Seismic evidence for stretched continental crust in the Japan Sea

Abstract

The Japan Sea is a major marginal sea in the western Pacific comprised of basins and bathymetric highs. Technically, these features were created by multiple‐rifting and spreading processes, as exemplified in the creation of the Japan Basin and Yamato Basin. However, there are smaller scale features for which seismic crustal models have been inadequate to infer or verify their origin and evolution. Recent advances in marine seismological surveys enable us to obtain heterogeneous seismic crustal structures in sufficient detail to characterize these features. In September of 1992, a seismic experiment was conducted to obtain the detailed seismic crustal structure in the southwestern part of the Japan Sea, including the basin and bank areas. Seventeen ocean bottom seismometers (OBSs) were deployed on two lines. Explosives and airguns were fired as controlled seismic sources. The model of the crust obtained in the northern Tsushima Basin is about 13 km thick including a 1.5‐km‐thick sedimentary layer. Beneath the bank area the crust is 22 km thick and the upper crust is relatively homogeneous with a P‐wave velocity of about 6 km/s. The bank is interpreted to be a stretched continental crust fragment.