HR: 0800h
AN: V21C-0623    [Abstracts]
TI: Magma Supply Path Beneath Mt. Asama Volcano, Japan
AU: * Takeo, M
EM: takeo@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1,Yayoi,Bunkyo, Tokyo, 113-0084 Japan
AU: Yamamoto, M
EM: makkie@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1,Yayoi,Bunkyo, Tokyo, 113-0084 Japan
AU: Ohminato, T
EM: takao@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1,Yayoi,Bunkyo, Tokyo, 113-0084 Japan
AU: Aoki, Y
EM: yaoki@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1,Yayoi,Bunkyo, Tokyo, 113-0084 Japan
AB: On September 1, 2004, a moderate eruption occurred at Mt. Asama, in central Japan, for the first time in the last 21 years. An earthquake swarm started on August 31, 2004, lasting just before the eruption. After this first eruption, 4 moderate eruptions had occurred by the end of November, 2004. Hypocenters of more than three hundreds volcanic earthquakes occurring from January 1, 2004 to January 31, 2005, were relocated using the double-difference earthquake location algorithm [Waldhauser, F. and Ellsworth, W. L., 2000] to get an accurate hypocenter distribution. The relocated hypocenter distribution reveals a sharp image of seismicity composed of two groups. The one group forms an WNW-SSE striking zone at a depth of 1 km to 1.5 km deep below the sea level. The eastern end of this seismic zone lies about 3 km deep beneath the summit crater and extends westward over 2 km in length. The other group forms a narrow vertical seismic zone extending from the eastern edge of the first group to just under the summit crater. Aoki et al. [2005] reveals that dike intrusions occurred several times before and after the summit eruption based on the GPS data. The east parts of intrusive dikes are overlapped with the first seismic group. Moreover, the top depths of these dikes coincide with the depth of the first group. The hypocenters of the swarm, which occurred just before the eruption, were also determined using the double-difference method. These hypocenters lie just under the vent ranging from 300 m to 800 m deep, suggesting that the shallowest part of the vent is broken during the swarm activity. These facts indicate that the hypocenter distribution before and after the eruption represents the magma supply path beneath Mt. Asama volcano. Before the eruption, long-period volcanic earthquakes were recorded using broadband seismographs located at the summit of Mt. Asama since October 17, 2003. The sources of these long-period events are located relatively shallow at a depth of a few hundreds meters, just under the crater. The activity of these long-period events had been synchronized with the activity of volcanic earthquakes until the last ten days of July 2004. However, the activity had dropped gradually, and no event has been observed since August 24, 2004, just 8 days before the first eruption.
DE: 7280 Volcano seismology (8419)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005