Upper Mantle Velocity Structure Estimated From Ps‐Converted Wave Beneath the North‐Eastern Japan Arc

Summary. 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.