HR: 0800h
AN: S51A-0133    [Abstracts]
TI: Heterogeneous Crustal Structure in the Focal Area of the 2000 Western Tottori prefecture Earthquake
AU: * Nakagawa, S
EM: shigeki@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1, Tennodai, Tsukuba city, Ibaraki pref., 305-0006 Japan
AU: Kawamura, T
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hirata, N
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Sato, H
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kurashimo, E
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Iidaka, T
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kato, A
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kasahara, K
AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1, Tennodai, Tsukuba city, Ibaraki pref., 305-0006 Japan
AB: On October 6, 2000, a large earthquake (Mw 6.6) occurred in the western part of Tottori prefecture, Honshu, Japan. Although the earthquake is a large intra-island-arc type event, a surface rupture is not clearly observed. The generation mechanism of such an intra-island-arc earthquake has not been clarified yet. The heterogeneous crustal structure is one of the key points to understand these earthquakes. To reveal the heterogeneous crustal structure in the fault area of this earthquake, we conducted earthquake observations in twice. In October 2000, right after this large earthquake, an urgent aftershock observation was carried out (Shibutani et al., submitted to EPS). As a part of this, we deployed a multi-channel seismic (MCS) array along and across the main fault area. The MCS array was operated for 85 hours to obtain quasi-continuous records of aftershocks, and 269 events were recorded by the MCS array. In April 2002, a seismic reflection survey was carried out along the fault (Nishida et al., 2002). At the same time, we conducted a seismic array observation using off-line seismic recorders in the same area to image the crustal structure. The array consisted of 145 seismometers distributed across a southeastern part of the main fault. The array was operated for about one month and 259 events were recorded. The common midpoint (CMP) reflection method is widely used to image the heterogeneous crustal structures. The sources and receivers are located at or near by the surface in the CMP method. Since the natural earthquakes are not located on the surface of the earth, we cannot use the usual CMP method. Therefore, we proposed a new method, the natural earthquake reflection profiling (NERP) method, to image the crust using natural earthquakes. By the NERP method, we calculate a common reflection point for sources in the subsurface and receivers on the surface to transform original data into zero-offset depth section. This method strongly depends on the hypocenter and origin time of earthquakes and the background velocity structure. To estimate these parameters accurate enough, the Joint Hypocenter Determination method was applied. We applied the NERP method to the observed seismic data by assuming both PP and SS reflections. Since both PP and SS profiles image the reflectors at the same depth, we can interpret that the reflectors are not ghost reflection images but real reflectors. The results are follows: Above a depth of 5 km no image is found because of few aftershocks in a shallow depth. Between depths of 5 and 9 km are several reflectors. Between depths of 9 and 14 km, the reflectors are not clearly found. At the depth of greater than 14 km, many clear reflectors are imaged. In the depth range between 5 and 9 km, some less-reflective zones are found. A very small slip occurred on the fault plane during the main shock in these less-reflective zones. Compared with the aftershock distribution and recent swarm-like seismic activities in 1989, 1990 and 1997, no earthquake occurred in these less-reflective zones. The P wave velocity in these less-reflective zones is slightly slower than that in the neighbor area. Those characteristics indicate that brittle fracture does not occur in the less-reflective zone but stably slip deformation is dominant there. It is possible that the stably slip areas are distributed in the upper crust, which may contribute the major characteristics of the 2000 Western Tottori prefecture earthquake.
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 7209 Earthquake dynamics and mechanics
DE: 0935 Seismic methods (3025)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting