Second microearthquake survey of the St. Lawrence Valley near La Malbaie, Quebec

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Abstract
A second microseismicity survey was conducted in the La Malbaie region during 2 summer months of 1974. Up to 19 stations were occupied simultaneously for most of the period. Thirty-four microearhquakes were located using at least four stations; the epicenters cover a region of about 70 × 40 km, centred in the St. Lawrence River. Thirty-three of the events are within the station configuration. All hypocenters are in the Precambrian rocks of the Grenville Province at an average focal depth of 11 km; none was located in the Ordovician sediments. Six fault-plane solutions were obtained, all having one nodal plane striking from north to northeast; thrust motion is predominant. On the basis of the confined and clearly delineated distribution of the hypocenters the influence of the Charlevoix impact structure on the length of the active zone is again proposed; this idea had already been advanced after the first microearthquake survey in 1970. The conjunction of this structure with Logan's line could also have some influence on the seismicity. Strikingly, both the focal parameters, and rate of occurrence of 1 event per 2 days observed in the two surveys are in agreement. Finally it is suggested, in view of the confined zone of microactivity, that the epicentral distribution of historical events could be regarded as less extensive than it appears.

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