HR: 1330h
AN: S52B-0139    [PDF]
TI: 3-D Attenuation structure beneath the Kanto District
AU: * Nakamura, R
EM: naka@tepsco.co.jp
AF: Tokyo Electric Power Services Co.,Ltd, 3-3-3 Higashiueno, Taito-ku, Tokyo, 110-0015 Japan
AU: Satake, K
EM: kenji.satake@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, Site C7 1-1-1 Higashi, Tsukuba, 305-8561 Japan
AU: Toda, S
EM: s-toda@aist.go.jp
AF: National Institute of Advanced Industrial Science and Technology, Site C7 1-1-1 Higashi, Tsukuba, 305-8561 Japan
AU: Uetake, T
EM: uetake.tomiichi@tepco.co.jp
AF: Tokyo Electric Power Company, 4-1, Egasaki-Cho, Tsurumi-Ku, Yokohama, 230-8510 Japan
AU: Kamiya, S
EM: kamiya@jamstec.go.jp
AF: Japan Marine Science and Technology Center, 2-15, Natsushima-Cho, Yokosuka, 237-0061 Japan
AB: Subsurface structure, both seismic velocity and attenuation, beneath the Kanto region is complex. The Pacific (PAC) plate subducts beneath Tokyo from the east and the Philippine Sea (PHS) plate subduct from southwest. Because of this complexity, three-dimensional (3D) velocity and attenuation structure should be examined and may be used for both reappraisal of historical earthquake sources and prediction of intensity distributions from future large earthquakes. 3-D attenuation structure for Japanese Islands was estimated from seismic intensity distribution (Hashida and Shimazaki, 1989, Tectonophysics). More detailed attenuation structure beneath the Tohoku region (Nakamura and Uetake, 2003), from a large number of strong-motion records, show that high Q in the Pacific plate and very low Q in volcanic zones. Here we report our estimate of 3-D attenuation structure estimated from dense seismic network. We obtained the Q structure at 10Hz in the Kanto area using strong motion records of the K-NET, KiK-net of National Research Institute for Earth Science and Disaster Prevention (NIED) and the 95-type seismometers of Japan Meteorological Agency (JMA). Study area is 138-$142\deg$E and 34-$38\deg$N. Block size is horizontally $0.2\deg$ and 30km in depth direction. Number of earthquakes and rays are 1649 and 22922, respectively, and 998 blocks are penetrated by the rays. We found several low Q zones, most of which correspond to the volcanic front. A significant NW-SE trend low-Q zone however lies under the non-volcanic Kanto area. This low-Q zone exists about $35.6\deg$N at the depth range of 0-30km, $36.0\deg$N at 30-60km and $36.4\deg$N at 60-90km. In the 3-D velocity model (Kamiya and Kobayashi, 2000, GRL), low-Vp and low-Vs with NW- SE trend and high Poisson_fs ratio are found at $36.0\deg$N at the depth of 40 km. The low-Q zone found in this study corresponds to this zone, and indicate wedge just above the subducting PHS plate. Such 3-D structure would play an important role to evaluate seismic intensity from earthquakes in the region.
DE: 5144 Wave attenuation
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7223 Seismic hazard assessment and prediction
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting