tephra emission

By 5 October, 14 ash eruptions had been recorded in the sequence that began 16 July (table 4). During clear weather in September, Vent 891 was observed emitting an ash-laden plume almost daily. On the 27th from 0900 to 1330, continuous ash emission produced a 3,000-m plume (the highest of the month) that deposited ash ~55 km E. September ash accumulation at AWS is shown in table 5.

Table 4. Eruption plumes at Aso, July-October 1989. The Japan Meteorological Agency uses the term "eruption" to distinguish voluminous ash ejections from low-level continuous ash emission. Colors are blackish gray (BG) or grayish white (GW). Courtesy of JMA.

    Date (1989)      Time       Activity

    16 Jul         1354         Ash plume to 2,500 m height.
                   1603         Ash plume to 1,000 m height.
                   1625         Ash plume to 1,000 m height.
    24 Jul         1620-1720    Ash plume to 2,000 m height.
    14 Aug         1050         Ash plume (BG) to 1,000 m height.
    04 Sep         1540         Ash plume to (GW) 2,500 m height.
                   1725         Ash plume to (GW) 2,500 m height.
    07 Sep         1650-1840    Ash plume to 3,000 m height.
    16 Sep         1400-1450    Ash plume to 2,200 m height.
    27 Sep         0900-1330    Ash plume to 3,000 m height.
    29 Sep         1330-1610    Ash plume to (GW) 1,500 m height.
                   1650-1930    Ash plume to (GW) 600 m height.
    30 Sep         0925-1830    Ash plume to (GW) 1,500 m height.
    01 Oct         1245-1630    Ash plume to (GW) 1,000 m height.
    02 Oct         1205-1650    Ash plume to (GW) 1,000 m height.
    09 Oct         1545         Block ejection to 150 m.
    10 Oct         1800         Block ejection to 150 m.
    12 Oct         1855         Block ejection to 180 m.
    13 Oct         1050-1220    Ash plume to 1,000 m height.
    18 Oct         0840         Ash plume to 1,500 m height.
    20 Oct         1635         Ash plume to 1,500 m height.
    21 Oct         1015         Ash plume to 1,000 m height.
    22 Oct         1100         Block ejection to 50-60 m above crater rim.
    23 Oct         1120         Block ejection to 300 m above crater rim.
    24 Oct         0715         Block ejection to 300 m above crater rim.
    25 Oct         1210         Block ejection to 300 m above crater rim.
    26 Oct         0715         Block ejection to 200 m above crater rim.
    27 Oct         0630         --
    28 Oct         0630         Ash plume to 1,000 m height; blocks to 100 m above crater rim.
    29 Oct         0630         Ash plume to 1,400 m height.
    03 Nov         1150         Block ejection to 100 m; 800-m ash plume.
                   1820         Block ejection to 150 m.
    05 Nov         1710         Ash plume to 600 m height.
    17 Nov         1135         Block ejection to 200 m above the rim.
    20 Nov         1030         Block ejection to 100 m above the rim; 1,400-m ash plume.
    21 Nov         1110         Block ejection to 80 m above the rim; 1,500-m ash plume.
    22 Nov         0840         Ash plume to 1,000 m height.
                   1230         Block ejection to 120 m above the rim; 1,400-m ash plume.
    23 Nov         2040         Block ejection to 300 m above the rim.
    24 Nov         1210         Block ejection to 700 m SSW of Crater 1.
    25 Nov         1355         Block ejection to 100 m above the rim.
    26 Nov         0107         --
                   0910         Block ejection to 200 m above the rim; 1,500-m ash plume.
    28 Nov         2217         Block ejection to 100 m above the rim.
    10 Dec         1525         Ash plume to 1,000 m height.
    17 Dec         1630-1710    Block ejection to 100 m; 300-m ash plume.
    28 Dec         1000-1640    Ash plume to 300 m height.

Table 5. Daily ashfall from Aso collected 1 km WSW of the volcano at Aso Weather Station, September 1989. Courtesy of JMA.

    Date (1989)   Ashfall (g/m2)

    05 Sep              73
    06 Sep           1,188
    08 Sep              18
    09 Sep               9
    11 Sep              11
    16 Sep              57
    17 Sep              48
    20 Sep              11
    24 Sep             306
    30 Sep           1,000

Red glow from vent 891 was seen almost daily from the rim of the crater. A maximum September vent temperature of 535°C was measured by an infrared radiation thermometer on the 6th. During a field survey 26-27 September, burning gas from the vent reached 50 m height.

The number of isolated volcanic tremor episodes (recorded by a seismometer 0.8 km W of Crater 1) increased in September, with 720 events on the 18th and 702 on the 27th (figure 15, top). On the 26th, the month's largest tremor amplitude, 7.7 µm, was recorded (figure 15, bottom).

Figure 15. Daily number of isolated tremor episodes (top) and average amplitudes of tremor episodes (bottom) at Aso, 1 March 1988-2 October 1989. Courtesy of JMA.

Information Contact: JMA.

10/1989 (SEAN 14:10) More explosions; stronger tremor; new vent

Recorded explosions became more frequent in mid-October, occurring almost every day during the second half of the month (see table 4), for a total of 29 since 16 July. Minor ash emission occurred on most days in October, causing ashfalls around the crater. Ash reached AWS during favorable wind conditions, and by 21 October had accumulated to 3 cm depth (11,409 g/m2). On 27 October, 8437 g/m2 of ash was deposited at AWS, and slight ashfall was observed at Kumamoto Local Meteorological Observatory, ~35 km SW of Aso. The heavy ashfalls damaged agricultural products. A zone within 1 km of the crater remained closed to tourists by the Aso Disaster Authority.

Block ejection, to 10 m height, from Vent 891 was first seen during a 6 October field survey. During 9 October fieldwork, a new vent (named 892) about 1-2 m in diameter was observed about 10 m N of Vent 891, intermittently ejecting fist-sized blocks to 20 m height. Ash emission continued from 891 vent, but no blocks were ejected. The next day, Vent 892 had grown to ~5 m in diameter. By 21 October, the new vent had enlarged further to 25 m across, coalescing with 891 the following day.

Flames from burning gases rising several tens of meters from the vent were often seen at night. Strong rumbling was sometimes audible at AWS, and rumbling was heard 10 km to the ESE (at Takamori) during the 28 October eruptive episode.

The number of isolated tremor episodes increased toward the end of October (figure 16). The amplitude of continuous tremor, recorded by a seismograph 0.8 km W of Crater 1, grew particularly large from mid-October, reaching a maximum of 12.8 µm on the 21st.

Figure 16. Daily maximum and average amplitudes of volcanic tremor episodes, 1 February-10 November 1989. Courtesy of JMA.

Information Contact: JMA.

11/1989 (SEAN 14:11) Frequent tephra ejection continues

Eruptive episodes have been recorded on 36 days since 16 July, including 11 days in November (see table 4). Minor ash emission, without recorded explosions, occurred on most days in November, causing ashfalls around the crater. The month's ash accumulation at AWS was 1,409 g/m2. The 23 November eruptive episode, accompanied by lightning, ejected blocks to 300 m above the crater rim; blocks had begun to be thrown over the rim as recorded explosions became more frequent in mid-October. During a 24 November field survey, fist-sized blocks were seen 700 m SSW of the crater. Fieldwork on 26 November revealed that the cone on the crater floor had disappeared and the wall between craters 1 and 2 had been removed. Felt shocks of intensity I (JMA Scale) occurred on 19 and 26 November, centered under the summit crater. The number of isolated volcanic tremor episodes and the amplitude of continuous tremor, recorded by a seismograph near AWS, remained large. Rumbling was audible every day at AAWS and was strong on 4 and 25 November.

Information Contact: JMA.

12/1989 (SEAN 14:12) Ash emission and seismicity decline

Activity was less vigorous in December than in November, with only three recorded explosions (see table 4), bringing the year's total to 39. Minor ash emission was observed almost daily. A total of 755 g/m2 of ash was deposited at AWS. The number of isolated tremor episodes recorded by a seismometer at AWS decreased in late December (figure 17). The amplitude of continuous tremor declined to a level similar to that of early September (figures 17 and 18). Rumbling was slightly audible at AWS during the month.

Figure 17. Daily number of isolated volcanic tremor episodes (top) and daily maximum and mean amplitudes of continuous tremor (bottom) at Aso, April-December 1989. Courtesy of JMA.
Figure 18. Monthly mean amplitude of continuous tremor at Aso, 1966-1989. Courtesy of JMA.

Information Contact: JMA.

02/1990 (BGVN 15:02) Block and ash ejections increase in late January; daily ash emission in February

Activity was relatively quiet in the first half of January, but increased in the second half of the month. A 21 January explosion ejected blocks to 300 m above the crater rim. Additional explosions occurred at 1645 on 1 February and 1320 on 7 February, the latter continuously ejecting blocks to 300 m above the crater rim. Minor ash emission was observed almost daily, causing ashfalls around the crater. A total of 30 g/m2 of ash was deposited in January and 3,167 g/m2 in February at AWS. Volcanic tremor amplitude increased from the end of January, but declined toward the end of February.

A pool of water was present in Vent 892 during fieldwork on 15 February. Mud ejection was observed for the first time since September 1989. Vent 892 began to develop in October, and has gradually enlarged to occupy half of the crater floor.

Information Contact: JMA.

05/1990 (BGVN 15:05) Strong block and ash ejection after 2 months of quiet, then continuous ash emissions

A large eruption occurred on 20 April after 2 months of quiet. On 13 February, activity decreased to emission of white vapor and weak mud ejection from a water pool in [Vent 892]. Because of the decreased eruptive activity, the area within 1 km of the crater (closed since 9 August 1989) was reopened by the mayor of Aso town on 23 March.

At 1323 on 20 April, tremor amplitude suddenly increased by about five times. An "Extra Volcanic Information" was issued by the Aso town office and the tourist viewing area was closed by the mayor (both at 1415). Tremor amplitude abruptly decreased at 1644 and remained low until the eruption began, similar to several previous episodes. At the onset of the eruption at 1708, tremor amplitude again suddenly increased. Ash and block ejection was accompanied by a loud rumbling noise which was strongest at around 1730. Blocks 0.5 m across were ejected to 300 m above the crater rim throughout the eruption. The ash cloud rose more than 1 km, and three electrical discharges (lightning) were observed between 1808 and 1815. Tremor amplitude decreased at around 1815 and the eruption is believed to have ended shortly thereafter, although the exact timing is unknown.

Ash from the eruption, wet due to rain, fell mainly N of the volcano where it affected traffic and caused agricultural damage. The towns of Aso and Ichinomiya lost electricity at around 1800, presumably due to a short circuit or leak caused by the wet ash. Service was restored to most homes by midnight, although some had no power until noon the next day.

The eruption deposited 1 m of ash on the crater rim, 0.5 m of ash 1 km N of the crater, and 4 kg/m2 at 6 km N of the crater. Fine ashfall was observed to 35 km W (at Kumamoto) and 35 km E (at Takeda). Blocks, mostly dense and angular, and fresh scoria fell N of the volcano. Lithic clasts 20 cm in diameter were scattered to 1 km N. The total tephra mass from the 20 April activity was estimated to be 1 x 109 kg (the total accumulated tephra mass from July 1989 through 20 April 1990 was 6 x 109 kg; table 6).

Table 6. Number of days/month on which ash was emitted at Aso and monthly ash accumulation measured by JMA at AWS, 1 km W of the crater, July 1989-May 1990. The April 1990 value is low, despite the large emission on 20 April, because of wind direction.

