MODELING P-RG CONVERSIONS FROM ISOLATED TOPOGRAPHIC FEATURES NEAR THENORESS ARRAY

Using the T-matrix, or extended boundary condition, method, we model s eismic-wave scattering from earth models with one-dimensional (1D) iso lated topographic features. The earth model is simple, but we are prim arily interested in free surface interactions. Scattering phenomena en countered in this model would presumably also operate in more realisti c two-dimensional (2D) models. This study is motivated by observations suggesting that surface topography generates coda waves, which are re latively 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-Raylei gh (S-Rg), and other mode conversions. For isolated topographic featur es, these conversions still exist. We modeled teleseismic P-Rg convers ions from isolated hills in order to understand and confirm the origin of observed conversions near the NORESS array. Parameter studies indi cate that P-Rg and critical P-P conversion amplitudes increase roughly -linearly with scatterer relief, but are insensitive to changes in tel eseismic ray parameter. Forward-scattered Rg waves are always two to t hree times larger than backscattered Rg waves. Poor fits to the observ ed P-Rg amplitudes (relative to the incident P waves) suggest that mor e realistic and complicated structure effects must be included. For ex ample, it is well known that P-wave amplitudes vary considerably acros s the NORSAR array due to subsurface structures.