Subducting plate boundary beneath the northeastern Japan arc estimated from SP converted waves

In the Tohoku District, the northeastern part of Honshu, Japan, a conspicuous phase is frequently observed between P and S phases in seismograms for events in the lower seismic plane of the double-planed deep seismic zone. This phase is dominant in the vertical component and has an apparent velocity almost equal to or slightly smaller than the P phase. We have never found this phase in seismograms for events in the upper seismic plane. From these characteristics, the phase is interpreted as SP, converted from S to P at the upper boundary of the descending oceanic plate (UBP). Using the travel times of the SP waves, we estimated the location of the UBP beneath the Tohoku District to be just above the upper seismic plane of the double-planed deep seismic zone. This position is consistent with those determined from the analyses of the travel times of the ScSp and PS waves, which are respectively the seismic wave converted from ScS to P and that from P to S at the UBP.