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PR 25

by R.L. Porcella, R.B. Matthiesen; R.D. McJunkin and J.T. Ragsdale

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A moderate-size earthquake (magnitude 5.7) occurred at 10:05 (local time) on August 6, 1979 in the central California coastal region (figure 1). The earthquake was instrumentally located in the Calaveras fault zone at Coyote Lake approximately 10 km north-northeast of Gilroy, California. Ground shaking caused minor damage in Gilroy, Hollister, and nearby communities. This seismic event is the largest to occur in the region since the magnitude 6.6 earthquake in 1911.

The following parameters, after Lee and others (1979), are for the main shock:
           Origin time: 17:05:22.3 06 Aug 79 (UTC)
           Epicenter: 37.11 N, 121.53W (± 1 km)
           Focal depth: 9.6 km (± 2 km)
           Magnitude : 5.7 (± 0.2), USGS network
                              5.9, Berkeley, Seismographic Station.

The right-lateral, strike-slip Calaveras fault zone is one of the principal active zones in the central California coastal region. The Calaveras and its associated faults form a part of a northwest trending zone that branches from the San Andreas fault south of Hollister (Jennings and others, 1975; Herd, 1978). The Calaveras fault zone extends more than 170 km from south of Hollister to Suisun Bay and appears to be related to historically active faults to the north. In the area of the August 6, 1979 earthquake, the Calaveras fault zone is within the Diablo range foothills east of the Santa Clara Valley. The fault zone is coincident with a northwest-trending valley in which Coyote Lake and Anderson Reservoir are impounded. South of Coyote Lake the Calaveras fault zone crosses the southern end of the Santa Clara Valley and passes through Hollister. North of Coyote Lake the Hayward fault branches westward from the Calaveras fault and trends parallel to the San Andreas fault. Right-lateral movement and fault creep have been documented by the offset of correlative rocks (Nakata, 1977), streams (Radbruch-Hall, 1974), and streets, sidewalks, and curbs (Rogers and Nason, 1971; Radbruch-Hall, 1974). At least one historic earthquake on the Calaveras fault has been accompanied by surface rupture.

More than 1,000 locatable aftershocks were recorded in the first fifteen days after the main shock. Most aftershocks occurred southeast of the main shock (figure 2). There was no prominent foreshock activity (Lee and others, 1979).