    Date        Eruption days    Ash accumulation (g/m2)

    Jul 1989          2                    36
    Aug 1989          1                   579
    Sep 1989          5                 2,740
    Oct 1989         17                29,177
    Nov 1989         11                 1,509
    Dec 1989          3                   755
    Jan 1990          1                    30
    Feb 1990          2                 3,168
    Mar 1990          0                     0
    Apr 1990          2                    95
    May 1990          9                12,837

Ash emission was almost continuous from the morning of 21 April through early June. Another vigorous eruptive episode occurred 27 April and such episodes became frequent at the end of May. Tremor remained at low amplitude for a few days following the 20 April eruption then gradually increased to high amplitudes in May.

Information Contact: JMA.

06/1990 (BGVN 15:06) Ash and block ejection; gradual increase then abrupt decrease in tremor amplitude

Intermittent eruptive activity has continued since 16 July 1989 from Crater 1. Eruptive episodes on 3, 5, 6, 7, 12, 13, and 18 June ejected ash clouds and blocks and were similar to those of previous months. Ash ejection was most active on 13 June, as tephra rose continuously to ~1,000 m throughout the day and the month's highest ash cloud rose 3,000 m above the crater. The 18 June activity was also strong, ejecting blocks 300 m W from the center of the crater. Ash accumulation at AWS . . . was 9,713 g/m2 in June, slightly lower than the May total of 12,837 g/m2 that was the heaviest monthly ashfall since October 1989.

Tremor amplitude gradually increased through the end of June after the abrupt drop that followed a larger eruption on 20 April. Short-period and large tremors recorded between 22 June and 1 July were thought to have been generated by small phreatic explosions. The bottom of the crater was occupied by a water pool after heavy rains 14-16 June and 26 June-4 July. Tremor amplitude was very small 1-9 July. AWS issued an "Extra Volcanic Information" on 4 July, warning of the potential for a sudden explosion following the abrupt decrease in tremor amplitude.

Information Contact: JMA.

07/1990 (BGVN 15:07) Sporadic eruptive activity stops; variable tremor amplitude

. . . Ash clouds and blocks were ejected [from Crater 1] on 7 days in June 1990, most recently on the 18th. However, no eruptions occurred in July. Tremor amplitude increased from 22 June to 2 July, decreased, then gradually increased for the remainder of the month. The bottom of Crater 1 was covered with a pool of water.

Information Contacts: JMA.

08/1990 (BGVN 15:08) Mud, water, and steam ejected from lake in active vent

Vent 892 on the NE floor of Crater 1 had been covered by a pool of water since the last noted ash ejection in the crater on 30 June. Frequent mud and water ejections, and white steam emissions occurred during July and August. A plume containing small amounts of ash, intermittently ejected to 100 m from a vent in the SW part of Crater 1, was noted along with strong rumbling during a visit 30 August. The number of tremor episodes gradually decreased toward the end of August and tremor amplitude was at low levels.

Information Contact: JMA.

09/1990 (BGVN 15:09) Scoria ejected from new vent

White steam emission from Crater 1 continued, punctuated by weak ash emissions on 1, 14, 16, 17, and 27 September. Plume heights ranged from a few hundred to 1,000 m above the crater. Vent 892, site of activity until June, was buried by mud and heavy rainfall in July. A visit to the crater on 17 September revealed the existence of a new vent (901), not visible the previous day, which ejected blocks and flame to 10 m. Scoria blocks were ejected to 30 m above the vent on 28 September, the first magmatic ejecta since 18 June (15:06). The amplitude and number of tremor episodes increased for several days around 20 September.

Information Contact: JMA.

10/1990 (BGVN 15:10) Weak ash emission and glow; increased tremor

No ash was erupted during October . . . . Crater 1 . . . continued to emit white steam that rose to 900 m above the crater. Weak ash emission was observed on 13 November, and glow from vents on the crater bottom was seen during fieldwork that night. The amplitude and number of volcanic tremor episodes increased in late October, reaching levels similar to September's and continuing at those levels through early November.

Information Contact: JMA.

11/1990 (BGVN 15:11) Tephra ejection resumes

Crater 1, active July 1989-June 1990 (table 7), weakly emitted ash on 12, 18-19, and 25-29 November; white steam was emitted steadily on other days. The highest plume observed in November reached 1,000 m above the crater. Ash had last been emitted on 17 September. The area within 1 km of the crater, which had reopened to tourists on 15 October, was closed on 12 November and remained closed in early December.

Table 7. Brief chronology of activity at Aso, January-14 December, 1990.

    Date (1990)    Activity

    Jan            Eruption continuing (since July 1989).
    07 Feb         Eruption ceased.
    Mar            Quiet, but white steaming continued.
    20 Apr         Eruption resumed.
    May-Jun        Frequent strong eruptions.
    Jul-Aug        Quiet, with occasional weak ash emissions.
    Sep            Occasional ash emissions; vent 901 opened.
    Oct            Quiet; white steam; tremor increased toward month's end.
    Nov            Ash emissions and glow resumed at mid-month. Stronger tremor continued
                     through the month.
    13 Nov         Glow resumed on crater floor.
    17 Nov         Scoria ejection resumed.
    24 Nov         Vent 902 had opened; 811°C temperature measured.
    04 Dec         Eruption at 0410 ejected 1,200-m ash cloud.
    06 Dec         Eruption; vent 903 had opened.
    07 Dec         Eruption.
    08 Dec         Eruption.
    13 Dec         Eruption.

Glow from many points on the crater floor was observed during a night visit on 13 November, the first crater glow seen since June. Glow remained visible through early December. During 17 November fieldwork, incandescent scoria was being ejected to 30 m height from a small vent on the crater floor. Scoria ejection had last been observed in June. By the 24 November crater visit, a vent 10 m across had developed on the crater floor and was ejecting blocks to 5 m height. The vent was named 902, the second new vent of 1990 . . . . An infrared thermometer detected a maximum temperature of 811°C in the vent.

Ash emission became frequent in early December. An eruption on 4 December at 1410 ejected a 1200-m ash cloud, December's highest (as of the 14th), and similar activity occurred on 6, 7, 8, and 13 December. Vigorous ash emissions had last occurred in June. Ejections of blocks and scoria were also more frequent and higher (to 150 m) in early December. A visit on 6 December revealed that a new vent . . . had opened near 902.

The amplitude and number of volcanic tremor episodes has gradually increased since October and remained high through November (figure 19).

Figure 19. Daily number of tremor events at Aso, January-8 December 1990. Longer arrows at top of figure mark eruptions, shorter arrows indicate weaker ash emissions. Courtesy of JMA.

Information Contact: JMA.

12/1990 (BGVN 15:12) Periodic tephra emissions from new vent

Steam and ash were emitted periodically throughout December, to a maximum height of 1,200 m, on 4 December. A crater visit on 6 December revealed a new vent (903), 30 m long and 10 m wide. The center of eruptive activity had moved to Vent 903 from Vent 902 . . . . The amplitude and number of tremor episodes had gradually increased since October, and remained high through early January 1991.

Information Contact: JMA.

01/1991 (BGVN 16:01) Frequent ash emission, but tremor declines

Crater 1 emitted ash on 8-9, 18, 20-21, 23, and 28-29 January. The highest cloud observed in January reached 700 m above the crater on the 29th. Continuous tremor amplitude, which had been high since October, declined toward the end of January.

Information Contact: JMA.

02/1991 (BGVN 16:02) Tremor amplitude and steam emission decline

Crater 1 weakly emitted ash on 5, 8, and 9 February, and steadily emitted steam to 100-200 m heights on other days, a decline from previous months. The highest steam emission of the month reached 300 m above the crater on the 7th. Continuous tremor amplitude, which had been high since October, has declined since the end of January. Activity continued at similar levels through early March.

Information Contact: JMA.

05/1992 (BGVN 17:05) Mud/water ejections from heating crater lake; tremor episodes

Isolated volcanic tremor episodes began to increase in October 1991, reaching about 100 events/day by the end of May. The increase in seismic activity followed a period of quiet after the July 1989-December 1990 eruptive phase. Ejections of mud and water, the first since June 1991, were observed within the active crater lake . . . on 23 April. Similar ejections, to 5 m height, were observed on 27 April, 1 and 27 May, and 2 June. The lake's surface temperature has been increasing since March-May 1991 when it was 20-30°C, reaching ~70°C (measured by infrared thermometer) in May. Weak mud ejections have been common in the past, during the period between eruptive phases when the crater is normally occupied by a lake, but have not been observed during the lowest levels of activity.

Information Contact: JMA.

06/1992 (BGVN 17:06) Explosions follow increased seismicity and heating of crater lake

Eruptions that occurred from Crater 1 during the night of 30 June-1 July were the first [strong explosions] since . . . December 1990. The daily number of isolated volcanic tremor episodes began to increase in October 1991, and had reached ~100/day by the end of May. Isolated tremor episodes rapidly became more frequent in late June, and the amplitude of continuous tremor also increased through the month.

Ejections of mud and water from the lake in Crater 1 were first noted on 23 April and were sporadically observed later in April and in May. The ejections became more vigorous in late June, increasing in height from 5 m on 24 June to 20 m on the 26th, 50 m on the 29th, and 150 m on the 30th. Surface temperatures of the lake water increased from around 20°C in May 1991 to 78°C in June 1992. Steam plumes also grew to 1,000 m height in late June.

Strong tremor episodes were recorded during the night of 30 June-1 July. During fieldwork at noon on 1 July, the crater was quiet, but many blocks to 0.8 m across had been scattered to 100 m from the crater's NE rim. The eruptions were not seen or heard, but seismic and air-vibration records suggested that they may have occurred at 2349 on 30 June and 0316 on 1 July.

Tremor decreased in early July, but remained at higher levels than in mid-June. Ejections of mud and water to heights of a few tens of meters occurred sporadically through early July, but no additional strong mud/water ejections or eruptions were reported.

Because of the increasing activity, the area within 1 km of the crater was closed to tourists on 24 June, and remained closed as of mid-July.

Information Contact: JMA.

07/1992 (BGVN 17:07) Phreatic activity and seismicity decline after block ejection

Blocks were ejected during the night of 30 June-1 July from Crater 1 for the first time since . . . December 1990. Vigorous steam emission followed for about 10 days, fed a plume to a maximum of 2 km height on 6 and 8 July, then gradually declined toward the end of the month (figure 20). Ejections of water, mud, and blocks that rose ~50 m above the surface of the crater lake were observed almost every day during July. The lake shrank rapidly in early July until it occupied only about 1/3 of the crater floor. The temperature of the lake surface (measured by infrared thermometer) reached 95°C on 4 July (figure 20), the highest since March 1991, but declined to around 60° by the end of the month. Isolated tremor episodes, which had peaked at ~2,000/day at the end of June, declined rapidly after the block ejection to 0-6/day (figure 20). The amplitude of post-eruption continuous tremor also declined (figure 21).

Figure 20. Daily number of tremor episodes (top), steam cloud heights (middle), and highest monthly surface temperatures of the crater lake (bottom) at Aso, January 1991-July 1992. A long arrow marks the 30 June-1 July eruption. Smaller arrows show weaker ash emissions. Courtesy of JMA.
Figure 21. Daily mean amplitude of continuous tremor at Aso, late 1988-July 1992. Long arrows mark strong explosions, short arrows indicate weak ash emissions. Courtesy of JMA.

Similar activity continued through mid-August, with weak mud ejections from the lake, steady steam emissions to 1,000 m height, and low-level seismicity. The lake expanded again to cover all of the crater floor by 5 August because of inflow of groundwater, precipitation, and weaker ejection activity.

