Three-dimensional seismic velocity structure around the focal area of the 1978 Miyagi-Oki earthquake

摘要

We performed a 3D seismic tomography around the rupture area of the 1978 Miyagi-Oki earthquake ( M 7.4) using both onshore and offshore seismic station data. In our results, the subducting oceanic crust and the mantle wedge of the overriding plate were imaged as the landward dipping low P-wave velocity (similar to 7.0 km/s) layer and the high velocity (similar to 8.0 km/s) layer above it, respectively. Most of the earthquakes were relocated along the plate boundary, including the mainshock and the aftershocks of 16 August 2005 Miyagi-Oki earthquake (M 7.2). The relationship between spatial extents of the rupture areas of the 1978 and 2005 earthquakes and the velocity variation in the mantle wedge suggests that the heterogeneity of the upper plate delimits the extent of the rupture area of large (M > 7) interplate earthquakes.