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The Philippine Sea plate exhibits an along-trench variation in structure, from an eastern volcanic arc to a typical oceanic crust in the west. The regional difference of intra-slab seismicity implies that this transition occurs around the Itoigawa-Shizuoka Tectonic Line (ISTL). However, the nature of the subducting slab in this region has not been studied in detail. Here, we investigate the structure of the Philippine Sea plate subducting beneath the ISTL. Using active source data, we found reflective portions at depths of 14–18 km. An amplitude evaluation for the reflection phases showed that the reflective zone has a P wave velocity as low as 3.0 km/s. Together with its slightly northwestward-dipping geometry, we interpreted that the low-velocity zone is at the plate boundary and can be attributed to high water content supplied by the slab. This feature is in great contrast to the collision zone further east, where the slab top is less reflective and the dehydration process is inactive. This structural difference also correlates well with the regional distribution of slab seismicity. The reflective zones we found are likely located at the down-dip end of the locked zone, where high slip deficit rates are currently observed. This may suggest that changing fluid pressures and the resulting frictional properties in the down-dip direction control the transition from a coseismic rupture zone to a deeper aseismic zone.

Transition from collision to subduction and its relation to slab seismicity and plate coupling