HR: 0800h
AN: V21C-0618    [Abstracts]
TI: Precise Hypocenter Determination for Volcanic Earthquakes Observed at Asama Volcano, Japan
AU: * Oikawa, J
EM: oikawa@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ida, Y
EM: yida@sci.u-hyogo.ac.jp
AF: Graduate School of Life Science, Univ. of Hyogo, 2167 Shosha, Himeji, Hyogo, Himeji, 671-2201 Japan
AU: Tsuji, H
EM: htsuji@eri.u-tokyo.ac.jp
AF: ERI, Univ. of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Hypocenters of volcanic earthquakes in Asama volcano were reexamined and precisely determined using additional four seismic stations that had been installed close to the summit crater from 1996 to 1999. The most significant result obtained in the present study is that the group of shallow earthquakes beneath the summit crater (B-type and N-type earthquakes), which show broad distribution within the depths of 2.5 km according to the old calculation without the four summit stations, are compactly concentrated into a shallower range within the depths of 600 m underneath the west side of the crater. In addition to the conventional method of the hypocenter determination, the double difference method was applied but its application turned out not to have a significant influence on the result. The summit stations were also quite effective in constraining the seismic velocity structure and the P wave velocity of Asama volcano was determined to be 3.5 km/s so as to minimize the variance between the observed and calculated travel times. Most volcano-tectonic earthquakes (A-type earthquakes) are distributed near the sea level under the west flank of the volcano. The precise hypocenter determination has revealed that there is a seismic gap between the shallow and flank earthquake groups. It is inferred that this seismic gap may represent a magma reservoir because the gap agrees with the location of the pressure source that was estimated from a previous study of the ground deformation.
DE: 7280 Volcano seismology (8419)
DE: 8419 Volcano monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005