Configuration of the Philippine Sea Slab and the Seismic Activity in the Tokai Region

The configuration of the Philippine Sea plate subducted under the Tokai region has been investigated using JMA hypocentral data in recent years. Characteristics of the shape obtained by this study are almost the same as the previously proposed models based on seismic data of Nagoya University and NIED, respectively, for the region to the east of Ise Bay. That is, in the area to the west of Suruga Bay, the slab deepens to the west, while it inclines to the north around Lake Hamana, and a valley-like configuration is observed near Omaezaki Point and its north-west region. The deepest portion of the Tokai slab exists beneath southeastern Gifu Prefecture. We calculated the gradient vector of the subducting slab at each point on the surface using the shape of the model. By comparing the gradient vector field with the seismicity in the slab, we found a coincidence between the regions of high seismicity and the zones where the gradient vector changes greatly. This coincidence suggests the relevance between the bending of the Philippine Sea plate and the occurrence of earthquakes in the slab. On the basis of the observation we propose that one of the main causes generating seismogenic stress in the slab in the Tokai region is conformation of the Philippine Sea plate to the shape of the subducted slab in the course of plate motion