abstract

Several thousand small earthquakes, with magnitudes ranging up to 4, occurred near Danville, California, during May, June, and July 1970. Seven temporary seismographs were installed near the epicentral region to augment an existing telemetered network within 1 day after the first felt earthquake. The dense concentration of 10 seismographs within 12 km of the seismic activity and the existence of a reversed seismic-refraction profile through the region permitted a very detailed study of this earthquake sequence. Over 400 events with magnitude greater than 0.5 were located with an average accuracy of ±1 km. The main earthquake region was approximately spherical: 1 km in radius, 6 km deep, and centered about 5 km southeast of Danville, 40 km east of San Francisco. Fault-plane solutions indicate that faulting associated with the earthquakes was of the right-lateral strike-slip type, with a strike of N 35°W.

The earthquakes cannot readily be related to any previously identified geological structure or fault. They suggest that active right-lateral faulting is not restricted to the mapped fault trace of the Calaveras fault. This result implies that not all of the sources of future earthquakes in the San Francisco Bay region can be indentified by surface mapping alone. The use of small earthquakes to map active faulting at depth may help to identify areas of potential earthquake hazards.

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