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We determine the upper boundaries of the Philippine Sea (PHS) plate in the Tokai District, Japan, based on a total of 11,340 relocated hypocenters and recalculated mechanism solutions of about twenty percent of hypocenters, for earthquakes observed during recent about 14 years. Variously oriented cross sections of hypocenters and P and T axes distributions are used to delineate the configuration of the PHS slab. The thickness of the Wadati-Benioff zone which nearly corresponds to the oceanic crust is about 5 to 7 km. The resultant overall geometry of the upper boundary of the slab is convex seaward and forms NW-to-NNW trending `slab valley' from the point of about 20 km depth beneath Pt. Omaezaki to the deepest tapered end of 70 km depth beneath central Honshu. Regional sinus shapes of the slab reasonably explain the features of seismicity and focal mechanisms along the plate boundary, in terms of the conservation of the subducted spherical shell of the PHS plate, and the change in the direction of relative plate motion between the PHS and the overriding plates in the past. A strong coupling is suggested along gently-dipping plate boundary in the direction of relative plate motion extending from the point beneath Lake Hamana to the point of 40 km depth beneath the southern end of the source area of the great 1891 Nobi intraplate earthquake. The slab subducting at the Suruga trough has rather steeply-dipping angle perpendicular to the trough axis. A remarkable landward-dipping cluster overlies the Wadati-Benioff zone at the mid to lower crustal depths beneath the western coast of Suruga Bay. The focal mechanisms of the clustered events show reverse-slip faultings with subhorizontal NW-SE oriented P axes and near-vertical T axes. A thin aseismic layer is intercalated between the cluster and the underlying Wadati-Benioff zone. From the history of accretion tectonics and the other geophysical and geological evidences in the South Fossa Magna, we infer the cluster is composed of underplated oceanic materials presenting an ocean-to-continent transitional seismic layer. The underplated materials may probably be dragging downward coupled with the underlying oceanic plate through the thin aseismic layer, in accordance with the large crustal subsidence just above the dipping cluster continuing since the 1854 Ansei Tokai earthquake. We interpret that the thin and probably partially-strong intercalated aseismic layer is the site of nucleation of the next `Tokai earthquake'. Another important feature is that the deeper Wadati-Benioff zone seems to continue to the shallower gently dipping seismic zone beneath the Suruga trough and the Izu Peninsula. This suggests an alternative tectonic process that a new convergent plate boundary is creating and extending seaward underneath the Suruga trough and the Izu Peninsula.

Geometry of the Philippine Sea Slab and the Convergent Tectonics in the Tokai District, Japan