HR: 0800h
AN: S31A-0200    [Abstracts]
TI: Seismic velocity structure and hypocentral distribution in the Western Nagano prefecture by using the dense seismic network data
AU: * Noda, S
EM: shunta@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Gokasho, Uji, 611-0011,
AU: Iio, Y
EM: iio@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Gokasho, Uji, 611-0011,
AU: Sekiguchi, S
EM: seki@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, 305-0006,
AU: Horiuchi, S
EM: horiuchi@smtp.bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, 305-0006,
AB: We carried out the seismic tomography to investigate the reason why the earthquake swarm occurs in the hypocentral region of the 1984 Western Nagano prefecture earthquake (Mj6.8). The seismic activity of the earthquake swarm is very high from 1976 to the present. A dense seismic network was installed in this region at 1995. The sampling frequency of this recording system is 10 kHz. The initial number of stations is 6. However, the number of stations gradually increased, the maximum number is 57. The average station spacing is 1 km or 2-4 km near the center or in the periphery of the network, respectively. We used 14,226 events that occurred between October 1995 and February 2005. The number of travel time data of P and S waves is 250,212 and 209,656, respectively. All arrival times are manually picked. The accuracies of P and S arrival times are about 1 ms and 30 ms, respectively. These accuracies are quite high because of the 10 kHz sampling frequency and good signal-to-noise ratio. We conducted an initial hypocenter determination and the 1 dimensional inversion before the 3 dimensional inversion (Seismic tomography), then we obtained more precise hypocentral distribution and velocity structure. The grid intervals in the 3 dimensional inversion are 1km*1km*1km in the vicinity of the area where most of events occur. We used the Pseudo Bending method (Um and Thurber, 1987) and the LSQR method (Paige and Saunders, 1982) for the ray tracing and the matrix calculation, respectively. To estimate the resolutions at each grid point, we carried out the checker board resolution test. We found that most of events are distributed along a lot of planes or lines. Those planes or lines decline almost the same dip angle or perpendicular to the dip angle. The relationship between the hypocentral distribution and the velocity anomalies shows a few distinct features as follows. First, the hypocenters are located around the upper part of the low P wave velocity anomaly. Second, the hypocenters adjoin to the low P and S wave velocities anomaly. Some velocity anomalies seem to reflect the existence of gas or fluid. Thus, the gas or fluid is related to the occurrence of the earthquake swarm in the Western Nagano prefecture.
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1219)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7270 Tomography (6982, 8180)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting