Spectral Studies of Shallow Earthquakes and Explosions: Implications for P/S Energy Partitioning, Stress Drop, and Discrimination
Final rept. 17 Dec 2005-17 Dec 2007
CALIFORNIA UNIV SAN DIEGO LA JOLLA INST OF GEOPHYSICS AND PLANETARY PHYSICS
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We compute and analyze P-wave spectra from 18,101 earthquakes and 1770 explosions recorded by 196 broadband seismic stations in southern California at epicentral distances up to 100 km. We use an online waveform database stored on a RAID system at Caltech, which provides complete access to the Southern California Seismic Network SCSN seismogram archive. We compute spectra using 1.28s noise and signal windows, positioned immediately before and after the P arrivals. After applying a signal-to-noise cutoff, we process the spectra using an iterative robust least-squares method to isolate source, receiver, and propagation path contributions. This corrects for first-order attenuation structure, as well as near-receiver site effects and any errors in the instrument response functions. Using the earthquake spectra and a simple source model, we compute an empirical Green s function to remove the tradeoff between the source terms and other terms in our model. Our observed earthquake spectra are fit reasonably well with a constant stress drop model over a wide range of moment. However, the explosion spectra show significant differences from the earthquake spectra and have generally steeper falloffs at high frequencies. We also compare P and S-wave amplitudes and find modestly smaller average S amplitudes for the explosions compared to the earthquakes. The best earthquakeexplosion discriminant is the RMS misfit to an omega-2 source model, which works for 90 of the events.
- Ammunition and Explosives