abstract

On August 1, 1975, a magnitude 5.9 (mb) (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.

First Page Preview

First page PDF preview
You do not currently have access to this article.