HR: 1340h
AN: S43A-1035    [Abstracts]
TI: Small Repeating Earthquakes Found at Kusatsu-Shirane Volcano, Japan
AU: * Yamawaki, T
EM: yamawaki@ksvo.titech.ac.jp
AF: Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Nogami, K
AF: Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Ohba, T
AF: Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Ogawa, Y
AF: Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Hirabayashi, J
AF: Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan
AB: Kusatsu-Shirane volcano is situated in the central part of Japan. Its major activity in the past 3000 years consists solely of recurrent phreatic explosions around the craters (Yugama, Mizugama and Karegama, etc). Recent explosions occurred in 1976, 1982, 1983 and 1989. Even now the volcano maintains fumarolic and seismic activity around the craters. We have monitored the seismic activity of the volcano with several stations including three borehole ones since 2001. Good signal-to-noise ratio data at borehole stations enable us to locate dominant earthquakes as small as M~ -2. Previous studies revealed two seismic clusters, one located under Yugama and Mizugama, and the other located to the south of these craters. It was identified at various tectonic settings, such as geothermal areas and subduction zones, that there exist repeating earthquakes (doublets and multiplets) which are repeatedly observed as similar waveforms. Such events are very useful to investigate small temporal changes in the crust which are caused by volcanic and/or tectonic activities, and can be an important tool to monitor volcanic activity. Therefore we search for seismic doublets and multiplets from very small earthquakes at Kusatsu-Shirane volcano. We analyze ~1600 earthquakes which occurred between July 2003 and May 2007. At each station, cross-correlation of seismic waveform is examined for all event pairs by using 2-second time window which includes P and S arrivals. Similar event pairs are picked up based on the maximum cross-correlation coefficient (≥ 0.8) at borehole stations and from these pairs are formed groups of repeating earthquakes if different pairs share the same event. We find three major groups (hereafter referred to as A, B and C) of repeating earthquakes which occur recurrently over the whole period and each group has more than 100 events. The groups A and B are found in the Yugama-Mizugama cluster. These groups are classified as high-frequency and low-frequency repeating earthquakes. Finally, the group C is found in the south cluster. The earthquakes are high-frequency events. These groups probably occur along the major magma-plumbing system under the volcano. We are also planning to incorporate results of geochemical observations such as fumarolic gas and lake water of the Yugama crater.
DE: 7280 Volcano seismology (8419)
DE: 8419 Volcano monitoring (7280)
SC: Seismology [S]
MN: 2007 Fall Meeting