P-WAVE VELOCITY STRUCTURE INSIDE THE SUBDUCTING PACIFIC PLATE BENEATH THE JAPAN REGION

Little is known of the internal structure of the subducting Pacific plate. Previous studies suggest the existence of an intermediate boundary within the subducting slab. The low density of seismological stations in oceanic regions has prevented high-resolution studies. Recently, seismological stations in Japan have accumulated digitally recorded seismograms. Beneath the Kanto region, central Japan, the Pacific plate is subducting with a high inclination compared with that of the Tohoku region, northern Japan. The ray paths from deep earthquakes to seismological stations are expected to penetrate more inner parts of the subducting plate. This region is suitable to resolve the velocity structure within the plate. In this study, travel-time residuals and seismograms of the deep-focus earthquakes that occurred beneath the Kinki region, southern Japan, are used to constrain various parameters related to the velocity structure model within the slab. The seismograms of deep-focus earthquakes at the geographically restricted stations located in the Kanto region, central Japan, show a remarkable high-frequency later arrival of the P wave. The later arrival was studied and recognized as the reflected wave at the intermediate boundary inside the plate. The velocity structure within the plate is also studied. Our best-fit model has a velocity of Jeffreys-Bullen (J-B) model plus 1.5% in the upper layer and J-B plus 2.5% in the lower layer.