Upper mantle velocity structure estimated from PS-converted wave beneath the north-eastern Japan Arc

The upper boundary of the descending oceanic plate is located by using PS-waves (converted from P to S at the boundary) in the Tohoku District, the north-eastern part of Honshu, Japan. The observed PS-P time data are well explained by a two-layered oceanic plate model composed of a thin low-velocity upper layer whose thickness is less than 10 km and a thick high-velocity lower layer; the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle. The estimated location of the upper boundary is just above the upper seismic plane of the double-planed deep seismic zone. This result indicates that events in the upper seismic plane, at least in the depth range from 60 to 150 km, occur within the thin low-velocity layer on the surface of the oceanic plate.