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OFFICE OF EARTHQUAKE STUDIES U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
Abstract
Twenty-six small aftershocks of the Oroville, California earthquake were recorded on three orthogonal components of a digitally recording seismograph with a 15-bit analog to digital converter at a sample rate of 400 samples/sec/channel. The magnitude of the events ranged from less than 1 to about 2.6, and hypocentral distance varied from about 3 to 14 km; depths were 3 to 10 km. For 14 of these events, both P and S waveforms were recovered. Fourier displacement spectra were calculated for the body waves and analyzed with respect to the Brune (1970, 1971) source model. Most spectra have a constant long-period level and falloff above a corner frequency at a rate proportional to 
–
. for these earthquakes is usually greater than 2 and frequently as high as 4 or 5 for spectra that have corner frequencies between 10 and 70 Hz. For 12 of the 14 events that recorded both P and S waves, the ratio of the P-corner to S-corner frequency is about 1.7, as predicted by Hanks and Wyss (1972). Stress drops vary from a few tenths of a bar to 27 bars and appear to increase with depth. One earthquake that fell significantly above the stress drop-depth curve (a large stress drop event) was followed by a ML = 3.4 earthquake in 2.5 to 3 days after a period of quiescence.
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