Scattering Effects for Teleseismic Plane Wave Propagation in a Heterogeneous Layer Over a Homogeneous Half-Space

An in-depth study into teleseismic wave propagation through a heterogeneous layer over a homogeneous half-space is conducted to help explain observed complex teleseismic particle motions and coda generation. Acoustic and elastic finite difference synthetics for more than 200 different models were computed and examined to reveal any relations between the model parameters and coda generation. Each model consisted of a scattering layer over a homogeneous half-space; the models varied in scattering layer thickness (L), heterogeneity correlation distance(s) (a, or a_{x}, a_ {z}), and heterogeneity standard deviation (sigma). Data from the NORESS seismic array were also analyzed in hope of determining the degree of heterogeneity in the crust and upper mantle below the array, and to locate regional scattering sources. Of particular interest are relations between the model parameters and the level and rate of scattered energy decay, apparent scattering attenuation, and the scattering sources/mechanisms and scattered wave types. The level of scattered energy is controlled by the ka ratio and the standard deviation of the perturbations. Energy levels increase with sigma regardless of the other model parameters. The rate of coda decay is found to be controlled by the model anisotropy where the model end members are (1) those with homogeneous plane layers, which exhibit the most rapid coda decay rates, and (2) those with isotropic heterogeneities, which exhibit the slowest coda decay rates. Apparent scattering attenuation of the direct pulse is found to be strongest for models with isotropic heterogeneities. Frequency-wavenumber analysis of the elastic synthetics revealed that the coda, for models with isotropic heterogeneities, is almost totally composed of wide angle S, and body-to-surface wave scattered energy, while the coda for models with homogeneous plane layers is composed of vertically propagating layer reverberations. The analysis of the NORESS data revealed similar body-to-surface wave scattered energy arriving approximately 15 secs after the initial arrival. Because of the importance of P-to -S and body-to-surface wave scattering, acoustic approximations should not be used to investigate the elastic scattering problem.