We determined the deep seismic structure of the seismogenic plate boundary zone of off-Sanriku, the northern part of the Japan Trench inner slope area. Travel time data from aftershocks of the 1994 Far-off Sanriku earthquake (M = 7.5) recorded both by ocean bottom seismographs and the local land seismic network enable us to image the three-dimensional seismic velocity structure of the region. Along the plate boundary, there exists a P-wave low-velocity layer of ca. 7 km s-1 and with thickness <10 km as the topmost layer of the subducting Pacific plate. The present analysis indicates that this layer, interpreted as the subducting oceanic crust, continues without any velocity variations at least down to ca. 40 km depth. In the overriding plate, we found a zone of considerable high P-wave velocity, ca. 8 km s-1, between the Pacific coastal line of the northeastern Japan arc at the longitude of 142.5°E. This manifests that the trenchward toe of the mantle wedge has significantly higher velocity than that beneath the land area of the arc. The landward limit of the high P-wave velocity toe corresponds to the down-dip limit of the seismogenic plate boundary. Further to the east, the P-wave velocity is ca 6 km s-1. The structure image obtained indicates that the nature of the overriding plate is significantly different along the plate boundary. In the eastern half of the seismogenic zone, the island arc crust and subducting oceanic crust are in contact with each other, whereas the high-velocity mantle wedge makes up the hanging wall of the deeper part of the seismogenic zone. (C) 2000 Elsevier Science B.V. All rights reserved.
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