Analysis and Simulation of 3D Scattering due to Heterogeneous Crustal Structure and Surface Topography on Regional Phases; Magnitude and Discrimination
Final rept. 29 Jan 2007-30 Jun 2009
URS GROUP INC PASADENA CA
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This report concentrates on generating physical explanation for commonly used discriminants. It begins with a brief review of broadband observations from earthquakes and an explosion occurring in North Korea. By comparing detailed earthquake modeling results, we confirm the existence of radiation pattern for body wave arrival onsets to high-frequency, 8 Hz. We also addressed the coda-levels for all three components and demonstrate that the PS spectral levels provide a useful discriminant, especially at large distances at frequencies 2 Hz. To simulate these features, we performed anelastic 3D finite-difference calculations on highly heterogeneous media for a range of source depths, receiver distances, and source-types. In this first stage, we investigated the effects of small-scale crustal heterogeneities including fault-damaged zones. In the second stage, we investigated the effects of surface topography including mini-basins combined with crustal heterogeneities. Our numerical experiments show that wave-path scattering is a major contributor to S and Lg coda from shallow explosions and depends on source depth. PLg ratios estimated at different frequencies indicate that it is indeed an excellent discriminant at high frequency. However, at frequencies below 2 Hz, explosions and earthquakes look the same for realistic crustal models containing shallow mini-basins. A second possible discriminant is the ratio P high frequency P low frequency or P 2 to 4 Hz P .1 to 2 Hz that works particularly well at small distances. This appears to be caused by the effect of pP on the shallow explosion. A product discriminant of P hf P lf dot PS should work best according to our simulations.