CRUSTAL STRUCTURE AND SEISMICITY BENEATH THE FORE-ARC OFF NORTHEASTERN JAPAN

Marine seismological and other data in the Japan Trench area (north of 38.5 degrees N) were used to infer the relation of subducting plate s tructure, seismicity, and focal mechanisms especially at the aseismic to seismogenic zone along the interface between the two interacting pl ates. Previous crustal models from ocean bottom seismographic refracti on (OBS) surveys were improved by taking into account the relative sei smic amplitude characteristics and constraining the shallow structure using multichannel seismic data. A wave speed discontinuity in these m odels is interpreted to be the contact zone of the crust of the overri ding plate and the subducting Pacific plate crust. Its dip angle incre ases to about 7 degrees, 110 km landward of the trench axis. A large i ncrease beneath the deep sea terrace is required to reach the well-def ined angle of 25 degrees beneath the Tohoku east coast. A large wave s peed gradient within layer 2, commonly observed under normal oceans, s eems to vanish beneath the inner trench slope at about 10-km depth. Wi thin the overriding plate, apparently brittle material with a P wave s peed of similar to 6 km/s can be found as near as 35 km from the trenc h axis. The upper limit of the seismogenic zone of interplate low-angl e thrust events is about 15-km depth from OBS seismicity and large-eve nt analyses. Both the strength of the crust of the overriding plate an d the characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates.