Source Spectra, Moment, and Energy for Recent Eastern Mediterranean Earthquakes: Calibration of International Monitoring System Stations
LAWRENCE LIVERMORE NATIONAL LAB CA
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In the past several years there have been several large Mw 7.0 earthquakes in the eastern Mediterranean region Gulf of Aqaba, Racha, Adana, etc., many of which have had aftershock deployments by local seismological organizations. In addition to providing ground truth data GT 5 km that is used in regional location calibration and validation, the waveform data can be used to aid in calibrating regional magnitudes, seismic discriminants, and velocity structure. For small regional events mb 4.5, a stable, accurate magnitude is essential in the development of realistic detection threshold curves, proper magnitude and distance amplitude correction processing, formation of an Msmb discriminant, and accurate yield determination of clandestine nuclear explosions. Our approach provides a stable source spectra from which Mw and mb can be obtained without regional magnitude biases. Once calibration corrections are obtained for earthquakes, the coda-derived source spectra exhibit strong depth-dependent spectral peaking when the same corrections are applied to explosions at the Nevada Test Site Mayeda and Walter, 1996, chemical explosions in the recent Depth of Burial experiment in Kazahkstan Myers et al., 1999, and the recent nuclear test in India. For events in the western U.S. we found that total seismic energy, E, scales as Mo 0.25 resulting in more radiated energy than would be expected under the assumptions of constant stress-drop scaling. Preliminary results for events in the Middle East region also show this behavior, which appears to be the result of intermediate spectra fall-off f-1.5 for frequencies ranging between approx. 0.1 and 0.8 Hz for the larger events. We developed a Seismic Analysis Code SAC coda processing command that reads in an ASCII flat file that contains calibration information specific for a station and surrounding region, then outputs a coda-derived source spectra, moment estimate, and energy estimate.
- Seismic Detection and Detectors