The area within 1 km of the crater . . . was reopened on 10 August.

Information Contact: JMA.

08/1992 (BGVN 17:08) Explosion ejects blocks

Heavy rains expanded the crater lake in early August but it shrank to cover only 20% of the crater floor later in the month as a result of daily ejections of mud, blocks, and water. Steam emissions were continuous, rising to 1 km on 10 and 24 August.

A strong eruption of water, blocks, and mud from Crater 1 at 1223 on 8 September ejected blocks to 200 m above the crater rim and a 2-km steam plume. Blocks up to 70 cm across were scattered to 300 m W of the crater rim. The eruption was similar in size to the previous eruption on 30 June-1 July, but eruption tremor of 30 µm amplitude was detected by a seismometer 800 m W of the crater, compared to the 9-µm amplitude recorded by the same instrument on 1 July. No changes in tremor activity were recorded before the eruption, although there was an increase in the amplitude of continuous tremor for 2 days after the eruption (figure 22). There were only 55 isolated tremor episodes in August compared to 1520 in July, with another 35 events recorded in September through the 15th.

Figure 22. Daily number of isolated volcanic tremor episodes (top), daily mean amplitude of continuous tremor (middle), and plume heights (bottom) at Aso, 1 June 1991-15 September 1992. Arrows mark explosive episodes. Courtesy of JMA.

Information Contact: JMA.

09/1992 (BGVN 17:09) Blocks ejected by explosive episode

An eruption from Crater 1 ejected blocks at 1223 on 8 September, the first such activity since a similar episode on 30 June-1 July. Another eruption at 1627 on 29 September scattered blocks 800 m SE and ejected a steam plume 2,000 m high. The number of blocks and the distance they fell from the crater were greater than for the eruptions of 8 September and 1 July. Eruption-tremor amplitude was 30.2 and 30.4 µm, respectively, for the September eruptive pisodes.

Steam was steadily emitted to a few hundred meters throughout September, and volcanic-tremor frequency was low. No anomalies in steam emission or tremor frequency were noted either before or after the eruptions. However, continuous-tremor amplitude increased for two days after the 8 September eruption. Weak ejections of mud, blocks, and water continued.

An area within 1 km of the crater has been closed to tourists since 24 August, and no damage was caused by the eruptions. Similar activity has continued through 14 October, but there have been no additional eruptions.

Information Contact: JMA.

10/1992 (BGVN 17:10) Explosion produces 2500-m plume and wet ashfall; weak ejections from crater lake

An eruption from Crater 1 occurred at 1340 on 26 October, the fourth this year and the first such activity since 29 September. A steam plume containing ash rose 2,500 m, and wet ash fell 1.5 km S of the crater. No blocks were jected. Eruption-tremor amplitude was 20 µm at the nearest seismometer, 0.8 km W of the crater (table 8).

Table 8. Eruptions at Aso during 1992. Courtesy of JMA.

    Date       Tremor      Plume        Range of blocks
              Amplitude    Height    (from center of crater)

    01 Jul      9 µm       unknown          300 m N
    08 Sep     30 µm       2 km             600 m E
    29 Sep     30 µm       2 km             700 m S
    26 Oct     20 µm       2.5 km            none

Steam was steadily emitted to a few hundred meters throughout October, and volcanic-tremor frequency was low. No changes in steam emission, tremor frequency, or earthquakes were noted before or after the eruption. Weak ejections of mud, blocks, and water to 15 m height continued in the crater lake, which ccupies half of the crater floor. Similar activity has continued through 14 November, without additional eruptions. The area within 1 km of the crater has been closed to tourists since 24 August. No damage was caused by the eruption.

Information Contact: JMA.

11/1992 (BGVN 17:11) Renewed block ejection; gas plume

No eruptions have been observed since a brief episode on 26 October from Crater 1. Ejections of mud, blocks, and water to 30 m height continued n the crater lake through November. Steam was steadily emitted to 500 m, reaching 1 km on 27 November (figure 23). Volcanic tremor and earthquake activity ere low. The area within 1 km of the crater . . . was re-opened on 12 November.

Figure 23. Steam-plume heights from Aso, May 1991-13 December 1992. Arrows mark explosive episodes. Courtesy of JMA.

Surface activity increased in December. Continuous low rumblings were heard beginning on 1 December, and on the 3rd, blocks were ejected to 200 m from the crater floor. An area of 1-km radius was again closed at 1400 on 3 December. Observations the following day revealed that a new vent (named 921) about 5 m across had developed in the central part of the crater floor, producing flames 10 m high and ejecting incandescent blocks to 5 m height. The ejections continued the next day, but activity was unconfirmed after 6 December. The continuous steam plume included minor ash 4-7 December but was white again on the 8th. The highest steam plume rose 1 km on 5 December but the plume was only a few hundred meters high after the 6th. Seismicity was relatively low, unchanged from November.

Information Contact: JMA.

12/1992 (BGVN 17:12) Mud/water ejection from crater lake; steam plumes; new vent

No eruptions have been observed since . . . 26 October . . . . Ejections of mud and water to 20 m height continued in the crater lake through December. Steam was steadily emitted to 500 m, reaching 1,000 m on 5 December. The steam plume contained minor ash 4-7 and 26-27 December. [Vent 921] emitted ash until 7 December, then became inactive and was no longer visible a few days later. Observations on 26 December, following an increase in rumbling the previous day, revealed that a new vent (named 922) about 30 m across had developed near the site of Vent 921, and was emitting ash and steam. Seismicity was relatively low, with tremor amplitude gradually increasing after the middle of the month. The area within 1 km of the crater, closed to the public on 3 December, was reopened on the 30th.

Information Contact: JMA.

01/1993 (BGVN 18:01) Block ejection and steam emission; seismicity remains high

Field reports confirmed that by 1 January the lake in Crater 1 had dried up. Steam was steadily emitted to ~500 m, with the plume containing ash 13-14 and 17-29 January. A small eruption occurred in the crater on 21 and 22 January, ejecting many scoria blocks to 10-50 m heights from Vent 922. This was the first eruption since 26 October and the first scoria eruption since June 1992. . . . Ejecta fell within the crater, which is 400 m across and 150 m deep. The steam plume, containing ash, rose 1,000 m on the 21st and 1,500 m the 22nd. Seismicity has been relatively high since mid-December, but no significant change was detected before or after the eruption.

Activity continued at the same levels through early February, with steam emission to a few hundred meters, occasionally containing ash.

Information Contact: JMA.

02/1993 (BGVN 18:02) Scoria eruption; seismicity declines

The eruptive phase . . . continued through February at Crater 1. At noon on 20 February, vent 922 ejected scoriae to 100 m above the vent, the first eruption observed during daily visits to the crater rim since 22 January. The ejecta, consisting of blocks up to 50 cm across, fell within the 400-m-wide, 150-m-deep crater. Incandescent fountaining, also to a height of 100 m, was observed that night. Inclement weather prevented visual observations in the following days and the ending date for the eruption is not known.

The amplitude of the continuous tremor, which increased in late December, persisted at an elevated level until 10 February, when it rapidly declined (figure 24). The decline was not accompanied by any observed change in surface phenomena. At 0600 on 20 February, the tremor amplitude increased. Tremor amplitude remained high during and after the eruption, abruptly returning to background level at 1215 on 25 February after a small block ejection. An area within 1 km of the crater has been closed since 18 January.

Figure 24. Daily mean amplitude of volcanic tremor (top) and height of steam plume (bottom) at Aso from 1 January 1992 to 8 March 1993. Arrows at the top indicate eruptions. Courtesy of JMA.

Ash was observed in the steam plume on 1-5, 12-13, 18-20, and 23-25 February. The plume rose to heights of 500 m, increasing briefly to 1,000 m n 19-20 February (figure 25).

Information Contact: JMA.

03/1993 (BGVN 18:03) Activity decreases; crater lake forms

Activity in March and early April was lower than in previous months. Rain created a small lake in part of Crater 1. On roughly half of the March visits, mud and blocks were seen being ejected a few meters above the lake. A white steam plume continually rose 200-500 m; it contained a minor amount of ash on 7 March. Seismic activity was low.

Information Contact: JMA.

05/1994 (BGVN 19:05) Mud ejected; tremor amplitude increases

Activity at [Crater 1] has been moderate since an explosion on 20 February 1993 ejected scoriae 100 m above the vent. During the daily rim visit on 2 May 1994, mud ejection was observed for the first time since 10 June 1993. However, the crater floor has been covered by water and frequent water ejections have been observed. Continuous tremor was registered at a seismic station 800 m W of the crater. Average amplitude of continuous tremor had been 0.2 µm through May, but on 7-9 June the average amplitude suddenly increased to >6 µm.

Information Contact: JMA.

06/1994 (BGVN 19:06) Volcanic tremor; water ejections from pond in crater floor

Crater 1 remained restless through June after a mud ejection on 2 May. The floor of the crater has been covered by a pool of water, but frequent water ejections have been noted during daily observations from the crater rim. Continuous tremor was registered at the seismic station 800 m W of the crater. During May, average tremor amplitude was around 0.2 µm. However, in early June, the amplitude increased suddenly. Continuous tremor became intermittent from 7 to 21 June, and isolated tremor occurred with a maximum amplitude >6 µm.

Information Contact: JMA.

07/1994 (BGVN 19:07) Crater 1 at Nakadake still restless

Crater 1 remained restless through July, but the intensity of activity became more moderate compared to the last two months. Through July the average amplitude of continuous tremor was around 0.1 µm.

Information Contact: JMA.

08/1994 (BGVN 19:08) Mud and stone ejections from crater floor

Activity from Crater 1 was moderate in August. However, at about 0800 on 11 September, intermittent mud ejection from the water-covered crater floor was detected seismically. Tremor registered at a station 800 m W of the crater had an amplitude of 4.8 µm. The seismic station detected similar activity on the evening of 12 September. During the daily crater visit on the morning of 14 September, several tens of stones were found outside the crater rim, within ~300 m of the crater center.

Information Contact: JMA.

09/1994 (BGVN 19:09) Explosions eject mud and blocks

Activity increased at Crater 1 during September. Tremor amplitude registered at a seismic station 800 m W of the crater was 4.8 µm at about 0800 on 11 September. Three hours later, the AWS (figure 25), issued a Volcanic Advisory noting that Aso was getting restless. Another tremor, which was large enough to be felt at AWS, occurred at 1148 later that day. The floor of Crater 1 was covered by a pool of water, and intermittent mud ejection took place. Several tens of volcanic stones were found outside of the crater rim within ~300 m from the center of the crater during a visit on the morning of 14 September. These rocks were ejected by an explosion on the evening of 12 September, based on seismic records. The area within 1 km of Crater 1 was placed off-limits on 11 September by local governments through the Board for Volcanic Disaster Reduction.

Figure 25. Summit area of Nakadake cone at Aso, showing numbered craters, the Aso Weather Station, and associated buildings (squares). Courtesy of JMA.

During the rest of September, mud ejection was intermittent and volcanic tremor was frequent. On 15 and 18 September, ejected mud rose 150 m above the bottom of the crater, almost to the crater rim. On 16 and 19 September, a plume rose to a height of 1,500 m above the crater rim. Tremor was felt by personnel at AWS on 11, 15, 21, 22, and 29 September, and 1 October. The 29 September event was registered 800 m W of the crater with an amplitude of 52 µm, which is the largest reading since tremor amplitude measurements began in 1969.

The 12 September ejection of stones beyond the crater rim was the first eruptive activity since February 1993; mud ejections have been reported since 2 May 1994.

Information Contact: JMA.

