Simulation of coda wave envelope in depth dependent scattering and absorption structure

Abstract

Coda waves are considered to be composed of scattered waves. For simplicity, previous models of coda wave generation have often assumed spatial uniformity of scattering and absorption. To account for possible depth dependent attenuation, the energy density of coda waves are synthesized using the Monte Carlo simulation method with multiple isotropic scattering assumption. Uniform velocity structure is assumed and impulsive point source is used. In depth dependent structures given in this paper, the synthesized envelopes give coda Q−1 decreasing with increasing lapse time, which has been observationally known well but has not been adequately explained with uniform models. The energy density becomes asymptotically close to a common decay curve irrespective of epicentral distances as the lapse time increases. However, the amount of the energy density from two sources is different at the same receiver at even the same lapse time for the same radiation energy when the focal depths are different. Theoretical basis of the conventional coda normalization method is examined in cases of the strong depth dependent attenuation structure.