Geophysical monitoring of Mt. Asama, Japan, during the 2008-9 eruptions (Invited)
Abstract
Mt. Asama is one of the most active volcanoes of the Japanese islands with recent eruptions in 2004, August 2008, and February 2009. Here we give an overview of the 2008 and 2009 eruptions and the associated phenomena in particular emphasis on ground deformation. Seismic activity of Asama has been low after the 2004 eruptions before starting to increase in July 2008, leading to minor eruptions in 10 and 12 August 2008. In particular, the number of volcanic earthquakes at a depth of several hundred meters below sea level to approximately 1 km west of the summit has significantly been increased since July 2008. In addition, the seismic activity to approximately 3 km west of the summit at a depth of about 1 km below sea level has been reawakened in September, 2008, after a dormancy since late 2005. These observations imply that the magma is fed from depth since July, 2008. The ground deformation field observed at contnuous GPS sites supports the hypothesis above. It shows that the extension to the west of the summit occurred between July 2008 and June 2009 with a quasi-constant rate. This observation is explained by dike intrusion to the west of the summit, as was observed during the 2004 eruptions. The location of the diking is imaged as a high velocity zone formed by repeating intrusions. Note, however, that the magnitude of the extension is up to 15 mm, less than a half of that in 2004 and the volume of the intruded dike is about 3 million cubic meters, also less than half of that associated with the 2004 eruptions. In addition, a GPS site located at the eastern rim of the crater marks the eastward deformation reaching up to about 30 mm during the latter half of 2008, which is impossible to explain by the dike intrusion to the west of the summit. This deformation indicates the presence of a shallow pressure source. The pressure source must be shallow, say, 1000 meters beneath the surface (1500 meters above sea level), because a next closest site (4 km from the summit) is unaffected by this pressure source. This pressure source is identified for the first time due to the installation of the continuous GPS site at the summit.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V32B..01A
- Keywords:
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- 1243 GEODESY AND GRAVITY / Space geodetic surveys;
- 8419 VOLCANOLOGY / Volcano monitoring