10/1994 (BGVN 19:10) Continued mud ejections and ash plumes from Nakadake crater 1

After ejecting mud and blocks on 12 September, Crater 1 remained restless in October (figure 26). The water-covered crater floor ejected mud intermittently, sometimes accompanied by ash plumes. In one case on 27 October, ejected mud flew >100 m above the crater bottom. Tremor amplitude (at Station A, 800 m W of the crater) generally remained <1 µm. Some larger tremor episodes exceeded 10 µm and were felt by personnel at the Aso Weather Station.

Figure 26. Seismicity and plume heights at Aso, January-October 1994. Earthquakes and tremor were registered at a station 0.8 km W of Nakadake cone. Plume heights were estimated by personnel at AWS. Courtesy of JMA.

Information Contact: JMA.

11/1994 (BGVN 19:11) Minor phreatic activity from crater lake

During November, no eruptive activity took place at Crater 1. Water and gas ejection from a pool of water on the crater floor was observed on 5 days in November (specifically, 2, 3, 6, 27 and 28 November). Tremor amplitude registered at a seismic station 800 m W of the crater was not greater than 0.5 µm, but in December the amplitude began to rise.

Information Contact: JMA.

12/1994 (BGVN 19:12) Intermittent mud ejection and large-amplitude tremor

Intermittent mud ejection from the pool of water on the floor of Crater 1 continued in December. Ejections on 18 and 23 December rose 150 m, almost to the level of the crater rim. Large-amplitude tremor was often registered, and 12 seismic events were felt at the AWS.

Information Contact: JMA.

01/1995 (BGVN 20:01) More mud ejections, tremor, and a white plume

Intermittent mud ejections from the lake in Crater 1 continued through mid-January. Mud ejections that rose 100 m on 2 January were accompanied by tremor, seven seismic events felt at the AWS, and a white plume rising to 2 km above the crater rim. At other times during January a white plume rose continuously to ~400 m above the crater rim from the water-covered crater floor, ~150 m below the rim. Large-amplitude tremor associated with the mud ejections was often registered, and one other event was felt on 4 January.

Information Contact: JMA.

03/1995 (BGVN 20:03) Mud ejection beyond the crater and an ash cloud to 1 km

Mud and water ejections continued during February from the shrinking pool of hot water in Naka-dake Crater 1. Similar ejections occurred on 13 and 17 March. The eruption on 17 March ejected mud and volcaniclastic materials within a 300-m radius, including some beyond the crater rim, and sent an ash cloud as high as 1 km above the crater rim. Large-amplitude tremor associated with the mud ejections was felt at the Aso Weather Station (AWS) on 14 and 19 February, and another nine times during March. An earthquake centered beneath the crater was also felt at AWS on 16 February.

Information Contact: JMA.

05/1995 (BGVN 20:05) Mud and water ejections from crater lake; tremor

During April and May, occasional water ejections took place from a hot water pool at the bottom of Naka-dake Crater 1. Water volume in the pool had decreased by 60% in late May. On 9 April mud and water ejections were observed at the bottom of the crater, in addition to a large-amplitude tremor felt at the Aso Weather Station. The daily number of isolated (short-duration) tremors increased in the middle of April, and during May a total of 1,128 were recorded from Station A, 800 m W of Crater 1.

Information Contact: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

07/1995 (BGVN 20:07) Water rises and covers the crater floor, minor water and mud ejections

During June, occasional water ejections took place from a hot water pool at the bottom of Naka-dake Crater 1. The volume of water in the crater increased towards the end of June such that by July Naka-dake's crater was completely covered with hot water. During July, the occasional water ejections were accompanied by the ejection of mud, the highest reaching 10 m.

In July, 791 isolated tremors were recorded at Station A, 800 m W of Crater 1. Continuous tremor occurred through early July, with a maximum amplitude of 8 µm. There were seven natural tremors during July, including four felt at the Aso Weather Station and three earthquakes. Only one large-amplitude tremor was recorded during June.

Information Contact: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

08/1995 (BGVN 20:08) Continued mud and water ejections; increasing tremor episodes

The bottom of Naka-dake Crater 1 remained covered with a pool of hot water throughout August. The central part of the lake was gray, changing to grayish white or green near the margins. Mud and water ejections were frequently observed; the highest rose 10 m. Isolated tremors increased late in the month (recorded 800 m W of the crater). Isolated tremor events totalled 2,613 during August, and five earthquakes were detected. Tremor events continued increasing in early September; by the 10th there had been >2,000 counted.

Information Contact: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

09/1995 (BGVN 20:09) Continued mud and water ejections and many isolated tremors

Throughout September the hot water pool on the floor of Naka-dake Crater 1 frequently ejected mud and water; the highest ejection rose 10 m. Many isolated tremors were recorded at Station A, 800 m W of the crater. The monthly total of isolated tremors was 6,618; only two earthquakes were detected. Continuous tremor with 0.2-0.8 µm amplitude was registered throughout the month.

Mud ejections have been reported since May 1994 (BGVN 19:05). The 24-km-wide Aso Caldera contains 15 central cones. One of these cones, Naka-dake, has erupted more than 165 times since 553 AD, the first documented historical eruption in Japan. Aso is located 75 km E of Unzen and 150 km N of Sakura-jima volcanoes.

Information Contact: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

10/1995 (BGVN 20:10) Isolated tremor; ejections of mud and water

During October the floor of Aso's active crater (Naka-dake Crater 1) remained covered by a pond of hot water. The pond's surface was disrupted by occasional fountaining up to 5-m high. Elevated tremor continued since last month, and some October days had over 200 earthquakes; the daily mean amplitude of continuous tremors sometimes reached over 0.5 þm. Personnel 800 m W of the crater (at Aso Weather Station) felt earthquakes at 1829 and 1909 on 11 and 22 October, respectively.

Information Contact: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

12/1995 (BGVN 20:11/12) Numerous isolated tremors

During November and December 1995 the floor of Naka-dake Crater 1 remained covered with hot water, yet there were few if any mud-and-water ejections. During November the number of isolated tremors reached 5,488; during December, 4,896. In addition, continuous tremor prevailed with amplitudes confined to 0.1-0.8 µm.

Information Contact: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

02/1996 (BGVN 21:02) Continuous tremor; crater floor still covered with water

The floor of Naka-dake Crater 1 remained covered with water in January. Seismic station A, 800 m W of the crater, recorded continuous tremor of 0.1-0.3 µm amplitude. In addition, there were 4,966 isolated tremors during the month.

Aso, a 24-km-wide caldera, produced pyroclastic-flow deposits during the Pleistocene that cover much of Kyushu. Its frequently active Naka-dake is one of a group of 15 intra-caldera cones.

Information Contact: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

05/1996 (BGVN 21:05) Crater glow

Red glow has been observed over part of the S wall of Naka-dake Crater 1 since 27 April. The floor of this crater was still covered with water in May. Aso, a 24-km wide caldera, produced pyroclastic-flow deposits during the Pleistocene that cover much of Kyushu. Naka-dake, one of the 15 intra-caldera cones of Aso's caldera, has erupted more than 165 times since 553 AD.

Information Contact: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

07/1996 (BGVN 21:07) Crater glows; water and mud ejected

Red glows were observed over the S wall of Naka-dake Crater 1 in May and June. The crater floor was covered with water in June, and weak water ejections were observed on 5-6 June. On 30 July, Crater 1 ejected mud on its SE side. Such ejections were also observed on 26 October 1995 (BGVN 20:10).

The 24-km-wide Aso Caldera contains 15 central cones. One of these cones, Naka-dake, has erupted more than 165 times since 553 A.D.

Information Contacts: Volcanological Division, Japan Meteorological Agency, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan

11/1997 (BGVN 22:11) Two tourists killed by volcanic gas on 23 November

Tomoki Tsutsui (Aso Volcanological Laboratory, Kyoto University) reported that a new fumarolic vent ~10 m in diameter formed on the S wall of Crater 1 in early November; later, small mounds of mud formed around the vent. Although Crater 1 had been quiet since 1993, hot greenish-gray water remained in the crater. Videos of Crater 1 taken by the Aso Volcano Museum recorded emissions of mud fragments and white fumes from the new vent, as well as a bubbling noise; other instruments detected low-level volcanic tremors.

According to news reports, inhalation of volcanic gas killed two men, aged 62 and 51 years, after they collapsed ~100 m S of Crater 1's rim at 0945 and 1040 on 23 November. Volcanic gas concentration around the crater is monitored using a sensor installed by the Japan Meteorological Agency in April 1997. Due to high levels of SO2 (~5 ppm), the Crater 1 overlook was closed on the morning of 23 November, but re-opened at 0900 when the SO2 level dropped to <2.5 ppm. A warning to tourists with poor health was issued. At 1049, the overlook was closed again because SO2 levels rose to ~8 ppm. The weather station at Aso had recorded no abnormal volcanic conditions.

Seventy-one people have been hospitalized due to inhalation of volcanic gases at Aso since 1980; of those, seven were killed. In June 1994, five junior high school students on a field trip collapsed near Crater 1.

Aso, a 24-km wide caldera, produced Pleistocene pyroclastic-flow deposits that cover much of Kyushu. Fifteen central cones form an E-W line on the caldera floor. Naka-dake, one of the intra-caldera cones, has erupted more than 165 times since 553 AD. Naka-dake has a group of craters (1.1 km long) including Crater 1 at the summit. Strombolian, phreatic, and phreatomagmatic eruptions are common in Crater 1. The 4 km2 100-m-deep Crater 1 is accessible by cable car, automobile, and on foot.

Information Contacts: Tomoki Tsutsui, Aso Volcanological Laboratory, Kyoto University, Choyo, Aso, Kumamoto, 869-1404, Japan (Email: tom@aso.kugi.kyoto-u. ac.jp); Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (Email: nakada@eri.u-tokyo.ac.jp, URL: http://hakone.eri.u-tokyo.ac.jp/vrc/VRC.html).

10/2003 (BGVN 28:10) Phreatic eruptions during 10-14 July cause ashfall 14 km away

Recent noteworthy activity at Aso consisted of elevated tremor in August 2002 and a phreatic eruption in July 2003. Seismicity recorded by the Japan Meteorological Agency (JMA) between January 2000 and April 2003 (table 1) was generally constant, with continuous volcanic tremor every month in addition to isolated tremor events. The number of tremor events was high through October 2000, during April 2002, and from August 2002 through March 2003. Also during this extended period, white plumes were observed approximately once a month, with two or more plumes occurring in July, October, and December 2002, and February and March 2003. These plumes were usually less than 500 m high.

Table 1. Seismicity at Aso between January 2000 and April 2003. The seismograph station is located ~ 13 km W of the summit. Courtesy of JMA.

    Month       Number of volcanic    Number of volcanic
                  earthquakes               tremors

    Jan 2000           19                    1,466
    Feb 2000           16                      926
    Mar 2000           73                    1,232
    Apr 2000    No JMA report received this month
    May 2000           39                      537
    Jun 2000           30                      802
    Jul 2000           29                    1,234
    Aug 2000           21                    2,104
    Sep 2000           36                    1,445
    Oct 2000           38                    1,448
    Nov 2000           43                      202
    Dec 2000           33                      129
    Jan 2001           51                       60
    Feb 2001          161                      739
    Mar 2001           76                      537
    Apr 2001           40                       81
    May 2001           40                       85
    Jun 2001           99                      188
    Jul 2001           84                      282
    Aug 2001           60                      471
    Sep 2001           40                       86
    Oct 2001           91                       32
    Nov 2001           52                       17
    Dec 2001           45                        5
    Jan 2002           38                        5
    Feb 2002           59                       20
    Mar 2002           20                       20
    Apr 2002          114                    1,138
    May 2002           91                       14
    Jun 2002          191                       36
    Jul 2002          238                       37
    Aug 2002          153                    4,413
    Sep 2002          144                    1,438
    Oct 2002          103                    1,440
    Nov 2002          652                    3,391
    Dec 2002          154                    8,496
    Jan 2003          122                    6,981
    Feb 2003          178                    4,183
    Mar 2003           92                    1,965
    Apr 2003           70                      474

Activity during August 2002. For the first time since 1992, isolated volcanic tremor events occurred at a rate of more than 300 events/day in Naka-dake Crater 1. These events were recorded between 5 and 21 August and totalled nearly 4,000 (table 2), with the highest number, 340 events, on 15 August. During this period, the water temperature of the pool in the crater remained between 57 and 60°C. On 14 August, infrared cameras measured the maximum temperature of the southern crater wall at 307°C; this increased to 314°C the following week.

