from California Geology, November 1979, Vol. 32, No. 11.




San Francisco District

California Division of Mines and Geology

This report is a summary of observations and interpretations of the Division's San Francisco District staff geologists sent to the Coyote Lake-Gilroy area following the earthquake of 6 August 1979. Charles F. Armstrong, Charles C. Bishop, Edward Bortugno, and Salem J. Rice, as well as the active-fault evaluation program staff, William A. Bryant, Trinda L. Bedrossian, and Earl W. Hart, made visits to the field area. The purposes were to inspect the Calaveras fault zone for signs of surface displacement resulting from the earthquake, to identify any associated land instability or geologic hazards, and to gather data for fault evaluation relative to Alquist-Priolo Special Studies Zones. The early observations of fault rupture are summarized by Hart and others (1979). An expanded report on the Coyote Lake earthquake, including a tabulation of these observations will be published in a future issue of California Geology.......EDITOR.

On Monday August 6, at 10:05 a.m. PDT, an earthquake of Richter Magnitude 5.9 (University of California Berkeley Seismograph Station) occurred near Coyote Lake, Santa Clara County, about 50 km (25 miles) southeast of San Jose (see map). The earthquake was shallow, having a focal depth of about 8 kilometers (5 miles). The epicenter was located about 3 kilometers east of the Calaveras fault zone along a trend of earthquake epicenters which have been attributed to the Calaveras fault (Armstrong and Wagner, 1976).


Map showing selected traces of the Calaveras Fault, earthquake epicenters, and locations of recent surface displacement.


The Calaveras fault zone is part of a system of active faults which branch from the San Andreas fault south of Hollister. The system may extend along the entire length of the northern Coast Ranges (Herd, 1978). Like the other faults in the system the Calaveras is a right-lateral, strike-slip fault. The Calaveras fault extends from a point south of Hollister northward at least 160 km (100 miles). The Calaveras fault is seismically active; numerous earthquake epicenters have been located along its trace.

With the possible exception of the 1861 San Ramon Valley earthquake, ground rupture accompanying an earthquake has never been observed on the Calaveras fault. Ground displacement in the form of aseismic creep has historically occurred along the Calaveras fault in the area from Anderson Lake to Hollister. An annual creep rate of 1.0 cm is well documented by creep-meters and offset fences, streets, curbs, sidewalks, and orchard rows.


The Coyote Lake earthquake was felt over the entire San Francisco Bay area and parts of north-central California. Tremors were felt as far away as Santa Rosa, Sacramento, and Reno, to the north and northeast; the Sierra Nevada and Fresno to the east; and Monterey and Carmel to the south. The earthquake caused only very minor damage in San Francisco although definite swaying sensations in highrise buildings were reported.

At the CDMG San Francisco District Office, in the Ferry Building, two distinct shock waves were felt. The first, a P-wave (pressure wave), began with a sharp jolt and lasted for 5 to 10 seconds. It was immediately followed by a rolling S-wave (shear wave) which lasted about twice as long. The seismograph at U. C. Berkeley showed that the S-wave came 13 seconds after the start of the P-wave.

The earthquake caused very little damage in the epicentral area, which is mountainous and sparsely populated, although trees were reported to have shaken violently. A U. S. Geological Survey (USGS) strong motion instrument at Coyote Dam recorded a horizontal acceleration of .28 g (28% of the acceleration of gravity). Much greater damage occurred in the nearby towns of Gilroy (9 km south), where a horizontal acceleration of .42 g was recorded (USGS), and Hollister (30 km south). The highest Modified Mercalli intensities (VII) were observed in the Gilroy and San Felipe Lake areas. Intensities were higher in these areas than in the epicentral area probably because these areas are on deep, alluvial-filled valleys, and the earthquake energy propagated southward.

SEE PHOTOS 1, 2, 3

Photo 1. Freshly opened, left-stepping, en echelon cracks in east Dunne Road on the northeast side of Anderson Lake. Looking south. Photo August 10, 1979 by Earl W. Hart. (Location number 5 on map).

Photo 2. Freshly open, left-stepping, en echelon cracks in asphalt street. Looking north across Third Street at the corner of Powell Street, Hollister. Photo August 10, 1979 by author. (Location number 3 on map).

Photo 3. Freshly open, left-stepping, en echelon cracks in asphalt on Shore Road. Fresh 2-3 cm scarp in soil in foreground indicates vertical displacement (east side down). Photo August 6, 1979 by author. (Location number 4 on map).


Ground displacement was observed along the Calaveras fault zone from Anderson Lake, north of the epicenter, southward as far as Hollister (see map), a distance of 39 kilometers (24.2 miles). The surface displacement was manifested as freshly opened left-stepping, en echelon cracks (which indicates right slip) in asphalt pavement and soil (See photos 1, 2, and 3). In at least one instance (number 1 on map) the displacement consisted of single cracks showing right-lateral, strike-slip movement. A maximum right-lateral displacement of 5-6 mm was observed at this location on the afternoon of August 6. By August 10 the offset had increased to about 10 mm.

Afterslip also occurred at other locations (e.g. numbers 2 and 3 on map) between August 6 and 10. At location number 2, fresh en echelon cracks appeared between August 20 and 25. A USGS monitored creepmeter at map location number 4 recorded a total of 14 mm of displacement over a two-week period starting several days prior to the earthquake; this is interpreted to be the result of several discrete creep events and a small amount of coseismic movement (Robert Burford USGS, oral communication). At this location (number 4 on map), dip-slip displacement (down to the east) of 2 - 3 cm occurred.

In all cases, the fault-related ground displacement occurred on previously mapped traces of the Calaveras fault (Rogers, unpublished report; Radbruch-Hall, 1974; Armstrong and Wagner, 1976; and Wagner, 1978) and were within Alquist-Priolo Special Studies Zones. Evidence of ground lurching, settlement, and slumping as a result of ground shaking was observed at many locations between Anderson Lake and Hollister.


A comparison between the August 6 event and other earthquakes of the same magnitude presents several problems. The apparent surface rupture length (39 km) of this earthquake is roughly twice the length of the aftershock zone, which would normally coincide with the slip surface. The observed surface displacements were sparsely distributed along the apparent surface rupture length. If the apparent length of surface rupture for the 6 August 1979 earthquake is compared with that of other earthquakes of the same magnitude (Bonilla and Buchanan, 1970, and Slemmons, 1977), it appears to be anomalously long --- at least twice as long as expected. If the maximum apparent surface displacement (.5 cm) from this earthquake is compared with that of other earthquakes of the same magnitude (Bonilla and Buchanan, 1970, and Slemmons, 1977), it appears to be anomalously low---1/10 to 1/20 of the displacement which would be expected.

It seems improbable that the fault rupture which caused the August 6 earthquake could have propagated to the ground surface. It is more likely that the observed surface displacement was earthquake-induced fault creep and post earthquake afterslip. Strain may have accumulated at the surface along the fault zone, and was released during and after the August 6 earthquake as spasmodic fault creep.


Armstrong, C. F., and Wagner, D. L., 1976, Environmental geologic analysis of the Diablo Range study area, southern Santa Clara County, California: California Division of Mines and Geology open-file report 78-11-SF, map scale 1:12,000.

Bonilla, M. G., and Buchanan, J. M., 1970, Interim report on worldwide historic surface faulting: U. S. Geological Survey open-file report.

Hart, E. W., Bryant, W. A., and Bedrossian, T. L., 1979, Observations of fault rupture associated with the August 6, 1979 earthquake and tabulation of observed data: California Division of Mines and Geology, in house memo of August 16, 1979 to C. F. Armstrong, 6 p., map scale 1:24,000.

Herd, D. G., 1978, Intracontinental plate boundary east of Cape Mendocino, California: Geology, v. 6. no. 12, p. 721-725.

Radbruch-Hall, D. H., 1974, Map showing recently active breaks along the Hayward Fault zone and the southern part of the Calaveras fault zone, California: U. S. Geological Survey Map 1-813, scale 1:24,000.

Rogers, T. H., unpublished report, Some aspects of the general geology and seismic geology of the Hollister and San Felipe quadrangle, San Benito, Santa Clara and Monterey Counties, California: California Division of Mines and Geology, map scale 1:24,000.

Slemmons, D. B., 1977, State-of-the art for assessing earthquake hazards in the United States, report 6; in Faults and earthquake magnitude: U. S. Army Engineer Waterways Experiment Station Miscellaneous Paper S-73-1.

Wagner, D. L., 1978, Environmental geologic analysis of the Diablo Range study area II, southern Santa Clara County, California: California Division of Mines and Geology open-file report 78-12-SF, map scale 1:12,000.