Three-Dimensional Velocity Structure and Precise Hypocenters in the Nikkou-Ashio Area, Central Japan Revealed by the DD-Tomography Method

HR: 0800h
AN: S51A-0989 [Abstracts]
TI: Three-Dimensional Velocity Structure and Precise Hypocenters in the Nikkou-Ashio Area, Central Japan Revealed by the DD-Tomography Method
AU: * Hagiwara, H
EM: hagiwara@eri.u-tokyo.ac.jp
AF: Earthquake research Inst. Tokyo Univ., Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Sakai, S
S51A-0989 AF: Earthquake research Inst. Tokyo Univ., Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Igarashi, T
S51A-0989 AF: Earthquake research Inst. Tokyo Univ., Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: The Nikko-Ashio area, Central Japan, is one of the most seismically active regions in Japan._@It locates on the volcanic front and a couple of volcanoes and faults exist. We detect a large amount of earthquakes about 8,000 a year by routine observation. The specific characteristics of the activities are as follows: 1) Shallow earthquakes occur within 3-10 km depth. 2) Those earthquakes separate into clusters. 3) Remarkable reflectors are detected about 15 km depth. 4) Deep low frequency earthquakes occur beneath that area. In this report we investigate the relationship between those seismic activities and velocity structure. We have clarified three-dimensional velocity structure in the Nikkou-Ashio area in January, 2002-March, 2005 from DD-tomography method applied to 76,580 absolute times and 656,369 double differences by routine observation at 57 stations in and around the Nikkou-Ashio area. Applying this velocity model to all arrival time data in January, 1991-June, 2005, we have also obtained precise hypocenters from DD-tomography method. The results indicate that a clear velocity image from the top of the crust to the upper-most mantle and the connection between the velocity structure and hypocenters. 1) Micro-earthquakes occur in the narrow range of 5-10 km depth. In that seismogenic zone, P-velocity is relatively high ( 5.8-6 km/sec ) inserted into two low velocity zones, the upper-most crust and the middle crust. 2) Low velocity dominates in 20-30 km depth. 3) About 30 km depth, in which deep low frequency earthquakes rise, S-velocity is extremely low and the ratio of Vp/Vs is high.
DE: 7200 SEISMOLOGY
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7270 Tomography (6982, 8180)
DE: 9320 Asia
SC: Seismology [S]
MN: Fall Meeting 2005