Inversion of tsunami waveforms for the estimation of a fault heterogeneity: Method and numerical experiments.

Abstract

A method for estimating fault heterogeneity by an inversion of tsunami waveforms is presented. The ocean bottom bathymetry, by which the velocity of tsunami wave is determined, is more accurately known than the seismic velocity structure, so that the effect on the propagation path can be precisely evaluated by means of numerical computation. Since the propagation velocity of tsunami is much smaller than any kind of seismic waves or rupture velocity, only a final slip distribution on a fault can be estimated. The amount of slip on each segment of the fault is obtained by an inversion of the observed tsunami waveforms by using the numerically computed waveforms from each segment as the Green's function. Several numerical experiments are carried out to examine the spatial resolution of the fault heterogeneity by the present method. It is clarified that the size of the segment needs to be more than eight times the grid size used for the computation of the Green's function, indicating that accurate bathymetric data and large computation are required to get a fine picture of the heterogeneous fault motion. Simulation of the inversion shows that the slip distribution on the fault can be estimated stably. The effect of tide gauge response is the only unknown parameter contained in tsunami records besides the source information, so it is necessary to investigate it before the present method is applied to actual tsunami records.