Geometry and relative motion of the Philippine Sea Plate and Pacific Plate beneath the Kanto‐Tokai District, Japan

Abstract

The recent acquisition of high‐density and improved seismic data by the Kanto‐Tokai (K‐T) Observational Network of the National Research Institute of Earth Science and Disaster Prevention requires the revision of conventional plate configuration models for the K‐T district. We propose a new interpretation of the configuration and relative motion of the Philippine Sea (PHS) plate, Pacific (PAC) plate, and Eurasian (EUR) plate on the basis of the distribution of hypocenters, velocity structure, and focal mechanisms in the K‐T district. The new model for the configuration of the PHS and PAC slabs clearly delineates the PHS slab subducting northwestward beneath the K‐T district from the Sagami and Suruga troughs and the PAC slab subducting westward beneath the PHS slab from the Japan trench. The thickness of the descending PHS slab is estimated to be 30 ± 5 km. The proposed configuration of the PHS slab is significantly different from those obtained by other researchers, including our own previous study. The new model is fully consistent with global plate motion data. The new model leads to an interpretation of the tectonic processes taking place beneath the K‐T district. For the Sagami trough wing of the PHS slab, the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the PAC slab; the southwestern portion carrying the Izu Peninsula has been bent upward, because of its buoyancy, producing the observed contorted shape. For the Suruga trough wing, the dip direction of the PHS slab gradually changes from northwestward at the eastern portion to northward at the western portion. The new model also indicates the presence of offset within the PHS slab between the Sagami and Suruga trough wings. This area is characterized by a low level of seismic activity, the absence of a high‐velocity structure, and the presence of Quaternary volcanoes. It seems reasonable to speculate that the distribution of Quaternary volcanic edifice construction is controlled by the location of the offset within the PHS slab. In view of this new model, the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined.