HR: 0800h
AN: S31A-0217    [Abstracts]
TI: The Metropolitan Seisimic network (MeSO-net) for Detection of Mega-thrust and Intra-slab Earthquakes beneath Tokyo Metropolitan Area, JAPAN
AU: * Kasahara, K
EM: kkasa@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Hirata, N
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Sakai, S
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Morita, Y
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Nakagawa, S
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Sasaki, S
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Obara, K
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki, 305-0006, Japan
AU: Tanada, T
AF: Hot Springs Research Institute of Kanagawa Prefecture, 586, Iryuda, Odawara-shi, Kanagawa, 250-0031, Japan
AB: In central Japan the Philippine Sea plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region, where it causes mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9). Assessment in Kanto of the seismic hazard produced by the Philippine Sea Plate (PSP) mega-thrust earthquakes requires identification of all significant faults and possible earthquake scenarios and rupture behavior, regional characterizations of PSP geometry and the overlying Honshu arc physical properties (e.g., seismic wave velocities, densities, attenuation), and local near-surface seismic site effects. The Kanto Basin is covered with thick Neogene to Quaternary sediments interfering precise observations. Furthermore culture noises in the Kanto region are quite high because of the highly concentrated population. Those environments prevent us to make a clear image of the PSP plate until now. To achieve stable high sensitivity seismic observation(Hi-net) avoiding surface ground noise, seismographs are installed at the bottom of the borehole of more than 100m depth(maximum 3,500m). The average interval of Hi-net over the Tokyo metropolitan area is, however, about 25km, which is not sufficient to image the internal structure of depicts of the PSP. The velocity structure in the Kanto region is elucidated by a temporary highly dense seismic array observation in the Boso peninsula using double-difference tomography. (Hagiwara et al., 2006) The results of this array revealed detailes layered structure: there are an oceanic crust and the slab of the Philippine Sea plate. The configuration of the top and bottom boundaries of the Philippine Sea Plate colliding with the Pacific plate was elucidated. It was shown that, although containing large noises, the highly dense array data enables us to depict a detailed underground structure. Thus we proposed highly dense seismic array observation in the Tokyo metropolitan area. This array is named the MeSO-net (Metropolitan Seismic Observation network ) with 400 stations and 2-5km station interval under the Special Project for Earthquake Disaster Mitigation in the Metropolitan Tokyo area (2007 - 2011), which we have just started. Deployment of the MeSO-net will be completed in four years (2007-2010). To achieve stable seismic observation avoiding surface ground noise by an easy method, seismographs are installed at the bottom of a borehole of 20m depth. The MeSO-net will give an accurate estimation of the plate boundaries of the PSP and the Pacific plates under the metropolitan area, resulting in clear understanding of the relation between a deformation of PSP and intra-slab M7+ earthquakes. The MeSo-net will also present a high resolution tomographic image to show a low velocity zone which may a possible internal failure of the slab, i.e., a possible source region of the M7+ intra-slab earthquake. Our Special Project will contribute the next assessment of the seismic hazard in the Tokyo metropolitan area.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting