Normal fault type events in the upper plane of the double-planed deep seismic zone beneath the Northeastern Japan arc.

The spatial change in the focal mechanism of the intermediate-depth earthquakes in the Tohoku District, the northeastern part of Honshu, Japan, is investigated in detail. In addition to general features of the distribution of focal mechanisms revealed by the previous studies, it is found that normal fault type events are dominant in the upper seismic plane of the double-planed deep seismic zone beneath the volcanic front, where the upper seismic plane is bending downward. The bending of the descending oceanic plate is probably the cause of these normal fault type events.