Numerical Investigation of Relative Contributions of Rg Scattering and Incomplete Dissipation to Lg Excitation,

Numerous mechanisms have been proposed over the years to explain how the regional Lg phase can be generated by nuclear explosions. Commonly quoted mechanisms include trapped pS phase, non-geometrical S, spall, multiple reflections of initial P rays, anisotropy, and various near-source, near-surface scattering processes. Recent observational studies e.g., Patton and Taylor, 1995 indicate that Rg plays an important role in Lg excitation for Yucca Flat explosions. Numerical experiments by Jih and McLaughlin 1995 readily demonstrate that rough topography and random heterogeneity can scatter significant Rg energy into body waves. In this study, an additional Rg-to-Lg conversion mechanism is presented and the relative effectiveness and importance of all three Rg-related mechanisms are examined. It might be anticipated intuitively that the net effect of anelasticity in the surface layers is solely to reduce the amplitude of incident seismic waves. Rg wave is particularly susceptible to such a mechanism since it is confined in the uppermost crustal layers. If the anelastic attenuating layer is only thick enough to dissipate the retrograde rolling near the surface, then the free surface would behave asymptotically like a fixed point. Beyond certain distance, the fundamental mode can no longer be sustained by such a waveguide, and accordingly any undissipated Rg energy would have to propagate in other wave types or modes. Linear finite-difference calculations show that this process couples the undissipated Rg energy into pure shear waves or higher modes, depending on the complexity of the structure. In terms of Rg-to-S to Rg-to-P ratio, this process appears to be more efficient than other near-surface Rg scattering mechanisms.