2001 Volume 53 Issue 4 Pages 337-355
The Japan Sea is a back-arc basin in the northwestern Pacific and is now one of the most well studied marginal seas in the world in terms of its seismic crustal structure. The Japan Sea seems to possess areas of various stages of back-arc opening process. Understanding of each stage is critical in reconstructing the back-arc opening process. One of the key areas from which we lack information on seismic structure is the geologically interpreted failed rifts. In September 1997, an airgun-ocean bottom seismometer (OBS) survey was carried out in the Kita-Yamato Trough, which splits the Yamato Rise into two parts and geologically shows a failed rift structure. Seven digital recording OBSs were deployed every 20km and a 20-liter airgun was used for controlled source. We determined a two-dimensional velocity structure model by forward 2-D ray tracing method. The crustal thickness is about 12km including 1.5km thick sediments. A thin (ca. 0.8km thick) layer with a P-wave velocity of about 4.2km/s underlies the sedimentary layers. The lower part of the upper crust has a velocity of 6km/s with a thickness of about 2km. The lower crust has a velocity of 6.6-6.9km/s and is about 7.8km thick. There is no seismic evidence for existence of velocity greater than 7.1km/s in the lowermost crust. The Moho was detected by reflected waves only so that the Pn velocity remains unconstrained. The crust of the Kita-Yamato Trough is suggested to be not related to large and rapid volcanic activities, because the crust has no high velocity layer in the lower crust indicative of such activities. Since the Trough is adjacent to the Yamato Rise, which is believed to have a continental crust, the Kita-Yamato Trough should have been formed by rifting/stretching of a continental crust. The crustal structure of the Trough is roughly similar to those found in the southern Yamato Basin and the northern Tsushima Basin, which are stretched continental crusts. The resemblance of the crustal structures may indicate that the origins and/or stages of back-arc rifting are similar in the three areas.
