Scattering of trapped P and S waves in the hydrated subducting crust of the Philippine Sea plate at shallow depths beneath the Kanto region, Japan

Abstract

We performed a detailed analysis of seismograms obtained during intraslab earthquakes beneath the Kanto region and revealed a strong lateral variation in the waveforms of highfrequency trapped P and S waves propagating through the subducting crust of the Philippine Sea plate. Significantly distorted spindle-shaped trapped P and S waves with large peak delays were observed in areas where the Philippine Sea plate is at shallow depths beneath the Kanto region. In order to interpret these seismic observations, in relation to the structural properties of the crust of the Philippine Sea plate, we conducted finite difference method simulations of high-frequency seismic wave propagation using various possible heterogeneous velocity structure models. Our simulation successfully reproduced the observed characteristics of the trapped waves and demonstrated that the propagation of high-frequency P and S waves is significantly affected by small-scale velocity heterogeneities in the subducting crust. These heterogeneities can be traced to a depth of approximately 40 km, before disappearing at greater depths, a phenomenon that may be related to dehydration in the subducting crust at shallower depths.