abstract

The locations of all instrumentally recorded earthquakes of magnitude greater than 412, which occurred in the St. Lawrence Valley near La Malbaie, Québec, in the period 1927 to 1978, have been reexamined, along with some smaller associated events. Although the cataloged epicenters scattered on both shores of the St. Lawrence River, the relocated epicenters fall into two distinct epicentral areas, which coincide with the ends of the 70-km-long northeast trending zone of recent microearthquake activity. This zone lies mainly beneath the St. Lawrence River between Ile aux Lièvres to the northeast and Ile aux Coudres to the southwest.

The major St. Lawrence earthquake of 1925 originally located (±40 km) close to the town of La Malbaie, near the middle of the current microearthquake zone, is relocated 30 ± 15 km northeast near Ile aux Lièvres. If the pattern of the past half-century continues, significant earthquakes may be expected only near the two ends of the zone.

Structures known at present from geological or geophysical data do not explain completely the preferential stress concentrations near Ile aux Lièvres and Ile aux Coudres. However, the Charlevoix impact crater and Logan's contact together may influence the concentration of regional stress producing the larger earthquakes just beyond the crater rim near Ile aux Lièvres.

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