Table 2. Daily number of isolated volcanic tremor events at Aso, August 2002. Courtesy of Japan Meteorological Agency.

     Date     Number of isolated
    (2002)      tremor events

    05 Aug           129
    06 Aug           238
    07 Aug           241
    08 Aug           137
    09 Aug           244
    10 Aug           304
    11 Aug           315
    12 Aug           335
    13 Aug           299
    14 Aug           336
    15 Aug           340
    16 Aug           287
    17 Aug           257
    18 Aug           208
    19 Aug           162
    20 Aug           104
    21 Aug      37 as of 1100

Activity during July 2003. JMA reported on 11 July 2003 that tephra had fallen at Aso that morning. According to the report, a tremor event with an intermediate amplitude was recorded at 1718 on 10 July. Staff from the Aso Weather Station confirmed that small amounts of tephra had been newly deposited at Hakoishi-Toge (Hakoishi Pass), ~ 6 km ENE of the Nakadake crater. Kazunori Watanabe (Kumamoto University) and other geologists surveyed the deposit on 11 July and estimated the total mass of ejected material at roughly 130 tons. Ash was deposited as far as 14 km from the crater. A small amount of fresh vesicular glass particles were noted in the ejecta under the microscope. According to Yasuaki Sudo (Aso Volcanological Laboratory, Kyoto University), who inspected the crater area, the event was a small phreatic eruption of mud. The deposit consisted of wet ash aggregates and was ~ 1 mm thick, even at the crater rim. A spray of mud was blown off the crater rim by strong winds to 10 km from the crater.

Seismic signals implied a series of small phreatic eruptions between 12 and 14 July. Then on 27 July continuous volcanic tremor started around 1400. Observations that day noted that the water in Crater 1 was gray and boiling in the center; the temperature of the water was 76°C.

Information Contacts: Volcanological Division, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.kishou.go.jp/english/); Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (Email: nakada@eri.u-tokyo.ac.jp, URL: http://hakone.eri.u-tokyo.ac.jp/vrc/VRC.html); Kazunori Watanabe, Kumamoto University, 40-1, Kurokami 2-chome, Kumamoto 860-8555, Japan (Email: wittoku@gpo.kumamoto-u.ac.jp); Hitoshi Yamasato and N. Uchida, Japan Meteorological Agency (JMA), Fukuoka District Meteorological Observatory, 1-2-36 Oohori, Chuo-ku, Fukuoka 810-0052, Japan (Email: yamasato@met.kishou.go.jp, n-uchida@met.kishou.go.jp); Tomoki Tsutsui and Yasuaki Sudo, Aso Volcanological Laboratory, Kyoto University, Choyo, Aso, Kumamoto, 869-1404, Japan (Email: tom@aso.kugi. kyoto-u.ac.jp, yas@aso.vgs.kyoto-u.ac.jp).

01/2004 (BGVN 29:01) June 2003 phreatic outbursts and a January 2004 mud eruption

Japan Meteorological Agency (JMA) reports for July 2003 noted ash-bearing eruptions from Aso (BGVN 28:10). The 10 or 11 July 2003 eruption was followed by seismically inferred phreatic eruptions a few days later, and a mud eruption on 14 January (the end of this report interval).

Seismic signals during 12-14 July implied there had been about five small phreatic eruptions. Continuous volcanic tremor started at ~ 1400 on 27 July. The JMA report of 28 July 2003 noted that seismometers had recorded ~ 100 isolated tremors. Earthquakes also occurred, at a rate of ~ 10 per day. On 28 July, lake water in Crater 1 was gray colored with a temperature of 76°C and with boiling regions in its central area.

A later JMA report noted a mud eruption in Crater 1 at 1541on 14 January, the first such eruption since July 2003. Associated tremor also occurred. Small amounts of very fine ash from this eruption were seen in Takamori, a town ~ 10 km ESE of the crater. The report noted that thermal activity had risen since last year, causing the water level in the crater to decrease about 40% below normal. The hazard status rose from 2 to 3, and accordingly, authorities restricted tourists from the area within 1 km of the crater.

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.kishou.go.jp/english/); Fukuoka District Meteorological Observatory, Japan Meteorological Agency (JMA), 1-2-36 Oohori, Chuo-ku, Fukuoka 810-0052, Japan (Email: yamasato@met.kishou.go.jp, n-uchida@met.kishou.go.jp).

09/2011 (BGVN 36:09) Small ash-bearing eruptions during May and to lesser extent in June 2011

After small ash-bearing eruptions, the Alert Level on Aso was raised from 1 to 2 (on a scale of 1-5) on 17 May 2011. Aso, the largest volcano in SW Japan, consists of a large, 24-km-diameter caldera located on the Japanese island of Kyushu (figure 27). Under normal conditions the area within the caldera is restricted and, with the raising of the Alert Level, authorities restricted entry within 1 km of Naka-dake cone, containing one of the active craters that comprise the Aso volcanic complex.

Figure 27. Map of the main islands of Japan; Mt. Aso is on the Island of Kyushu. Map from Wordtravels.com.

Fumarole temperature from hydrogen isotopic ratios. The temperature of fumarole gas is a primary observation used at many volcanoes. Tsunogai and others (2011) describe a method of using hydrogen isotope ratios to determine fumarolic temperature. Because the isotopes can be collected at a distance from the vent and without entering the crater, this method offers several advantages. Aso was one of the volcanoes where this technique was applied because it has a deep crater that makes direct sampling and at-vent measurements impractical. Direct sampling of gases is potentially far more hazardous. Infrared measurements may suffer bias when cooled, outgassed material, such as ash, obstructs the hotter portions of the plume. Such measurements could understate the emission's radiant heat and thus its temperature.

We previously published a topographic map depicting the Aso caldera and the location of Naka-dake within the caldera (BGVN 19:09 ), one of 17 central cones. Of these, Naka-dake is the most active. Naka-dake has a crater lake at its summit that contributes to its tendency towards phreatic and mud eruptions.

Aso resides in a National Park of the same name. Naka-dake is easily accessible by public transport and is a popular tourist destination. The rims of the active crater area contain parking and viewpoints accessible by toll road or the Arcosan Ropeway (steel-cabled aerial tramway). At the rim, massive concrete structures offer some protection from falling ballistics in the case of sudden explosions. Another attraction in the area is the Aso Volcano Museum, which features a webcam and photos of phreatic eruptions at Aso.

Aso has been highly active in recent years, but rarely to an extent where it has become dangerous to people. Aso erupted from 10 June 2003 to 14 Jan 2004 (BGVN 29:01). During that time Aso mainly erupted mud, associated with volcanic tremors, and a small amount of ash. A rise in thermal activity in the area may have been a contributing factor in the eruption (Volcano Research Center, University Tokyo). On 14 April 2005, the volcano erupted again, forcing five tourists to be evacuated after hundreds of small earthquakes were detected in the prior two weeks.

May-June 2011 unrest. The latest series of eruptions began on 6 May 2011 with mud erupting about 5-10 m from the hot caldera lake. On 13 May the temperature of fumarolic emissions in the caldera had increased. The Japan Meteorological Agency noted that "A small volcanic flame [glow?] has been observed at nights at the crater pits in the center of Naka-dake." On 15 May, Naka-dake erupted a small amount of ash, with the plume rising to an altitude of 2.1 km. One approach to measuring areas of elevated temperature involves infrared photographs. Visible and infrared photos of Naka-dake crater documented temperature increases in the crater from 21 April to 15 May (figure 28). The temperature of fumarolic emissions in the crater reached around 370°C (the temperature measurement method was not disclosed).

Figure 28. Visible (left) and infrared (right) images on two different days (contact JMA for temperature scales). a) 21 April 2011 and b) 15 May 2011. Courtesy of JMA.

On 16 May, an eruption sent one plume to an altitude of 1.8-2.1 km and another ash plume to 2.4 km, according to the Tokyo Volcanic Ash Advisory Center. A video depicting the plume that day can be found on Youtube (Asahi.com, 2011). The video shows aerial footage of the plume, which is bent downwind. The emission is constant but not vigorous.

Rocks ejected from Naka-dake on 17 May landed in restricted areas, and ash plumes rose to an altitude of 1.8 km. Ash plumes continued rising to similar altitudes through the end of May, and small scale eruptions continued through 10 June, accompanied by low level seismicity. No additional plumes were reported through mid-October. The website of the Mt. Aso Ropeway noted that entry restrictions ended on 20 June 2011, allowing them to carry passengers.

References. Asahi.com, 2011, Naka Erupting, YouTube (URL: http://www.youtube.com/watch?v=uBKJnM2JIZs), posted 16 May 2011.

Tsunogai, U., Kamimura, K., Anzai, S., Nakagawa, F., and Komatsu, K., 2011, Hydrogen isotopes in volcanic plumes: Tracers for remote temperature sensing of fumaroles, Geochimica et Cosmochimica Acta, v. 75, no. 16, p. 4531-4546.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Volcano Research Center, VRC-ERI, Univ. Tokyo, (URL: http://hakone.eri.u-tokyo.ac.jp/vrc/erup/aso.html); Wordtravels, (URL: http://www.wordtravels.com/Travelguide/Countries/Japan/Map).

08/2012 (BGVN 37:08) Minor mud ejections resumed in 2011, the first since 2008

This report summarizes Japan Meteorological Agency (JMA) monthly reports (available in English since October 2010) covering the interval April 2011 to September 2012, with a separate subsection largely focused on aviation reports of Aso plumes emitted at Naka-dake crater during mid-2011. During this reporting interval Naka-dake continued to degas and emit small ash plumes. Eruptions of mud resumed after a hiatus of several years (February 2008 to April 2011).

Aso (also called Aso-san) is a caldera with dimensions ~17 km E-W by ~25 km N-S encompassing an area of ~350 km2. Figure 29 indicates the location of Aso in relation to other Holocene Japanese volcanoes and landmarks in the region.

Figure 29. A map of the major volcanoes of Japan. Aso is shown on the left side, on the island of Kyushu. Courtesy of the U.S. Geological Survey.

Aso’s most recent series of eruptions began in April 2011, with minor phreatic (mud-bearing) eruptions from Naka-dake’s crater lake. These eruptions were accompanied by minor ash plumes, rock ejections, an increase in the temperature of fumaroles (BGVN 36:09), and continuous, small-amplitude tremor.

Field observations during April 2011-June 2011. In April 2011, a small phreatic (mud-bearing) eruption 5-10-m-high was observed in Naka-dake’s crater lake; the lake’s temperature was 67°C. Volcanic seismicity remained at a relatively low level. A photo from 21 April 2011 shows a white steam plume (figure 30A).

