Laboratory Tests of the Single- and Multiple-scattering Models for the Generation of Seismic Coda Waves

Abstract

To investigate the scattering property of the earth's crust using seismic coda waves, scattering models for coda generation were first tested using an ultrasonic technique and sufficiently large 2-D models of scattering media. Next, short-period coda waves from local small earthquakes that occurred in the central Kinki district of southwest Japan were analyzed with the multiple-scattering model for coda generation given by Gao et al. The principal results obtained are (1) that the experiments demonstrate that the multiple-scattering model of Gao et al. for coda generation explains the time decay of the amplitude of coda consisting of waves scattered by small-scale crack-like heterogeneities in the earth's crust very well; (2) that when the lithosphere is assumed to be a scattering medium with randomly distributed heterogeneities, the isotropic scattering coefficient, g, of the earth's crust in the central Kinki district is nearly proportional to f^1.0 in the frequency range of 2<f<16 Hz; and (3) that from the weak dependence of g on the frequency, such as g∝f^0.7-1.2, for the earth's crust in seismically active regions, it is suggested that the heterogeneities responsible for short-period coda generation may be numerous cracks in the crust that have lengths of less than about 100m.