Earthquake processes: Seismotectonics of the 1984 Round Valley, California, earthquake sequence and radiated energy and the seismic spectrum
AuthorSmith, Kenneth D
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The November 23, 1984 ML5.8 Round Valley, California earthquake was one in a series of moderate earthquakes that have taken place in the Bishop-Mammoth Lakes, California area since 1978. The aftershock sequence exhibits conjugate strike slip faulting. A near vertical plane striking N30°E is the main shock with its conjugate structure striking N40°W and dipping 55°NE. Aftershocks defining the deeper near vertical plane concentrate around the periphery of a 36 to 49 km2 area which is interpreted to represent the slip surface of the main shock. The source dimensions and stress drops of 87 aftershocks (ML2.8 to ML4.2) of the sequence were determined applying a P-wave pulse width time domain deconvolution technique. Stress drops determined from the pulse width method were interpreted within the context of the conjugate faulting pattern that characterized the sequence. The stress drop for each event is mapped with accuracy within the aftershock distribution. The composite velocity squared source spectra of the main shock and a number of significant events in the literature were integrated in order to determine the seismic energy radiated during fault rupture. The high frequency spectral shape and the character of the source spectrum at the comer frequency are critical to the energy calculation. Savage and Wood (1971) proposed a model in which the final stress level was less than the dynamic stress level and that this was the result of "overshoot" during fault failure. The composite source spectra of the studied earthquakes were compared to that predicted by the Haskell (1966) and Brune (1971) models and their velocity squared spectra were integrated to determine the radiated energy, apparent stress and Savage and Wood Inequality. In all cases, the ratio of twice the apparent stress to the stress drop is greated than or equal to one, violating the Savage and Wood (1971) inequality, providing evidence against the "overshoot" source model.
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conjugate strike slip faulting
vertical plane striking
near vertical planes
domain deconvolution technique
pulse width method
high frequency spectral shape