A LARGE-SCALE THREE-DIMENSIONAL SEISMIC STRUCTURE UNDER THE JAPAN ISLANDS AND THE SEA OF JAPAN

Abstract

A large-scale three-dimensional seismic velocity structure down to 650 km under the Japan Islands and the Sea of Japan has been investigated to clarify the subduction of lithospheric plates by applying the inversion method of Aki and others. About 3500 data of P-wave travel time residuals observed at 330 Japanese and foreign stations from 20 intermediate and deep-focus earthquakes around Japan were used to determine the velocity anomalies in a number of blocks and source corrections simultaneously. This analysis has well revealed lateral heterogeneities in the upper mantle above 450km. A high velocity zone corresponding to the descending Pacific plate has been clearly identified, particularly in the north Honshu arc, indicating good correlations with the horizontal and vertical distributions of a number of earthquakes around there. It was found that intermediate and deep-focus earthquakes take place mainly in the upper portions of the descending plate with a thickness of about 100km. The highest velocity anomalies are +5-6%(50-250km), +3%(350km), +2%(450km) within the plate, and about -2% down to 350km over a region on the continental side of the descending plate.