Improved Seismic Velocity Structure in Southwestern Japan Using Pronounced sP Phases Recorded for Intraslab Earthquakes

Abstract

Intraslab earthquakes generate sP phases that are clearly observed at epicentral distances of 60 km or farther in southwestern Japan, while the phases are less easily seen in other regions of Japan. The amplitudes of the sP phase are sometimes larger than those of the direct S , especially at a distance of 130 km or farther. We show that the strong sP -phase arrivals in southwestern Japan are due to the shallow crustal structure of the Chugoku area, at the bounce point of the depth phase. In this study we use sP phases to refine the slab and crustal structures in this region. The comparison of data to synthetic seismograms calculated with existing one-dimensional (1D) models show some discrepancies in terms of the travel times and amplitudes. The synthetics calculated with an improved 1D model better match the observed waveform data. The revised 1D models are used to construct a 2D model that better represents the configuration of the slab and variation of velocity discontinuity depths. For stations where the travel times still show large differences using the improved 1D model, the synthetics calculated with a 2D model better account for the travel times of sP and S waves in southwestern Japan.