A three‐dimensional seismic wave speed model in southwestern Japan from combined use of waveform modeling and travel‐time tomography

Abstract

Recent three‐dimensional (3‐D) travel‐time tomographic models of southwestern (SW) Japan image the subducting Philippine Sea plate (PHSP) and low velocity anomalies with long‐wavelength spatial resolution. However, the agreement between the synthetics calculated with the existing 3‐D model and data is not satisfactory forsP and S waves in the higher frequency range (≥0.2 Hz). The unsatisfactory agreement can be attributed to the parameters of the initial wave speed model for the tomographic inversion, as well as the difficulty in detecting S‐wave travel times accurately. In order to improve the wave speed model, we first constructed a 3‐D reference model by forward modeling using waveform data from an intraslab earthquake and then performed repeated travel‐time inversions using the 3‐D reference model as the initial model. The newly derived high‐resolution model 3DM_SWJ can produce synthetics that are substantially in better agreement with the data than previous models. The distribution of highP‐wave toS‐wave speed ratios (VP/VS) calculated from model 3DM_SWJ shows short‐wavelength heterogeneities that could have been formed in relation to the subduction of the Philippine Sea Plate beneath the Eurasian plate. This combined use of forward waveform modeling for slab and crust configuration and repeated tomographic inversion of travel‐times provides a better 3‐D structural model of seismic properties for understanding tectonic features related to the subduction process, in addition to better estimating ground motions.