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P-wave travel-time variations before the August 1, 1975 Oroville, California earthquake

U.S. GEOLOGICAL SURVEY, MENLO PARK, CALIFORNIA 94025
CALIFORNIA DEPARTMENT OF WATER RESOURCES, SACRAMENTO, CALIFORNIA 95814

Abstract

On August 1, 1975, a magnitude 5.9 (m_b) (BRK, M = 5.7), normal dip-slip earthquake occurred 10 km south of Oroville, California. P arrivals for teleseismic and regional sources at the few seismographs in the area have been carefully timed to an accuracy of ±0.02 sec and the relative residual technique has been applied to these data. The data cover the period from August 1968 through March 1976. A significant delay of about 0.1 sec in travel-time residuals for Russian nuclear blasts was observed over a 3-year period preceding the Oroville earthquake at station ORV 10 km north of the epicenter. A 0.1-sec delay in travel-time residuals for U.S. nuclear blasts occurred after the Oroville event at station MGL, 40 km north of the main shock's epicenter. P arrivals from deep Tonga-Fiji earthquakes have also been analyzed but reveal no systematic time variations beyond ±0.05 sec from their mean values. P arrivals from moderate-size earthquakes along the San Andreas fault system in central California proved to be an unsatisfactory source of data because of ambiguities created by multiple P-phase arrivals and the emergent nature of the arrivals. The sparse station coverage does not allow adequate delineation of the extent and character of the anomalous P velocity zones, but the data do provide some limitations. The postearthquake travel-time delay at MGL may be precursory to a future earthquake or may only be related to the redistribution of stress in the Oroville area.

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