Radiative Transport Modeling of High Frequency Regional Seismograms for Event Discrimination

Abstract:

The differences between earthquakes and explosions are largest in the highest recordable frequency band. In this band, scattering of elastic energy by small-scale heterogeneity (less than a wavelength) can equilibrate energy on components of motion and stabilize the behavior of the Lg wave trapped in Earth's crust. Larger-scale deterministic structure (greater than a wavelength) still assumes major control over the efficiency or blockage of the Lg and the efficiency of other regional phases. We model high frequency regional seismic wave codas (2-4 Hz) for the combined effects of the large-scale 3-D (deterministic) and the small scale (statistical) structure with a radiative transport algorithm. The algorithm propagates packets of body wave energy with ray theory a through a large-scale deterministic 3-D structure, and includes the effects of multiple scattering by small-scale statistical structure. Coda envelopes are synthesized to illustrate sensitivities to variations in the parameters describing small-scale statistical heterogeneity, intrinsic attenuation, Lg blockage due to large-scale variations in crustal thickness, and the effects of tectonic release estimated from the seismograms of nuclear tests. We predict that event discriminants based on P/Lg amplitude ratios will best separate earthquake and explosion populations at frequencies 2 Hz and higher.