A preliminary study of the Coyote Lake earthquake of August 6, 1979, and its major aftershocks

The August 6, 1979 earthquake (magnitude 5.7 + 0.2) was located about 1 km east of the Calaveras fault trace near Coyote Lake, at a depth of about 10 km. The spatial distribution of the aftershocks in the first 15 days and the focal mechanism of the main shock and selected aftershocks suggest that faulting took place primarily to the southeast of the main shock along a 25-km segment of the Calaveras fault zone at a depth of 4 to 12 km. The motion was right-lateral strike-slip along a nearly vertical fault plane. Discontinuous surface rupture was observed along a 14.4-km length of the recently active trace of the fault southeast of Coyote Lake, but no surface faulting was seen in the epicentral area. A maximum 5-mm fault offset was recorded near San Felipe Lake, about 16 km south of the main shock.

There was no prominent foreshock activity. The cumulative number of aftershocks (N) versus magnitude (M) could be described by the equation log N= 3.33 - 0.75 M, in the magnitude range between 1 and 3.5. The number and size of aftershocks appeared to be much less than that expected for a main shock of magnitude 5.7.