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The Fort Ross earthquake sequence, March and April, 1978

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

On March 31, 1978, an earthquake of coda magnitude 3.3 (M_L = 3.7 BRK) occurred 5 km off the coast of northern California near Fort Ross. A single foreshock preceded the earthquake and approximately 60 aftershocks followed. Locations based on P- and S-wave arrival times indicate that the earthquakes occurred offshore, west of the San Andreas fault in the vicinity of a fault that is visible on acoustic reflection profiles. All earthquakes had hypocentral depths less than 8 km. A fault-plane solution from P-wave first motions suggests that the focal mechanism of the main shock consisted of nearly equal components of dextral and vertical movement on a plane striking northwest. Other events in this sequence had first-motion patterns strikingly different than the main shock, indicating that more than one type of faulting occurred during the sequence. Seismic moments computed for three aftershocks ranged from 3.8 x 10¹⁹ to 1.1 x 10¹⁹ dyne-cm.