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The Owen Fracture Zone southeastern boundary of the Arabian Plate.

Oceanic crusts subducting into the mantle were detected by analyses of mantle earthquakes in the Kanto district, central Japan. Earthquakes in a depth range of 30–80 km in the southwestern Ibaraki prefecture were relocated by a modified method of joint hypocentre determination. We obtained a good distribution map of hypocentres and focal mechanisms. A boundary layer between the Eurasian and Philippine Sea plates was found, where thrust earthquakes with P axes in the NW-SE direction and of which low-angle nodal planes dip northward occurred in a layer with a thickness of about 4 km. The P-wave velocity in this layer is estimated to be less than about 7.0 km s-1. Another boundary layer between the Philippine Sea and Pacific plates exists about 15 km beneath the above boundary layer. Thrust earthquakes with P axes in the EW direction and of which low-angle nodal planes dip westward occurred in this layer that also has a thickness of about 4 km. Furthermore, normal-faulting earthquakes of which the P axes are almost perpendicular to the boundary layers were found immediately outside the boundary layers. Combining these results, we can conclude that the boundary layers are oceanic crusts that exist at the top of the subducting Philippine Sea and Pacific plates. ‘Weak-zone-normal compression’ was proposed to result from the existence of low-velocity oceanic crusts at plate boundaries. This hypothesis can be explained, because only stress caused by relative motion of the two plates is consumed inside the weak oceanic crust. The remnant stress after subtracting the above stress from the tectonic one applied at the plate boundary is the stress of which the P axis is normal to the plate boundary.