06_3

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6-3. Seismic observations at the plate boundaries around Japanese islands

　The plate boundaries at the subdution zones around Japanese islands have generated destructive earthquakes in past. To study the earthquake generation mechanism in viewpoints of physical properties and role of water at the plate boundaries, we have carried out seismic experiments using ocean bottom seismometers since 1995. At the Nankai Trough, we found the structural discontinuity between the 1944-Tonankai and the 1946-Nankai earthquakes. At the Japan Trench, we observed strong seismic reflections from the plate boundary at 38°40′and 39°N for the aseismic region (Fig. 1). This observation can be interpreted by existence of fluid, which might release strains at the plate boundary. By the experiment at the Izu-Bonin Trench, we estimated the sepentinized mantle wedge (Fig. 2). The serpentine at the plate boundary reduces occurrence of large earthquakes. By the experiment at 39-41°N of the forearc slope of the Japan Trench, we found the structural discontinuity at the lower crust at 40°N, where coincides to the major moment release regions for the 1968- Tokachi- Oki and the 1994- Sanriku-Haruka- Oki earthquakes.

Fig.1. Seismic reflections were intense for the aseimic zone located at 38°40′-39°N.

Fig.2. Serpentinized materials with low seismic velocities seem to exist at the Izu-Bonin subducting plate boundary of 31°N.

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