Structural Anomaly at the Boundary between Strong and Weak Plate Coupling in the Central‐western Nankai Trough

To investigate structural characteristics associated with variations in slip behavior on the plate boundary in the Nankai subduction zone, we conducted seismic reflection surveys in the central-western Nankai Trough. Data were processed by pre-stack depth migration. The resulting depth map of the top of the subducting Philippine Sea plate shows detailed topographic features of the plate boundary, including subducting seamounts and large-scale undulations of the plate surface. We report the presence of a structurally anomalous region, potentially with low velocity, in the overriding plate at the boundary between the zone of large coseismic slip during the 1946 Nankai earthquake and the area producing slow earthquakes. This anomaly appears to be related to depth-dependent variations in plate boundary slip style and plate coupling in the central-western Nankai Trough.