日本列島周辺の海底地殻構造 地殻構造から見た海溝‐島弧‐背弧系

Abstract

The results of the recent seismic refraction surveys using an Ocean Bottom Seismograph (OBS) array have clarified fine crustal structures of the trench=island-arc= back-arc systems around the Japan Islands, namely the Kurile, the Northeastern Japan, the Southwestern Japan, the Izu-Ogasawara and the Ryukyu. The present paper tries to look over theseismic velocity structures of the crust and of the uppermost mantle beneath the trenches, the island arcs and the back-arc basins revealed by the OBS studies in the last decade.New OBS data have shown regional variations in the crustal structures of the subduction zones. The crustal structures around trenches vary not only from a trench to another but also along one trench. The dip angle of a subducting oceanic lithosphere, the spatial extent of an accretionary prism and the shallow structure of a continental slope characterize each subduction zone and are deeply related to its tectonic evolution. Interesting similarity and dissimilarity are found between the structures of the Bonin Trough, a intra-arc rift, and of the Okinawa Trough, a back-arc rift, providing a clue to understanding of the mechanism of the rifting in the island-arc crust. We also note the crustal structure of the Yamato Basin, a part of the Sea of Japan (an inactive back-arc basin), a curious structure in which the crust has a composition similar to the normal oceanic crust but has a twice thickness.