Abstract
Seismic observations with a dense network using telephone telemetries have been done in the eastern part of Yamanashi Prefecture to study the configuration of northern tip of the Philippine Sea plate and the seismotectonics in this area. Hypocenter distributions obtained from this network indicate that most of earthquakes beneath the eastern part of Yamanashi Prefecture occur in a narrow zone with a thickness of about 10km and dipping toward the northwest. No events were found in the western or downward extension of this zone. Weak S to P converted waves are detected by making seismic record sections composed of many seismograms from many earthquakes observed at each station. Time axes of these seismograms were shifted so that theoretical arrival times of S to P converted waves were aligned. Conversion points of these phases are located at the upper limit of the narrow zone of high seismicity beneath the eastern part of Yamanashi Prefecture, showing the existence of a velocity discontinuity at this location. The present result indicates this velocity discontinuity corresponds to the upper boundary of the Philippine Sea plate and most of the earthquakes in the zone of high seismicity dipping toward the northwest are occurring within the subducting Philippine Sea plate.
