Three-dimensional seismic structure of subducting lithospheric plates under the Japan islands

Abstract

The three-dimensional seismic structure of subducting lithospheric plates under the Japan Islands has been investigated in detail by using an inversion method developed by Aki and co-workers. The present analysis clearly reveals the subduction of the Pacific plate, with a gradual narrowing in breadth down to about 600 km beneath the Sea of Japan, and also gives some indication of the Philippine Sea plate in the uppermost mantle, less than 50 km under the southernmost part of Japan. The upper boundary of the descending Pacific plate shows sharp velocity contrasts with respect to the overlying low-velocity zone, while the lower boundary appears to have a transitional nature. Large positive Bouguer anomalies in northeastern Japan may be explained, in part, by the effects of the subducting Pacific plate, but those over the Sea of Japan cannot be accounted for by lateral heterogeneities in the upper mantle.