Estimation of three-dimensional boundary shape of the Osaka sedimentary basin by waveform inversion

Abstract

SUMMARY We present a method for estimating the boundary shape of a 3-D basin velocity structure by waveform inversion, and we apply the method to real seismic data observed in the Osaka sedimentary basin in Japan. The initial model is the 3-D Osaka basin velocity structure model developed by Iwata et al. (2008. Basin and crustal velocity structure models for the simulation of strong ground motions in the Kinki area, Japan. J. Seism., 2, 223–234) based on bedrock depth information obtained from various geophysical exploration surveys. We adopt representation of the boundary shape of the seismic bedrock using a 2-D cubic B-spline function. The spline coefficients are estimated by minimizing the L2-norm of the difference between the observed and synthetic waveforms. The target of the inversion is the set of velocity waveforms in the period range of 3–10 s of the real seismic data observed in the Osaka basin during two regional MW5.0 earthquakes that occurred 60–90 km to the east and southeast of the Osaka basin; they contain later phases with large amplitudes and long durations. We first perform a series of synthetic tests to validate the method and examine the effect of the choice of time window in the waveform inversion. We show that the later phases of the ground motion have a larger influence on the inversion and are affected by the structure of a wider area relative to the body-wave portion. We then apply the method to the real data in two successive steps: Step-1 involves the body-wave portion only and Step-2 involves the body-wave and later phases. We propose a multi-event inversion method in which ground motion data from two earthquakes with different azimuths are solved simultaneously. The updated basin velocity structure model obtained in our inversion analysis reproduces the ground motion better than the initial model at most of the stations. The bedrock depth of the updated model is smaller than that of the initial model in the central part of the basin, which is consistent with the bedrock depth information inferred from deep borehole data.