HR: 1340h
AN: S13D-1103    [Abstracts]
TI: Crustal Structure and Micro-seismic Activity Along the Atotsugawa Fault System, Central Japan
AU: * TAKEDA, T
EM: takeda@ni.aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: KUWAHARA, Y
EM: y-kuwahara@aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: MIZUNO, T
EM: takashi.mizuno@aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: IMANISHI, K
EM: imani@ni.aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: OKADA, T
EM: okada@aob.geophys.tohoku.ac.jp
AF: Tohoku University, Research Center for Prediction of Earthquakes and Volcanic Eruptions Graduate School of Science, TOHOKU UNIVERSITY Aoba-ku, Sendai, Miyagi, 980-8578 Japan
AU: ITO, K
EM: ito@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: WADA, H
EM: hiroo@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: HARYU, Y
EM: haryu@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1,Tennodai,Tsukuba-shi, Ibaraki, 305-0006 Japan
AB: Detailed seismic velocity structure along the Atotsugawa fault system, central Japan, was obtained to infer the structure effect on microseismic activity and fault creep movement. The Atotsugawa fault system consists of subparallel, right-lateral and strike-slip faults (e.g. the Atotsugawa fault, the Ushikubi fault) trending ENE-WSW. The baseline survey indicates a possibility of creep movement of 1.0-1.5mm/year in the middle part of the Atotsugawa fault, where the microseismic activity is very low comparing the other parts of the fault. Thus we clarify crustal structure in the whole fault system by seismic tomography, especially the structure effect on seismic activity along the fault or fault creep. We applied the Double-Difference Tomography method (Zhang and Thurber, 2003), which can make inter-hypocenter structure clear. We used data of local routine network and dozens of temporary stations that were deployed especially in the vicinity of the fault system. In order to avoid local concentration of ray paths, we adjusted the number of used hypocenter data so as to equalize seismic distribution, but except the fault system area. Consequently the number is 522, in particular 287 along the fault system. After tomography analysis, we checked resolution of the result and confirmed to be reliable down to 12km deep in the middle of the fault system. We summarize features of the obtained results as follows. In the cross section along the Atotsugawa fault, there is low velocity zone below 10km deep, which continues from middle lower part of the fault up to near-surface of the eastern fault edge. The micro-earthquakes correspondingly distributes along the upper boundary of the low velocity zone. The western part of the fault has a little higher Vp/Vs ratio than the eastern part, which is corresponding to the higher seismicity. The fault system does not have large-scale low velocity structure along the whole fault system. By several synthetic tests, it is sure that the width is less than at least several kilometers even if such a large-scale low velocity structure exists. In the plane view, low velocity area is localized in the shallow part (down to $\sim$2-4 km) between the Atotsugawa and the Ushikubi fault, which is in close vicinity to the creep area. From the obtained velocity structure, we cannot find the evidence of the Atotsugawa fault creep.
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting