Fore arc structure and plate boundary earthquake sources along the southwestern Kuril subduction zone

[1] The geologic structure and gravity anomalies of the fore arc wedge along the southwestern Kuril arc were compared with the sources of seismic waves and tsunamis of the 1952, 1973, and 2003 plate boundary earthquakes. The source areas are generally located beneath fore arc basins in the upper slope which has been subsiding, except for the tsunami source of the 1952 earthquake, while the slip amount varies beneath the basins. Large slip of seismic sources of the 1952 and 2003 earthquakes occurred under the low gravity anomaly zone of a fore arc basin and a structural ridge along the N-S-oriented collision zone between the Kuril and NE Japan fore arcs, where there is inferred to be a zone of a strong basal erosion of the overriding plate. The tsunami sources are located under the subsiding area to the east of the collision zone. These comparisons indicate that the source areas are primarily restricted to the region under the subsiding area of the upper slope, and the lateral change of slip amount in the source areas is related to the collision tectonics of the fore arc sliver. The 1952 tsunami source extended seaward, which was presumably induced by the slip of the larger seismic source under the upper slope. The lack of large-scale slope failure or splay faults in the middle to lower slope suggests that the slip along the plate boundary is responsible for tsunami generation. A subducting seamount presumably prevented the seaward propagation of the rupture beneath the middle to lower slope.