Effect of Scattering on Azimuthal Variation of Ground Motion Patterns for Earthquake and Explosion Events

An ideal underground explosion having a spherical elastic radius in a homogeneous medium radiates elastic energy isotropically as P waves. In contrast, shear dislocation earthquakes radiate both P and S waves, each having a four-lobed azimuthally dependent radiation pattern. In a homogenous Earth with no scattering, these differences in elastic radiation patterns enable amplitude ratios such as Pg/Lg to be used as effective source discriminants. By comparing observed local and regional seismograms with those synthesized by a radiative transport algorithm, this study investigates the disruption of radiation patterns and P to S conversions induced from scattering by combined deterministic and statistical heterogeneity. The results are used to establish empirical relationships between parameters describing the heterogeneity power spectrum and the distance range and frequency band at which azimuthal radiation patterns become indiscernible.