The Parkfield earthquake produced an instrumentally recorded acceleration of 0.5g but caused little structural damage because of the short duration of the maxima. Remeasurements of geodetic lines established in the area six months prior to the earthquake indicate approximately seven in. of right-lateral movement along the San Andreas fault.
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.
The characteristics of the Oroville, California earthquake sequence of 1975 are presented. Historically, the Oroville area is one of low seismicity; the largest earthquake in the region occurred with a magnitude (ML) 5.7 in 1940 some 50 km north of Oroville. The first foreshock of the sequence occurred on June 28, 1975. Twnety-one foreshocks (ML ≧ 1.6), the largest of magnitude 4.7, preceded the magnitude 5.7 main shock of August 1. All foreshocks and aftershocks of ML ≧ 3.0 were located using seismographs operated by the University of California at Berkeley, USGS, and the California Department of Water Resources. The aftershock region covers an area approximately 14 by 10 km southeast of the city of Oroville. The depth distribution of the earthquakes indicates a west dipping fault plane. The b value of 0.61 shows the sequence to be rich in larger magnitude aftershocks. Similar b values have been determined for other aftershock sequences in California, such as a sequence near Coalinga in August 1975. The aftershock occurrence rate follows an Omori relation with n(t∞t−0.70. Apparent variability in the earthquake mechanisms of the series makes interpretation of composite fault-plane solutions difficult, but the data indicate normal faulting striking NNW and a west dip of about 30°–40°, the Sierra Nevada moving up with respect to the Great Valley.
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