Explosion-Source Energy Partitioning and Lg-Wave Excitation: Contributions of Free-Surface Scattering

A 2D boundary-element numerical simulation approach and a local slowness analysis method for an embedded array are used to quantify effects of topographic scattering on near-source energy partitioning for simple underground explosion sources. Various parameters, including free-surface models with different root mean square (rms) random topographic fluctuations and correlation lengths, source depths from 0.25 to 3 km, and Q values from 50 to infinity are included in the numerical simulations, with energy responses of different phases being determined as functions of frequency. The results reveal that for a crustal model with a relatively high surface velocity, near-source free-surface scattering provides an important coupling mechanism that can impart additional explosion energy to the Lg wave. At relatively low frequencies, and for a moderately rugged free-surface, the Rg-to-Lg transfer is quite efficient, while at higher frequencies or for a very rugged free surface, the body wave to Lg transfer may dominate the process. The Rg excitation functions, source depth, and topographic correlation length all contribute frequency dependence to the Lg excitation function. The presence of a low Q value within the uppermost crust severely attenuates the high-frequency energy transferred to the Lg wave.