Abstract

Using the T-matrix, or extended boundary condition, method, we model seismic-wave scattering from earth models with one-dimensional (1D) isolated topographic features. The earth model is simple, but we are primarily interested in free surface interactions. Scattering phenomena encountered in this model would presumably also operate in more realistic two-dimensional (2D) models. This study is motivated by observations suggesting that surface topography generates coda waves, which are relatively large-amplitude arrivals following major seismic phases. For sinusoidal periodic surfaces, large-amplitude anomalies in the surface displacement spectrum correspond to P-to-Rayleigh (P-Rg), S-to-Rayleigh (S-Rg), and other mode conversions. For isolated topographic features, these conversions still exist. We modeled teleseismic P-Rg conversions from isolated hills in order to understand and confirm the origin of observed conversions near the NORESS array. Parameter studies indicate that P-Rg and critical P-P conversion amplitudes increase roughly-linearly with scatterer relief, but are insensitive to changes in teleseismic ray parameter. Forward-scattered Rg waves are always two to three times larger than backscattered Rg waves. Poor fits to the observed P-Rg amplitudes (relative to the incident P waves) suggest that more realistic and complicated structure effects must be included. For example, it is well known that P-wave amplitudes vary considerably across the NORSAR array due to subsurface structures.

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