Abstract

The St-Donat earthquake of February 18, 1978, occurred in a diffuse zone of significant seismicity in western Quebec. A field survey detected two aftershocks (graphic) in a 3 day period following the magnitude mb(Lg) 4.1 main shock. These were followed by a magnitude mb(Lg) 3.4 earthquake on February 23, after the field stations were removed. Main shock and aftershock activity occurred within a small active volume located at 46 °19.2′N, 74°06.6′W, depth 7 km, with an estimated uncertainty of 1 km on all three hypocentral parameters. The main shock seismic moment was calculated to be 0.8 × 1022 dyne∙cm (8 × 1022 μN∙cm).The tremor was felt over an area of 70 000 km2 in western Quebec, eastern Ontario, and northern New York State with a maximum radius of perceptibility of 185 km and a maximum intensity of V in the St-Donat area. A comparison of the isoseismals of this and three other recent earthquakes of magnitude near 4 suggests a variation of maximum intensity inversely with focal depth; a significant area of intensity V is only observed when the focal depth is less than 10 km. There is also an indication of a systematic change in focal depths across the western Quebec zone, from shallow (upper crustal) depths north of Montreal to deeper (midcrustal) depths north of Ottawa.The P-nodal solution indicates almost pure thrust motion on a plane striking N20°W and dipping either 40°NE or 50°SW. The deviatoric compression axis is nearly horizontal, in a west-southwest direction. Similar results from the 1975 Maniwaki, Quebec, earthquake suggest that a uniform stress condition exists throughout western Quebec.

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