Three-dimensional velocity structure beneath the Kanto district, Japan.

Abstract

Three-dimensional velocity structure under the Kanto district, Japan, is determined by inversion of the P-wave arrival time data from the earthquakes occurring under this district using the method originally due to AKI and LEE (1976). It is found that the crustal structure (0-32km depth) is dominated by a low velocity zone centered in Tokyo Bay, which coincides with the low Bouguer anomaly as well as the thick surficial layer. In the mantle just beneath the crust (32-65km depth), the velocity is high in northeastern and southwestern Kanto and is low in northwestern Kanto and the northeast part of Chiba prefecture. The velocity variation in this depth range is estimated to be 6-7% (±2.5%). At depths of 32-65km, the northeast high velocity zone corresponds to the subduction of the Pacific plate. The southwest high velocity zone correlates with the seismic zone inclining from the Sagami trough toward the northeast. It has been suggested that this seismic zone corresponds to the subduction of the Philippine Sea plate from studies on hypocenter distribution, seismic intensity distribution and focal mechanism solutions. The high velocity characteristic found in the present study confirms this suggestion. The low velocity zone beneath the northeast part of Chiba prefecture roughly coincides with the low-Q zone found in the spectral study of shear waves. We suggest that this low-V, low-Q zone is caused by the downward bending of the Pacific plate.