Subducting oceanic crust on the Philippine Sea plate in Southwest Japan

It is well-known that subcrustal earthquakes occur in Southwest Japan owing to subduction of the Philippine Sea (PHS) plate. Observing the earthquakes at a single station in the Chugoku district, we frequently find a distinct pair of later P and S phases after weak initial P and S phases, respectively. These later phases were observed only for earthquakes with particular source-receiver geometries; the events in the west Seto Inland sea, epicentral distances from 120 km to 240 km, and the source-receiver direction almost parallel to the strike of the Nankai trough. The feature of initial and later phases was summarized as follows: (1) The later phases are observed for subcrustal earthquakes occurring at depths between 40 km and 60 km, but not for those below the depth, (2) "Single-station" apparent velocities of initial P and S phases are 8.0 km/s and 4.9 km/s, and they are somewhat higher than ordinary velocities of the uppermost mantle. (3) Single-station apparent velocities of later P and S phases are, on the other hand, 6.6 km/s and 3.8 km/s, and they are comparable to the velocities of the continental lower crust and the main layer of oceanic crust. These features could not be explained unless the subcrustal earthquakes take place within the low-velocity oceanic crust overlying the high-velocity oceanic plate. This suggests that the oceanic crust subducts beneath the west Seto Inland sea as a part of the PHS plate.