Energy Transport in a Multiple-Scattering Crustal Waveguide: Role of Polarization

Abstract:

Seismic waves radiated by small crustal earthquakes are prone to multiple mode conversions caused by reflexion-transmission at interfaces, and scattering by small-scale heterogeneities in the bulk of the medium. The goal of this study is to clarify the role of wave polarization in the complex interplay between volume and surface scattering in crustal waveguides. To carry out this task, we have incorporated a rigorous description of polarization in the context of Monte-Carlo simulations of the multiple-scattering process. In this approach, elastic P and S energy is transported by seismic phonons whose polarization properties are described by a 5-dimensional Stokes vector. This representation allows us to track the complex changes of polarization that are caused by both scattering and deterministic reflections and to incorporate a realistic description of the polarization at the source for explosions and dislocations. To shed light on the wave content of the regional short-period seismic wavefield, we investigate the asymptotic partitioning of seismic energy onto P, SV and SH polarizations in the coda. Equipartition theory in full elastic space predicts an even energy distribution among SV and SH waves, both being more excited than P waves by a factor (Vp/Vs)^3. Our numerical results indicate that a form of energy stabilization among P, SH and SV modes occurs in the crustal scattering waveguide, with partitioning ratios that depend both on the scattering properties of the medium and on the velocity model. Compared to the full-space case, the partitioning ratio between S and P waves can increase by as much as 35% in favor of the former. This phenomenon is explained by fact that a fraction of P wave energy can always be transmitted into the mantle at the Moho, in sharp contrast with the case of S waves. A significant shift of polarization (~15%) in favor of SV waves is also observed and is currently under investigation. The role of the initial polarization at the source has also been examined. In the case of events with equal magnitude but different source mechanisms, we find that the energy level can differ by as much as 30% in the coda. These findings are in sharp contrast with the accepted view that the coda excitation is independent of the source process.