Heterogeneous structure across the source regions of the 1968 Tokachi-Oki and the 1994 Sanriku-Haruka-Oki earthquakes at the Japan trench revealed by an ocean bottom seismic survey

To study the physical properties along the subducting plate boundary at the Japan Trench, a seismic study was carried out using ocean bottom seismometers (OBSs) and artificial sources was carried out. The 250 km long survey line with an almost N-S strike crosses the two major focal zones of the 1968 Tokachi-Oki earthquake and the 1994 Sanriku-Haruka-Oki earthquake. Ray tracing, and non-linear inversion were used to analyze the data obtained by the OBSs. P-wave velocity structure down to the island-arc-Moho was obtained. The result shows distinct heterogeneities along the plate boundary at 40°10′N, which are located exactly at the southern boundary of major moment release regions of the above two earthquakes. P-wave velocities for the crust north of 40° 10′N are approximately 7% slower than those for the south. The depths of the island-arc-Moho vary from 15 km in the south to 21 km in the north below the sea surface. Three explanations of the velocity heterogeneities are presented in terms of the migration of water.