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Aftershocks of the 22 November 1977 earthquake at Willits, California: Activity in the Maacama fault zone

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
U.S. GEOLOGICAL SURVEY NATIONAL CENTER, RESTON, VIRGINIA 22092

Abstract

In late 1977, a moderate-sized earthquake and associated aftershock sequence occurred along the Maacama fault zone in the northern Coast Ranges near Willits, California. The main shock, M_L = 4.8, was located on the east side of Little Lake Valley. An 11-station temporary seismic network recorded aftershocks for 2 weeks after this event. The aftershock locations are along the southwest member of two northwest-southeast-trending seismically active lineations that are situated about 30 km apart as defined by CALNET data. Four possibly unrelated events occurred about 20 km east-northeast of the main aftershock zone, about half-way between the two seismic lineations. A b value of 1.1, typical for an aftershock sequence, was obtained for events above a magnitude of 1.35. Seismic activity was centered about 5 km east-northeast of Willits, where 83 microearthquakes define a volume of activity delimited by an epicentral area of about 20 km² at depths from 5 to 14 km. The northwestern part of this volume is along the Rocktree Valley fault, a strand of the Maacama fault zone. Fault-plane solutions of first motion and locations for 75 events define a right-lateral, strike-slip plane striking N29°W and dipping about 76° to the northeast. A composite focal mechanism of events in a small southwestern section of the aftershock volume shows right-lateral, strike-slip planes rotated clockwise about 45° from the orientation of the composite of the main group. Projection of planes downward from mapped fault traces suggests that the aftershocks are related to a complex geometry of faulting. The Willits earthquake and its aftershocks may manifest the movement of the northeast component of a right echelon step that forms a pull-apart basin in Little Lake Valley.