Eruption of Asama volcano

(Photo by Karuizawa Weather Station, 15 September 2004)

Revised on 23 June 2005

Asama volcano, which is one of the most active volcanoes in Japan, erupted at 20:02 JST, 1 September 2004.
The first eruption was an explosive eruption at the summit crater and many blocks were ejected explosively.
Japan Meteorological Agency updated the Volcanic Activity Level as 3 (Eruption).

Fresh lava emerged in mid-September and continuous eruption took place in 15-18 September. Coordinating Committee for Prediction of Volcanic Eruption (CCPVE) was appealing cautions by saying that there is the possibility of more explosions.

Since the spring of 2005, seismicity and thermal activity have gradually declined, JMA updated the Level as 2 (Active) on 23 June. However, CCPVE commented "There is a possibility of eruptive activity in future, therefore, careful watch is still necessary" on the same day.

Major eruption of Asamayama in 2004

time date(JST)Air shock
(Pa at 8km)
20:02 1 SeptemberExplosion205Volcanic quake, tilt changeVolcanic bombs and ashfall
15-18 SeptemberContinuous Max 9.4Lava dome emerged Ashfall at Tokyo
19:44 23 SeptemberExplosion72.3Tilt change
12:17 29 SeptemberExplosion29.6Tilt change
23:10 10 OctoberSmall explosion18.9
20:59 14 NovemberExplosion73Volcanic quake, tilt changeVolcanic bombs and ashfall

Seismic and infrasonic records associated with the continuous eruption

Tilt change preceding explosion

In the present activity of Asama volcano, very small tilt changes were observed preceding some explosions.
At a tiltmeter set 2.5km distant from the summit, very small tilt changes were observed several hours to one day before some explosions. They are indicated as expansion of a pressure source beneath the active crater.
(Left figure)
Tilt change and hourly number of volcanic earthquakes. Small tilt change (the white arrow) was detected just before the explosion (the arrow). Please click the figure to see other examples.

Definition of Volcanic Activity Level of Asama of JMA

LevelVolcanic activityExamples
5Very large eruption
Pyroclastic flow and lava flow
1108, 1783
4Large eruption
Volcanic bombs far from the crater, pyroclastic flow, very large air-shock
1973,1958,1950 etc.
3Small-Middle sized eruption
Explosive eruption and its possibility
1983,1973 etc.
Volcanic quakes, high volcanic gas, very small eruptions
Low level of seismicity and fumarole
No sign of volcanic activity
The volcanic activity level will be revised to new alert level since December 2007. The difinition is different from this.

Sequence of the Level of Asama by JMA

2003.11. 4 / LEVEL 2 / Start of announcement of the Volcanic Activity Level by JMA
2004. 7.20 / LEVEL 1 / Low level of seismicity
2004. 7.31 / LEVEL 2 / High level of seismicity and volcanic gas emission
2004. 9. 1 / LEVEL 3 / Explosive eruption
2005. 6.21 / LEVEL 2 / Seismicity and volcanic gas emission declined
2006. 9.22 / LEVEL 1 / Seismicity and volcanic gas emission declined


Asama page of Earthquake Research Institute, Tokyo Univ.
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