Slab Structure beneath the Japanese Islands and Earthquake Generation

Abstract

A dense nationwide seismic network recently constructed in Japan has been yielding large volumes of high-quality data that have made it possible to investigate the seismic structure in the Japanese subduction zone with unprecedented resolution. We introduce a precise configura- tion of the Philippine Sea and Pacific plates subducting beneath the Japanese Islands, which was recently obtained by seismic tomographic imaging, precise earthquake hypocenter determina- tions, and focal mechanism studies. Seismic tomographic studies show that the Philippine Sea plate subducting beneath southwest Japan is continuous throughout the entire region, from Kanto to Kyushu, without disruption or splitting even beneath the area north of the Izu Penin- sula. The estimated geometry of the subducted Pacific and Philippine Sea slabs shows a broad contact zone between the two slabs located directly beneath the Kanto plain. It further shows the wavy configuration of the Philippine Sea slab subducting beneath the entire region of south- western Japan. Contact between the Philippine Sea plate and the Pacific plate causes anoma- lously deep interplate and intraslab earthquake activity in Kanto. Moreover, the interplate cou- pling coefficient estimated from repeating earthquake data shows a distinct change across the northeastern edge of this slab contact zone, suggesting that the overlying plate controls large- scale interplate coupling. High-resolution studies of spatial variations of intraslab seismicity and the seismic velocity structure of the slab crust strongly support the dehydration embrittle- ment hypothesis for the generation of intraslab earthquakes.