日本列島周辺の海底地質 その4 日本海

抄録

Topographies of the Japan Sea can be divided roughly into three parts; the continental borderland lying from Hokkaido to San'in Region, the Yamato Ridge and the Korea Plateau of the continental relic, and the deep-sea basin in the northern Japan Sea. The deepest part of the Japan Sea is about 3700m deep located near the eastern margin of the Japan Basin. The age of the Japan Sea is estimated to be younger than 30Ma. The recent topographic features of the Japan Sea have been built mainly by the large-scale deformation and reorganization of topography since late Pliocene. The continental borderland, east of the Toyama Trough, in the northeastern area has very complicated topographies with folding and faulting. In contrast to it, the continental borderland of southwestern area has been comparatively calm in structural activity since late Pliocene. Erosional terraces in late Pliocene to early Pleistocene are reserved around the summit of the Yamato and Oki Ridges and on the basement covered by Quaternary strata forming a marginal terrace. The eastern margin of the Japan Sea is structurally the most active zone, where the lithospheric convergence occurs by the strong compression. The main belt of convergence in the eastern margin of the Japan Sea can be traced by the distribution of epicenters and aftershock areas of great earthquakes of M7 class along the Okusiri Ridge, the Mogami Trough, and the valley of the Sinano River.