P-wave velocity structure in the northern part of the central Japan Basin, Japan Sea with ocean bottom seismometers and airguns

In 1996, an airgun-ocean bottom seismometer survey was carried out in the northern part of the central Japan Basin. The crustal thickness in the central part is about 9 km, including a sedimentary layer with thickness of 1.5 km, and increases eastward. The obtained crustal structure is slightly different from those of typical ocean basins. The thickness and velocity of less than 6.5 km/s in the upper part of the crust do not correspond to that of a typical oceanic crust and the clear linear geomagnetic anomaly around this survey line has been unconfirmed. Although, this crust could be interpreted to be either anomalous thick oceanic crust formed at spreading centers influenced by a mantle plume or thinned continental crust at ocean-continental boundaries in passive margins, we prefer the latter as a conclusion, that is, it may be formed by thinning of a continental crust rather than by the melt of mantle plumes during the opening of the Japan Sea. In addition, the difference of the crustal structures in the study area and the northeastern Japan Basin where the crust is typical oceanic, indicates that the process of crustal formation may differ in the northern part of the central Japan Basin from in the northeastern Japan Basin.