Deep seismic structure in the margin of the southwestern Yamato Basin, Japan Sea by ocean bottom seismographic experiment

To reveal the formation process of the southwestern Yamato Basin in the Japan Sea, which is a back-arc basin in the northwestern Pacific, it is important to obtain the detailed deep seismic structure of the transition area between the Japan Island arc and the southwestern Yamato Basin. However, there are few seismic structures in the transition area of the southwestern Japan Sea. In this study, we investigate the detailed transitional structure and discuss the formation process of the southwestern Yamato Basin. This study is a part of an integrated onshore-offshore wide-angle seismic experiment conducted from the Shikoku Island to the southwestern Japan Sea in 2002. Thirty-five ocean bottom seismographs (OBSs) were deployed with a spacing of about 6 km along the offshore survey line (approximately 170 km) spanning the Oki Trough, Oki Ridge and the southwestern Yamato Basin in the southwestern Japan Sea. Seismic signals from an airgun array (12,000 cu. inch) and ten land-explosives were recorded by the OBSs. Also, multi-channel seismic (MCS) reflection data were acquired along the offshore survey line. The results indicate that the thickness of the crust beneath the southwestern Yamato Basin and the Oki Ridge is estimated approximately 13 km and 20 km, respectively. The sedimentary layer has approximately 1 to 1.5 km thick in the Basin area and approximately 0.3 km in the Ridge area. Two crustal layers are identified beneath the sedimentary layer. The upper and lower crustal layers have approximately 4 km and 8 km thick, respectively, in the Basin area, whereas those have approximately 9 km and 11 km thick, respectively, in the Ridge area. From the southwestern Yamato Basin toward the Oki Ridge, the upper crustal layer thickens steeply, however, the lower crustal layer thickens gently. This suggests that the role of the upper crustal layer seems to be different from that of the lower crustal layer during the formation of the southwestern Yamato Basin.