Configuration of subducting Philippine Sea plate and crustal structure in the central Japan region

A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3-5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0-6.4 and 6.6-6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20-35 km. The dip angle of the Philippine Sea plate was estimated to be 26degrees for a depth range of about 20-26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26-33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20-26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes. (C) 2004 Elsevier B.V. All rights reserved.