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Crustal structure and seismicity beneath the forearc off northeastern Japan

Marine seismological and other data in the Japan Trench area (north of 38.5°N) were used to infer the relation of subducting plate structure, seismicity, and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting plates. Previous crustal models from ocean bottom seismographic refraction (OBS) surveys were improved by taking into account the relative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data. A wave speed discontinuity in these models is interpreted to be the contact zone of the crust of the overriding plate and the subducting Pacific plate crust. Its dip angle increases to about 7°, 110 km landward of the trench axis. A large increase beneath the deep sea terrace is required to reach the well-defined angle of 25° beneath the Tohoku east coast. A large wave speed gradient within layer 2, commonly observed under normal oceans, seems to vanish beneath the inner trench slope at about 10-km depth. Within the overriding plate, apparently brittle material with a P wave speed of ∼6 km/s can be found as near as 35 km from the trench axis. The upper limit of the seismogenic zone of interplate low-angle thrust events is about 15-km depth from OBS seismicity and large-event analyses. Both the strength of the crust of the overriding plate and the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates.