The Upper Boundary of the Philippine Sea Plate beneath the Western Kanto Region Estimated from S-P and P-S Converted Waves

Although the subduction of the Philippine Sea plate has generally been investigated by using microearthquake seismicity and determining the three-dimensional seismic wave velocity structure, the configuration of the slab in the western Kanto region is not well known because of low seismic activity. To detect the plate boundary, we placed five seismic stations in the western Kanto region. Clear later phases (X1 and X2-phases) were observed between the P and S arrivals. The seismograms of earthquakes occurring in eastern Yamanashi prefecture show a clear later phase (X1) on the vertical component. The X1-phase, identified as the S to P converted wave at the upper boundary of the descending Philippine Sea plate, constitutes seismological evidence for the existence of the Philippine Sea slab to 20km depth beneath the western Kanto region. The S-P converted points are located in a restricted region along the depth direction, however, necessitating analyses of other phases for slab geometry delineation. The seismograms from earthquakes occurring beneath the east coast of the Izu Peninsula showed a clear later phase (X2-phase) dominant on the horizontal components. The observed values of the X2-P time and locations of known velocity discontinuities are consistent with the later phase being a P to S converted wave at the upper boundary of the subducting Philippine Sea plate. The upper boundary estimated from travel time data of the converted wave is located at depths of 28-35 km. Seismological evidence for the existence of the subducting Philippine Sea plate beneath the western Kanto region is shown in this study. The subducting Philippine Sea plate was found to exist at depths shallower than 20km and deeper than 25km from the S-P and P-S converted wave data in the aseismic western Kanto region, respectively. The location of the boundary is estimated to be at depths from 10km to 35km.これまでに,フィリピン海プレートの形状は,震源分布や3次元速度構造のインバージョン等の手法を用いて求められてきた.しかしながら,関東地方の西側地域は地震活動がきわめて低く,その形状を求めることは困難であった