Neotectonics of Southwest Japan due to the right-oblique subduction of the Philippine Sea plate

Abstract

The forearc region of Southwest Japan is divided into five equivalent regions called structural units. Each unit is 120 to 150 km long and composed of a forearc basin fringed by an inverted L-shaped structural high extending from each of five promontories on the Pacific coast to the outer ridge zone along the Nankai Trough. The structural units coincide, in both dimension and location, with the source areas of plate-boundary carthquakes caused by low-angle thrusts with a right-lateral component. The topographic and structural features of the unit also correspond to the pattenr of crustal movements associated with the earthquakes. Form these facts, it is concluded that the structural units have been formed fundamentally by accumulation of coseismic crustal movements. The second Paleo-Seto inland sea, which existed east of the present-day Seto sea in Pliocene to Middle Pleistocene time, was composed of three right-handed en echeleon basins elongated in the NE-SW direction oblique to the Median Tectonic Line. The paleogeographic evidence shows that the second Paleo-Seto inland sea was formed as a right-lateral shear zone in association with the right-lateral strike-slip faulting of the MTL in the same manner as the present Seto inland sea. These characteristic neotectonic movement in the forearc region and along the MTL are attributed to the right-oblique subduction of the Philippine Sea plate at the Nankai Trough since the earliest Pliocene about 5 Ma.