SEISMIC CODA WAVE ENVELOPE IN DEPTH-DEPENDENT S-WAVE VELOCITY STRUCTURE

Coda waves are considered to be composed of scattered waves. For simpl icity, previous models to generate coda waves have often assumed spati al uniformity of seismic wave velocity. To account for a depth-depende nt S wave velocity structure, energy densities of coda waves are synth esized using a Monte Carlo simulation method, based on the multiple is otropic scattering assumption for media consisting of several layers. The wave energy propagation and the scattering process are simulated b y random walks of many particles. Reflection/transmission is determine d by Snell's law and the ray parameter is preserved at the interface o f layers. On the other hand, scattering changes the ray parameter. The energy densities are determined by the overall effect of reflection/t ransmission, scattering, absorption and geometrical spreading along th e ray paths. The results of the simulation show that the energy densit y depends on the focal depth of the sources even for the late part of the coda. This suggests that the conventional coda normalization metho d may not work well in case of a strong depth-dependent velocity struc ture. (C) 1997 Elsevier Science B.V.