Abstract

The focal mechanism has been determined for an intermediate-magnitude earthquake in the most seismically active region in Eastern Canada. On August 19, 1979, an mb (Lg) 5.0 earthquake occurred in the Charlevoix seismic zone, under the St. Lawrence River near the towns of St. Fidele and La Malbaie, Quebec. Maximum reported intensity in the epicentral region was MMV; the steepest gradient in the isoseismal contours is in a southeasterly direction, where there are northeast-southwest trending structures of the Appalachian Province. The P-wave nodal solution indicates thrust faulting on both nodal planes, in general agreement with the solutions from microearthquakes in the same epicentral region. Focal depth of aftershock activity has an average value of 9.5 km, with a spread of ±1 km; our preferred estimate of the focal depth of the main shock is 10 ± 2 km. From an analysis of surface wave data of the main shock, an estimate of focal depth falls in the range 6–11 km, in agreement with aftershok data. A mean value of the aftershock area is 3.5 km2, which is taken to represent the fault area of the main shock, and under the assumption of a circular fault, the stress drop of the main shock is estimated to be 50 bars. Because of the presence of zones of weakness in the epicentral area, the direction of maximum principal stress is only loosely constrained (within northeast to southeast quadrant).

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