The Affect of Realistic Geologic Heterogeneity on Local and Regional P/S Amplitude Ratios Based on Numerical Simulations
Regional seismic discriminants based on high-frequency P/S ratios reliably distinguish between earthquakes and explosions. However, P/S discriminants in the 0.5 to 3 Hz band (where SNR can be highest) rarely perform well, with similar ratios for earthquake and explosion populations. Variability in discriminant performance has spawned numerous investigations into the generation of S-waves from explosions. Several viable mechanisms for the generation of S-waves from explosions have been forwarded, but most of these mechanisms do not explain observations of frequency-dependant S-wave generation. Recent studies have focused on the affect of near-source scattering to explain the frequency-dependence of both S-wave generation and P/S discriminant performance. In this study we investigate near-source scatter through numerical simulation with a realistic geological model We have constructed a realistic, 3-dimensional earth model of the southern Basin and Range. This regional model includes detailed constraints at the Nevada Test Site (NTS) based on extensive geologic and geophysical studies. Gross structure of the crust and upper mantle is taken from regional surface-wave studies. Variations in crustal thickness are based on receiver function analysis and a compilation of reflection/refraction studies. Upper-crustal constraints are derived from geologic maps and detailed studies of sedimentary basin geometry throughout the study area. The free surface is based on a 10-meter digital elevation model (DEM) at NTS, and a 60-meter DEM elsewhere. The model extends to a depth of 150km, making it suitable for simulations at local and regional distances. Our simulation source is based on the 1993 Non-Proliferation Experiment explosion at the NTS. This shot was well recorded, offering ample validation data. Our validation tests include measures of long-period waveform fit and relative amplitude measurements for P and S phases. Our primary conclusion is that near-source topography and geologic complexity in the upper crust strongly contributed to the generation of S-waves from the NPE shot. When either geologic heterogeneity or topography is removed from the model, simulated amplitudes of regional S-waves are diminished. We also find that deeper sources scatter less energy off of topography and upper-crustal structures, resulting in diminished S-wave amplitudes with increasing source depth.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 878605
- Report Number(s):
- UCRL-CONF-213525; TRN: US200611%%351
- Resource Relation:
- Conference: Presented at: Seismic Research Review, Palm Springs, CA, United States, Sep 20 - Sep 22, 2005
- Country of Publication:
- United States
- Language:
- English
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