The seismically quiescent boundary between the Philippine Sea plate and the Eurasian plate in central Japan

Abstract

The boundary between the Philippine Sea (PHS) and Eurasian (EUR) plates, which is presently seismically quiescent, is delineated on the basis of seismological data from the Kanto-Tokai (K-T) district during the period from January 1980 to April 1993. Most of the earthquakes in the K-T district occur either in the overriding plate (the EUR plate) or within the subducting slab (the PHS plate) at depths of less than around 30 km. The plate boundary between the PHS and EUR plates is characterized by a very low level of seismic activity; i.e., the plate boundary itself is presently seismically quiescent, and the seismicity consists of intra-plate events above or below the plate boundaries. In the Tokai district, earthquakes in the overriding (EUR) plate are mostly characterized by strike-slip faulting with an E-W-directed compressional axis, while those within the subducting (PHS) slab are characterized by strike-slip faulting with a N-S-directed compressional axis. In the Kanto district, the earthquakes are mostly characterized by faults with NW-SE-directed compressional axes. The above difference between the Tokai and Kanto districts is thought to be due to the difference in the slab configuration and/or regional stress conditions. Considering the results of the present paper together with geodetic data, the seismically quiescent plate boundary at depths from 10 to 30 km suggests strong coupling between the PHS and EUR plates, and is consistent with strain accumulation during an inter-seismic period.