(Photo by Karuizawa Weather Station, 15 September 2004) Japanese 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) Precursor Remarks 20:02 1 September Explosion 205 Volcanic quake, tilt change Volcanic bombs and ashfall 15-18 September Continuous Max 9.4 Lava dome emerged Ashfall at Tokyo 19:44 23 September Explosion 72.3 Tilt change 12:17 29 September Explosion 29.6 Tilt change 23:10 10 October Small explosion 18.9 20:59 14 November Explosion 73 Volcanic quake, tilt change Volcanic 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 Level Volcanic activity Examples 5 Very large eruption Pyroclastic flow and lava flow 1108, 1783 4 Large eruption Volcanic bombs far from the crater, pyroclastic flow, very large air-shock 1973,1958,1950 etc. 3 Small-Middle sized eruption Explosive eruption and its possibility 1983,1973 etc. 2 Active Volcanic quakes, high volcanic gas, very small eruptions 1 Calm Low level of seismicity and fumarole 0 Dormant 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 LINKS Asama page of Earthquake Research Institute, Tokyo Univ. Back to home