SEISMIC EVIDENCE FOR STRETCHED CONTINENTAL-CRUST IN THE JAPAN SEA

The Japan Sea is a major marginal sea in the western Pacific comprised of basins and bathymetric highs. Tectonically, these features were cr eated by multiple-rifting and spreading processes, as exemplified in t he creation of the Japan Basin and Yamato Basin. However, there are sm aller scale features' for which seismic crustal models have been inade quate to infer or verify their origin and evolution. Recent advances i n marine seismological surveys enable us to obtain heterogeneous seism ic crustal structures in sufficient detail to characterize these featu res. In September of 1992, a seismic experiment was conducted to obtai n the detailed seismic crustal structure in the southwestern part of t he japan Sea, including the basin and bank areas. Seventeen ocean bott om seismometers (OBSs) were deployed on two lines. Explosives and airg uns were fired as controlled seismic sources. The model of the crust o btained 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 2 2 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 c ontinental crust fragment.