Abstract: Here we report our estimate of 3-D attenuation structure by dense seismic network. We obtained the Qs structure at 5Hz in the Kanto area using strong motion records of the K-NET and KiK-net of National Research Institute for Earth Science and Disaster Prevention (NIED). We found several low Qs zones, most of which correspond to the volcanic front. A significant NWSE trend low-Qs zone however lies under the non-volcanic Kanto area. This low-Qs zone exists about 36.0deg N at 20-60km. In the 3-D velocity model [Kamiya and Kobayashi, 2000], low-Vp and low-Vs with NW- SE trend and high Poisson’s ratio are found at 36.0deg N at the depth of 40 km. The low-Qs zone found in this study corresponds to this zone, and indicate wedge just above the subducting PHS plate.

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, threedimensional (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, 1989). More detailed attenuation structure beneath the Tohoku region (Nakamura and Uetake, 2004), from a large number of strong-motion records, show that high Qs in the Pacific plate and very low Qs in volcanic zones. Here we report our estimate of 3-D attenuation structure estimated from dense seismic network. We obtained the Qs structure at 5Hz in the Kanto area using large number of strong motion records.

Figure caption: Estimated Qs distribution at 20-40km (left) and 40-60km (right) in depth beneath the Kanto district. Thick solid line indicates the location of the cross section shown in Figure 3. Solid triangles denotes the active volcanoes and gray dotted line denote the upper boundary of the Philippine sea plate after Ishida[1992].