Figure 30. (A) A photo taken by a field survey team on 21 April 2011 shows a white steam plume rising from the crater floor. (B) A photo taken on 16 May 2011 shows a grayish plume venting from the crater floor. Courtesy of JMA.

From 3 to 10 May, continuous small-amplitude tremor was detected. Seismicity, including isolated-pulse events, remained relatively low during this time. On 6 and 9 May, field surveyers observed a small 5-10-m-high phreatic eruption from the hot crater lake (locally called “Yudamari”).

A camera installed by the Aso Volcano Museum detected a small volcanic ash emissions from within the crater beginning on 13 May. Six cameras provide live image feeds to the Aso Museum website. There are also many videos showing Aso and Naka-dake on YouTube.

On 13 May, a field survey found increased fumarole temperatures in the crater, and a video camera revealed incandescence on multiple nights. According to JMA, a small eruption occurred on 15 May followed by minor ashfall, which extended 2 km NE of the crater. A field survey on 15 May recorded a temperature of ~370°C at a fumarole in the crater.

Another eruption occurred on 16 May, producing a grayish plume that rose 500 m above the crater rim. As a result of this increased activity, the Alert Level was raised from 1 to 2 (on a scale from 1-5). A field surveyer later the same day saw a gray plume rise 800 m above the crater rim (figure 30B). Small-scale eruptions occurred intermittently on the 17th. The lake water volume was low around this time, ~10-20% of its full volume.

A 9 June field survey revealed a decrease in fumarole temperatures from ~370°C on 15 May to ~160°C on 9 June. After 10 June, eruptions ceased and the lake water volume increased from 60% full on 12 June to 80% full on 17 June (figure 31). The rising lake level suggested a decrease in activity. Consequently, the Alert Level was lowered from 2 to 1 on 20 June. Seismicity, including isolated-pulse events, remained at relatively low levels.

Figure 31. (A) Photo taken on 9 June 2011 showing the bottom of Naka-dake crater. Note the absence (or near absence) of the crater’s lake. (B) Photo taken on 22 June 2011 showing the presence of the steaming crater lake just about two weeks after the photo in (A) was taken. Courtesy of JMA.

Plume heights and drift directions during May-June 2011. We summarize reports from the Tokyo Volcanic Ash Advisory Center (VAAC) issued between 15 May and 9 June 2011 (table 3). Many plumes contained ash. Notice that the plume heights are stated as altitudes above sea level (as compared to heights above the crater rim, as in the other sections of this report).

Table 3. Summary of plumes at Aso between 15 May and 9 June 2011. Smaller plumes may not have been recorded or were omitted. In most cases, the presence of ash in the plume was noted; in other cases ash may have been present but not recorded. ‘-’ indicates data not reported. Data provided by Tokyo VAAC and JMA.

Date (2011)         Plume altitude        Drift        Ash?    Pilot/JMA report

15 May                2.1 km               NE          Ash          Pilot
16 May                1.8-2.1 km            -           -            JMA
16 May                2.4 km                N          Ash          Pilot
17-18 May             1.8 km              E, SE        Ash           JMA
18 May                3 km                  -          Ash          Pilot
18-22 May             1.5-2.1 km        N, NE, SE      Ash           JMA
25, 27-28, 31 May     1.5-1.8 km       NW, N, E, S     Ash           JMA
1-7 June              1.5-2.1 km     NW, N, NE, E, S    -            JMA
8-9 June              1.5-1.8 km       NW, N, NE, E     -            JMA

Field observations during October 2011-June 2012. In October 2011, white plumes rose on average less than 200 m above the crater rim, with a maximum of 300 m. The lake water volume during September and October was at about 90% full, and the September and October lake-surface temperatures were 47-56°C and 49-58°C, respectively. Based on field surveys made on 3, 17, and 20 October, the sulfur-dioxide (SO2) flux was ~300-500 tons/day, compared to ~300 tons/day in September. Volcanic seismicity remained low. Tremor, detected 13 times during September, was absent during October. The total magnetic intensity measured at the NW rim of the Naka-dake crater had increased since December 2010, but was static during June 2011 through October 2011. No change was detected by GPS measurements.

The next JMA monthly report on Aso discussed activity during May and June 2012. Because of heavy rains after 15 May, the lake water volume had increased to ~70% full, and during the course of the month the volume was in the range 60-80% full. Then in late May, the lake level begain to drop, and continued into at least mid-June.

The lake surface temperature was 63-72°C in May and 67-73°C in June. The highest temperature of fumaroles along the southern crater wall was 246-260°C, compared to 228-267°C in May. Scientists conducting a field survey at night on 22 June noted that part of the S crater wall was incandescent.

In June 2012, white plumes rose an average of 600 m above the crater rim. There were 621 isolated cases of tremor in June, approaching a 2-fold increase over some of the previous months, but only amounting to a duration of a few minutes per month. Isolated volcanic tremor and seismicity remained low but had slightly increased overall after February 2012, with most hypocenters located at shallow depths under Naka-dake. No change was detected by GPS measurements. The total magnetic intensity began to increase again in June 2012.

Lake levels during July-September 2012. In July, heavy rains caused the lake level to rise to 80-90% full (from 30-70% full in June). The volume remained high in August and September (90-100% full). During June-July the lake surface temperature decreased slowly, from 58-66°C in July to 57-61°C in August and to 54-59°C in September. Steam emissions from the crater occurred in July and August, but stopped by September.

Crater temperatures during July-September 2012. The highest temperature of the S wall of Naka-dake-Daiichi crater decreased in July, but rose slightly in August and September (213-250°C in July, 241-249°C in August, and 250-283°C in September). A field survey on 24 September revealed that the hot areas had not changed since the previous survey on 22 June. On 23-26 September, weak glow in the crater was recorded at night by a thermal camera. Officials assumed the glow was caused by the hot crater wall.

July-September 2012 seismicity. Both isolated volcanic tremor and other seismicity returned at low levels during July-September 2012. 621 volcanic tremors occurred in June, 669 in July, 1,025 in August and 867 in September. 669 volcanic earthquakes occurred in July, 951 in August, and 978 in September. Other seismic events occurred 369 times in June, 626 in July, and were not reported in August or September. Few short-term tremors occurred (4 in June, none in July, 2 in August, and 1 in September). Most hypocenters were located at shallow depths (2-4 km) and in an area ~6 km NE of Naka-dake.

Based on field studies, sulfur dioxide levels were elevated during May-September 2012 (600-800 t/d in May, ~400 t/d on 10 July, and 500-700 t/d on 19 and 24 September). The total magnetic intensity at the NW rim of Naka-dake-Daiishi crater increased between December 2010 and September 2012, which officials suggested might signify a temperature rise underneath the crater.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Aso Volcano Museum (URL: http://www.asomuse.jp/); Volcano Discovery (URL: http://www.volcanodiscovery.com/); Earth Observation Research Center (Japan) (URL: http://www.eorc.jaxa.jp/en/index.php).

The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 cu km of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 AD. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

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

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
2014 Jan 13 2014 Feb 19 Confirmed 1 Historical Observations Naka-dake
2011 May 15 2011 Jun 9 Confirmed 1 Historical Observations Naka-dake
2008 Feb 17 2008 Feb 17 Confirmed 1 Historical Observations Naka-dake
2005 Apr 14 2005 Aug Confirmed 1 Historical Observations Naka-dake
2004 Jan 14 2004 Jan 14 Confirmed 1 Historical Observations Naka-dake
2003 Jul 10 2003 Jul 14 Confirmed 1 Historical Observations Naka-dake
1994 May 2 1995 Nov Confirmed 2 Historical Observations Naka-dake
1992 Apr 23 1993 Jun 10 (?) Confirmed 2 Historical Observations Naka-dake
1989 Apr 5 1991 Feb 9 Confirmed 2 Historical Observations Naka-dake
1988 May 26 ± 5 days 1989 Jan Confirmed 1 Historical Observations Naka-dake
1984 Apr 13 ± 3 days 1985 Jun 24 Confirmed 1 Historical Observations Naka-dake
1983 Jul 1983 Oct Confirmed 1 Historical Observations Naka-dake
1981 Jun 15 1981 Jun 15 Confirmed 1 Historical Observations Naka-dake
1980 Sep 24 1980 Sep 24 Confirmed 1 Historical Observations Naka-dake
1979 Jun 5 ± 4 days 1980 Mar 8 Confirmed 2 Historical Observations Naka-dake
1977 Apr 11 1978 Aug Confirmed 2 Historical Observations Naka-dake
1975 Oct 1976 Jan 13 Confirmed 1 Historical Observations Naka-dake
1973 Jan 1975 Jun Confirmed 2 Historical Observations Naka-dake
1970 Apr 21 1972 Sep Confirmed 2 Historical Observations Naka-dake
1967 May 1969 Dec Confirmed 1 Historical Observations Naka-dake
1964 Oct 1966 Dec Confirmed 2 Historical Observations Naka-dake
1964 May 14 1964 May 14 Confirmed 1 Historical Observations Naka-dake
1963 Nov 10 1964 Jan Confirmed 2 Historical Observations Naka-dake
1963 Apr 21 1963 Jul Confirmed 1 Historical Observations Naka-dake
1960 Sep 1962 Nov Confirmed 1 Historical Observations Naka-dake
1960 Jan 1960 Apr 9 Confirmed 2 Historical Observations Naka-dake
1959 Jul 1959 Oct 2 Confirmed 1 Historical Observations Naka-dake
1957 Oct 1958 Dec Confirmed 2 Historical Observations Naka-dake
1957 Apr 12 1957 Apr 12 Confirmed 1 Historical Observations Naka-dake
1956 Dec 21 1956 Dec 21 Confirmed 1 Historical Observations Naka-dake
1956 Aug Unknown Confirmed 2 Historical Observations Naka-dake
1956 Jan 3 (?) 1956 Jan 13 Confirmed 2 Historical Observations Naka-dake
1955 Jul 25 1955 Jul 28 Confirmed 1 Historical Observations Naka-dake
1954 May 26 1954 May 26 Confirmed 1 Historical Observations Naka-dake
1953 Dec 1953 Dec Confirmed 1 Historical Observations Naka-dake
1953 Apr 27 1953 Jul 30 Confirmed 2 Historical Observations Naka-dake
[ 1952 ] [ Unknown ] Uncertain     Naka-dake
1951 May 4 1951 Aug Confirmed 2 Historical Observations Naka-dake
1950 Nov 1951 Jan Confirmed 2 Historical Observations Naka-dake
1949 Dec 26 1950 Apr 15 Confirmed 2 Historical Observations Naka-dake
1948 Apr 9 1948 Dec Confirmed 1 Historical Observations Naka-dake
1947 May 26 1947 Sep Confirmed 2 Historical Observations Naka-dake
1946 Dec 30 1946 Dec 30 Confirmed 2 Historical Observations Naka-dake
1946 Apr 29 1946 Jun 24 Confirmed 2 Historical Observations Naka-dake
1945 Sep 16 1945 Sep 19 Confirmed 2 Historical Observations Naka-dake
1943 Dec 9 1944 Feb Confirmed 2 Historical Observations Naka-dake
1943 Jun 21 1943 Jun 24 Confirmed 2 Historical Observations Naka-dake
[ 1942 Jun 8 ] [ 1942 Jun 22 ] Uncertain 1   Naka-dake
1940 Apr 20 1941 Aug 8 Confirmed 2 Historical Observations Naka-dake
1938 1939 Aug 11 Confirmed 2 Historical Observations Naka-dake
1937 May 7 1937 May 13 Confirmed 2 Historical Observations Naka-dake
1937 Jan 13 1937 Jan 13 Confirmed 2 Historical Observations Naka-dake
1936 Aug 8 1936 Aug 14 Confirmed 2 Historical Observations Naka-dake
1936 Feb 5 1936 Feb 5 Confirmed 2 Historical Observations Naka-dake
1935 Jan 7 1935 Oct 8 Confirmed 2 Historical Observations Naka-dake
1934 Jul 16 Unknown Confirmed 2 Historical Observations Naka-dake
1932 Jun 1933 Sep 28 Confirmed 2 Historical Observations Naka-dake
[ 1931 Oct 18 ] [ Unknown ] Uncertain 1   Naka-dake
1930 Sep 3 1930 Sep 6 Confirmed 2 Historical Observations Naka-dake
1928 Sep 6 1929 Oct 23 Confirmed 2 Historical Observations Naka-dake
1926 Sep 21 1928 Jan 13 Confirmed 2 Historical Observations Naka-dake
1925 Jan 6 Unknown Confirmed 2 Historical Observations Naka-dake
1923 Jan 1923 Sep 17 Confirmed 2 Historical Observations Naka-dake
1920 Unknown Confirmed 2 Historical Observations Naka-dake
1919 Apr 1919 May Confirmed 2 Historical Observations Naka-dake
1918 Jan 16 Unknown Confirmed 2 Historical Observations Naka-dake
1916 Apr 19 Unknown Confirmed 2 Historical Observations Naka-dake
1914 Jan 13 Unknown Confirmed 2 Historical Observations Naka-dake
1911 1912 Confirmed 2 Historical Observations Naka-dake
1910 Apr 3 Unknown Confirmed 2 Historical Observations Naka-dake
1909 Apr Unknown Confirmed 2 Historical Observations Naka-dake
1908 Jan 17 1908 Jan 29 (?) Confirmed 2 Historical Observations Naka-dake
1907 Dec 12 Unknown Confirmed 2 Historical Observations Naka-dake
1906 Jun 7 Unknown Confirmed 3 Historical Observations Naka-dake
1898 Aug (?) 1899 Apr (in or after) Confirmed 2 Historical Observations Naka-dake
1897 Feb 24 ± 4 days Unknown Confirmed 2 Historical Observations Naka-dake
1894 Mar 6 1894 Aug 30 Confirmed 2 Historical Observations Naka-dake
1884 Mar 21 1884 Jun Confirmed 2 Historical Observations Naka-dake
1874 Feb 7 Unknown Confirmed 2 Historical Observations Naka-dake
1872 Dec 1 1873 Jun 8 Confirmed 3 Historical Observations Naka-dake
1856 Mar 18 1856 Jun 13 Confirmed 2 Historical Observations Naka-dake
1854 Feb 26 Unknown Confirmed 2 Historical Observations Naka-dake
1838 Mar 4 Unknown Confirmed 2 Historical Observations Naka-dake
1837 Oct 8 Unknown Confirmed 2 Historical Observations Naka-dake
1835 May 1 Unknown Confirmed 2 Historical Observations Naka-dake
1830 Aug 11 1832 Aug Confirmed 2 Historical Observations Naka-dake
1830 Feb 16 1830 Mar Confirmed 2 Historical Observations Naka-dake
1829 Jun Unknown Confirmed 2 Historical Observations Naka-dake
1828 Jun Unknown Confirmed 2 Historical Observations Naka-dake
1827 Nov 12 1828 Jan Confirmed 2 Historical Observations Naka-dake
1827 May Unknown Confirmed 2 Historical Observations Naka-dake
1826 Oct 3 1826 Nov 22 Confirmed 2 Historical Observations Naka-dake
1816 Jun 9 1816 Jul 6 Confirmed 2 Historical Observations Naka-dake and west flank (Yunotani)
1815 Feb 10 1815 Oct Confirmed 2 Historical Observations Naka-dake
1814 Unknown Confirmed 2 Historical Observations Naka-dake
1806 Jun (?) 1806 Oct (?) Confirmed 2 Historical Observations Naka-dake
1804 Sep 5 Unknown Confirmed 2 Historical Observations Naka-dake
1781 1788 Confirmed 1 Historical Observations Naka-dake
1772 1780 Confirmed 2 Historical Observations Naka-dake
1765 Jan (?) 1765 Oct (?) Confirmed 3 Historical Observations Naka-dake
[ 1753 ] [ 1754 ] Uncertain 2   Naka-dake
1709 Feb 13 Unknown Confirmed 2 Historical Observations Naka-dake
[ 1708 Sep 17 ] [ Unknown ] Uncertain 2   Naka-dake
1691 Apr 1691 Aug Confirmed 2 Historical Observations Naka-dake
1683 Jun Unknown Confirmed 2 Historical Observations Naka-dake
1675 Feb 16 Unknown Confirmed 2 Historical Observations Naka-dake
[ 1671 ] [ Unknown ] Uncertain 2   Naka-dake
1668 Aug 1669 Confirmed 2 Historical Observations Naka-dake
1668 Feb Unknown Confirmed 2 Historical Observations Naka-dake
1649 Jul 1649 Aug Confirmed 2 Historical Observations Naka-dake
1637 Sep 29 1637 Oct 5 Confirmed 2 Historical Observations Naka-dake
1631 Dec Unknown Confirmed 2 Historical Observations Naka-dake
1620 Jun 3 Unknown Confirmed 2 Historical Observations Naka-dake
1613 Aug 8 Unknown Confirmed 2 Historical Observations Naka-dake
1612 Aug 12 Unknown Confirmed 2 Historical Observations Naka-dake
1611 Unknown Confirmed 2 Historical Observations Naka-dake
1598 Dec 1599 Confirmed 2 Historical Observations Naka-dake
1592 Unknown Confirmed 2 Historical Observations Naka-dake
1587 Unknown Confirmed 2 Historical Observations Naka-dake
1584 Aug Unknown Confirmed 2 Historical Observations Naka-dake
1583 Dec 14 Unknown Confirmed 2 Historical Observations Naka-dake
1582 Feb 17 Unknown Confirmed 2 Historical Observations Naka-dake
1576 Nov 15 Unknown Confirmed 2 Historical Observations Naka-dake
1574 Unknown Confirmed 2 Historical Observations Naka-dake
1573 Unknown Confirmed 2 Historical Observations Naka-dake
1564 Dec Unknown Confirmed 2 Historical Observations Naka-dake
[ 1563 May 3 ] [ Unknown ] Uncertain 2   Naka-dake
1562 Mar Unknown Confirmed 2 Historical Observations Naka-dake
1558 1559 Confirmed 2 Historical Observations Naka-dake
1542 Apr 29 Unknown Confirmed 2 Historical Observations Naka-dake
1533 Jul 17 Unknown Confirmed 2 Historical Observations Naka-dake
1522 Feb 15 Unknown Confirmed 2 Historical Observations Naka-dake
1506 Apr 6 Unknown Confirmed 2 Historical Observations Naka-dake
1505 Feb Unknown Confirmed 2 Historical Observations Naka-dake
1485 Jan 5 Unknown Confirmed 2 Historical Observations Naka-dake
1473 May 16 1474 Apr 15 ± 45 days Confirmed 2 Historical Observations Naka-dake
1438 Jan 9 1438 Feb 18 Confirmed 2 Historical Observations Naka-dake
1434 May 10 1434 May 18 Confirmed 2 Historical Observations Naka-dake
[ 1390 ] [ Unknown ] Uncertain 2   Naka-dake
[ 1388 Oct 16 ] [ Unknown ] Uncertain 2   Naka-dake
1387 Jun 19 Unknown Confirmed 2 Historical Observations Naka-dake
1377 May 6 Unknown Confirmed 2 Historical Observations Naka-dake
1376 Jun 20 Unknown Confirmed 2 Historical Observations Naka-dake
1375 Dec 20 1376 Jan 31 Confirmed 2 Historical Observations Naka-dake
[ 1369 ] [ Unknown ] Uncertain 2   Naka-dake
[ 1346 ] [ Unknown ] Uncertain 2   Naka-dake
1343 Unknown Confirmed 2 Historical Observations Naka-dake
1340 Feb 3 1340 Feb 25 Confirmed 2 Historical Observations Naka-dake
1335 Feb 7 1335 Mar 26 Confirmed 2 Historical Observations Naka-dake
1331 Dec 1333 Jun Confirmed 2 Historical Observations Naka-dake
1331 Apr 1331 Apr Confirmed 2 Historical Observations Naka-dake
1324 Sep 7 Unknown Confirmed 2 Historical Observations Naka-dake
1305 May 2 Unknown Confirmed 2 Historical Observations Naka-dake
1286 Aug 30 Unknown Confirmed 2 Historical Observations Naka-dake
1281 Jul 1281 Aug Confirmed 2 Historical Observations Naka-dake
1274 Unknown Confirmed 2 Historical Observations Naka-dake
1273 Aug Unknown Confirmed 2 Historical Observations Naka-dake
1272 Nov 29 Unknown Confirmed 2 Historical Observations Naka-dake
1272 Apr 16 Unknown Confirmed 2 Historical Observations Naka-dake
1271 Jan 5 Unknown Confirmed 2 Historical Observations Naka-dake
1269 Aug Unknown Confirmed 2 Historical Observations Naka-dake
1265 Dec 1 (?) Unknown Confirmed 2 Historical Observations Naka-dake
1240 Unknown Confirmed 2 Historical Observations Naka-dake
1239 Feb 8 Unknown Confirmed 2 Historical Observations Naka-dake
[ 1229 Dec 31 ± 365 days ] [ Unknown ] Uncertain 2   Naka-dake
[ 0986 Sep 2 ] [ Unknown ] Uncertain 2   Naka-dake
0867 Jun 20 (?) Unknown Confirmed 2 Historical Observations Naka-dake
0864 Nov 9 Unknown Confirmed 3 Historical Observations Naka-dake
[ 0796 Aug ] [ Unknown ] Uncertain 0   Naka-dake
[ 0553 ] [ Unknown ] Uncertain 3   Naka-dake
0440 ± 75 years Unknown Confirmed 3 Radiocarbon (corrected) Naka-dake, N2S tephra
0630 BCE ± 50 years Unknown Confirmed   Radiocarbon (uncorrected)
1270 BCE ± 75 years Unknown Confirmed   Radiocarbon (uncorrected)
1350 BCE (?) Unknown Confirmed 3 Radiocarbon (corrected) Komezuka
1650 BCE (?) Unknown Confirmed 3 Radiocarbon (corrected) Ojo-dake, OjS tephra
1830 BCE ± 75 years Unknown Confirmed   Radiocarbon (uncorrected)
2050 BCE (?) Unknown Confirmed 3 Radiocarbon (corrected) Kishima-dake, KsS tephra
2150 BCE (?) Unknown Confirmed   Radiocarbon (corrected) ACP-1 tephra
2350 BCE ± 75 years Unknown Confirmed   Radiocarbon (uncorrected)
2550 BCE (?) Unknown Confirmed 1 Tephrochronology Jigoku explosion crater, Ikph1 tephra
2850 BCE (?) Unknown Confirmed 0 Tephrochronology Naka-dake
3610 BCE ± 50 years Unknown Confirmed   Radiocarbon (uncorrected)
8050 BCE (?) Unknown Confirmed 1 Ar/Ar Jigoku explosion crater, Ikph2 tephra

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.



Synonyms
Aso


Cones
Feature Name Feature Type Elevation Latitude Longitude
Akai Cinder cone 33 m 32° 46' 30" N 130° 49' 39" E
Eboshidake
    Eboshi-dake
Stratovolcano 1337 m 32° 52' 0" N 131° 4' 0" E
Hontsuka Cone 574 m 32° 57' 0" N 131° 4' 0" E
Ikenokubo Tuff ring
Janoo
    Zyanoo
Cone 754 m 32° 54' 0" N 131° 2' 0" E
Jigoku Spa Vent
Kishimadake
    Kizima-dake
    Kishima-dake
Stratovolcano 1270 m 32° 53' 0" N 131° 4' 0" E
Komezuka
    Kometsuka
Cinder cone 954 m 32° 54' 0" N 131° 3' 0" E
Kusenrigahama Pyroclastic cone 1160 m 32° 53' 0" N 131° 4' 0" E
Maruyama
    Maru-yama
Stratovolcano 1186 m 32° 52' 0" N 131° 6' 0" E
Nakadake
    Naka-dake
Stratovolcano 1500 m 32° 53' 0" N 131° 6' 0" E
Naraodake
    Narao-dake
Stratovolcano 1331 m 32° 53' 0" N 131° 6' 0" E
Nekodake
    Neko-dake
Stratovolcano 1408 m 32° 53' 0" N 131° 9' 0" E
Ojodake
    Ozyo-dake
    Ojo-dake
Stratovolcano 1238 m 32° 54' 0" N 131° 4' 0" E
Okamadoyama
    Okamado-yama
Stratovolcano 1150 m 32° 51' 0" N 131° 4' 0" E
Omine Cinder cone 409 m 32° 49' 56" N 130° 55' 38" E
Takadake
    Taka-dake
Stratovolcano 1592 m 32° 52' 51" N 131° 6' 23" E
Takanoobane Cone
Washigamine Stratovolcano 32° 53' 0" N 131° 7' 0" E
Yomineyama
    Yomine-yama
Stratovolcano 913 m 32° 51' 0" N 131° 2' 0" E


Craters
Feature Name Feature Type Elevation Latitude Longitude
Aso Caldera


Domes
Feature Name Feature Type Elevation Latitude Longitude
Yunotani Dome 800 m 32° 53' 0" N 131° 2' 0" E


Thermal
Feature Name Feature Type Elevation Latitude Longitude
Akamizu Thermal 440 m 32° 55' 0" N 130° 59' 0" E
Aso-Uchinomaki Thermal 480 m 32° 58' 0" N 131° 3' 0" E
Jigoku Thermal 740 m 32° 51' 0" N 131° 2' 0" E
Tarutama Thermal 760 m 32° 52' 0" N 131° 2' 0" E
Yunotani Thermal 800 m 32° 53' 0" N 131° 2' 0" E
Kometsuka scoria cone, on the NW side of the central cone complex of Aso volcano on the island of Kyushu, was constructed about 1800 years ago. Explosive eruptions building the cinder cone were accompanied by lava flows that traveled down the flank of this complex of stratovolcanoes in the center of Aso caldera. A broad moat separates the central cone complex from the caldera walls, which form the horizon.

Copyrighted photo by Dick Stoiber, 1981 (Dartmouth College).
Kometsuka scoria cone (lower left) was formed about 1800 years ago on the west flank of the central cone complex of Aso caldera. Clouds hover over a broad moat forming the caldera floor surrounding the cone complex. The NW wall of the 24-km-wide Aso caldera rises above the clouds. The caldera was formed during four major explosive eruptions during the Pleistocene between 300,000 and 80,000 years ago.

Photo by Ichio Moriya (Kanazawa University).
The 24-km-wide Aso caldera was formed during four major explosive eruptions during the Pleistocene. These produced voluminous pyroclastic flows that swept over much of Kyushu. A group of 17 central cones, seen here from the ENE caldera rim, was constructed in the central part of the caldera. One of these, Naka-dake, is one of Japan's most active volcanoes, with historical eruptions dating back to 553 AD. The summit crater of Naka-dake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Photo by Norm Banks, 1981 (U.S. Geological Survey).
Steam rises from an incandescent vent on the floor of the crater of Naka-dake on April 25, 1985. Moderate eruptions that ejected mud and ash had begun in April 1984 and continued intermittently until June 1985. Frequent ash eruptions occurred from March 1 to the night of May 5-6, 1985.

Photo by Japan Meteorological Agency, 1985.
A gray eruption column rising above the crater of Aso volcano on October 11, 1989 is observed by tourists from a nearby parking lot. Minor ash ejection 30-50 m high began on April 5 and 27, 1989. Tephra emission remained at a high rate in May and June, and intermittent vigorous eruptions that began on July 16 were continuing in 1990. The ash column reached 3000 m on September 7 and 27, and heavy ashfall in October damaged crops. Intermittent eruptive activity continued until February 9, 1991.

Photo by Yukio Hayakawa, 1989 (Gunma University).

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. 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.

Aoki K, 2008. Revised age and distribution of ca. 87 ka Aso-4 tephra based on new evidence from the northwest Pacific Ocean. Quat Internatl, 178: 100-118.

Fujii J, Nakajima T, Kamata H, 2001. Paleomagnetic directions of the Aso pyroclastic-flow and the Aso-4 co-ignimbrite ash-fall deposits in Japan. Earth, Planets, Space, 53: 1137-1150.

Green J, Short N M, 1971. Volcanic Landforms and Surface Features: a Photographic Atlas and Glossary. New York: Springer-Verlag, 519 p.

Ikebe S, Fujioka M, 2001. "Yunotani Catastrophe" of 1816, in Aso volcano, southwest Japan--historical records of steam explosion. Bull Volc Soc Japan (Kazan), 46: 147-163 (in Japanese with English abs).

Ikebe S, Watanabe K, Miyabuchi Y, 2008. The sequence and style of the 1988-1995 eruptions of Nakadake Aso volcano, Kyushu, Japan. Bull Volc Soc Japan (Kazan), 53: 15-33 (in Japanese with English abs).

Iki T, 1926. Geological notes on the Aso volcano. Pan-Pacific Sci Cong Guidebook Excur, E-4: 1-14.

Japan Meteorological Agency, 1996. National Catalogue of the Active Volcanoes in Japan (second edition). Tokyo: Japan Meteorological Agency, 502 p (in Japanese).

Japan Meteorological Agency, 2013. National Catalogue of the Active Volcanoes in Japan (fourth edition, English version). Japan Meteorological Agency.

Kaneko K, Kamata H, Koyaguchi T, Yoshikawa M, Furukawa K, 2007. Repeated large-scale eruptions from a single compositionally stratified magma chamber: an example from Aso volcano, Southwest Japan. J Volc Geotherm Res, 167: 160-180.

Kawakatsu H, Kaneshima S, Matsubayashi H, Ohminato T, Sudo Y, Tsutsui T, Uhira K, Yamasato H, Ito H, Legrand D, 2000. Aso94: Aso seismic observation with broadband instruments. J Volc Geotherm Res, 101: 129-154.

Kuno H, 1962. Japan, Taiwan and Marianas. Catalog of Active Volcanoes of the World and Solfatara Fields, Rome: IAVCEI, 11: 1-332.

Masuda N, Watanabe K, Miyabuchi Y, 2004. Rhyolite to dacite lava flows newly-discovered on the western slope of Aso Central Cones, southwestern Japan. Bull Volc Soc Japan (Kazan), 49: 119-128 (in Japanese with English abs).

Miyabuchi Y, 2010. Eruption age of Komezuka at Aso volcano, Japan. Bull Volc Soc Japan (Kazan), 55: 219-225 (in Japanese with English abs).

Miyabuchi Y, 2009. A 90,000-year tephrostratigraphic framework of Aso volcano, Japan. Sed Geol, 220: 169-189.

Miyabuchi Y, Hoshizumi H, Takada H, Watanabe K, Xu S, 2003. Pumice-fall deposits from Aso volcano during the past 90,000 years, southwestern Japan. Bull Volc Soc Japan (Kazan), 48: 195-214 (in Japanese with English abs).

Miyabuchi Y, Hoshizumi H, Watanabe K, 2004. Late-Pleistocene tephrostratigraphy of Aso volcano, southwestrn Japan, after deposition of AT Ash. Bull Volc Soc Japan (Kazan), 49: 51-64 (in Japanese with English abs).

Miyabuchi Y, Ikebe S, 2008. The February 2008 ash deposit from the Nakadake crater, Aso volcano, Japan. Bull Volc Soc Japan (Kazan), 53: 201-206 (in Japanese with English abs).

Miyabuchi Y, Ikebe S, Watanabe K, 2008. Geological constraints on the 2003-2005 ash emissions from the Nakadake crater lake, Aso volcano, Japan. J Volc Geotherm Res, 178: 169-183.

Miyabuchi Y, Masuda N, Watanabe K, 2004. Geologic history of the western part of post-caldera central cones of Aso volcano, southwestern Japan, based on stratigraphic relationships between lava flows and airfall tephra layers. Bull Volc Soc Japan (Kazan), 49: 267-282 (in Japanese with English abs).

Miyabuchi Y, Watanabe K, 1997. Eruption ages of Holocene tephras from Aso volcano, southwestern Japan, inferred from 14C ages of buried andisols. Bull Volc Soc Japan (Kazan), 42: 403-408 (in Japanese with English abs).

Miyabuchi Y, Watanabe K, 2000. Phreatic explosions and ejecta around Jigoku Spa, southwestern part of the central cones of Aso volcano, Japan. Bull Volc Soc Japan (Kazan), 45: 25-32 (in Japanese with English abs).

Miyabuchi Y, Watanabe K, Egawa Y, 2006. A pyroclastic flow deposit occurring at the northeastern foot of Nakadake, Aso volcano (Japan) and its stratigraphic significance. Bull Volc Soc Japan (Kazan), 51: 231-243 (in Japanese with English abs).

Nakano S, Yamamoto T, Iwaya T, Itoh J, Takada A, 2001-. Quaternary Volcanoes of Japan. Geol Surv Japan, AIST, http://www.aist.go.jp/RIODB/strata/VOL_JP/.

Ono K, Kubotera A, Ota K, 1981. Aso volcano. In: Kubotera A (ed) {Symp Arc Volc Field Excur Guide to Sakurajima, Kirishima and Aso Volcanoes, Part 3}, Tokyo: Volc Soc Japan, p 33-52.

Ono K, Watanabe K, 1985. Geologic map of Aso volcano. Geol Surv Japan, 1:50,000 geol map and text (in Japanese with English summary).

Ono K, Watanabe K, Hoshizumi H, Ikebe S, 1995. Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan. J Volc Geotherm Res, 66: 137-148.

Tanaka Y, 1993. Eruption mechanism as inferred from geomagnetic changes with special attention to the 1989-1990 activity of Aso volcano. J Volc Geotherm Res, 56: 319-338.

Yamasaki T, Hayashi M, 1976. Geologic background of Otake and other geothermal areas in north-central Kyushu, southwestern Japan. In: {Proc 2nd United Nations Symp Devel Use Geotherm Resour, San Francisco}, Washington D C: U S Government Printing Office, 1: 673-684.

Volcano Types

Caldera
Stratovolcano(es)
Cinder cone(s)

Tectonic Setting

Subduction zone
Continental crust (> 25 km)

Rock Types

Major
Andesite / Basaltic Andesite
Dacite
Rhyolite
Basalt / Picro-Basalt

Population

Within 5 km
Within 10 km
Within 30 km
Within 100 km
75,559
75,559
234,591
6,507,519

Affiliated Databases

Large Eruptions of Asosan Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
WOVOdat WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
EarthChem EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).
Smithsonian Collections Search the Smithsonian's NMNH Department of Mineral Sciences collections database. Go to the "Search Rocks and Ores" tab and use the Volcano Name drop-down to find samples.