Abstract

The Charlevoix region is historically the most active earthquake zone in eastern Canada. Understanding the links between its seismicity and the faults of the region is important for the assessment of earthquake risk along the St. Lawrence Valley. The region has been monitored by a microseismic array since 1977, yielding accurate locations of the hypocentres. Previous analyses of data from the array indicated a relationship between the earthquakes and the St. Lawrence Valley paleorift faults. As a sequel to previous studies, the relationships between the seismic activity and the faults of the region were reexamined through the use of the composite P-nodal solutions, in an effort to clarify the nature of faulting in the seismic zone. The microseisms were partitioned into subsets of events on the basis of geological and hypocentre-trend considerations. The main objectives of this paper are to delineate the details of faulting within the Charlevoix region and to determine the effect of the impact crater on the nature of faulting in this area.Assuming a constant 6.2 km/s velocity model and using a data set of 107 events, composite fault-plane solutions were computed. The composite P-nodal solutions indicated that the Charlevoix impact crater modifies to a certain extent the focal-mechanism characteristics. Events outside the impact crater were found to be quite consistent in their polarity distribution on the focal sphere, suggesting similarity in their focal mechanisms. The composite mechanism of these events suggests a relationship between the earthquakes and the north–south faults mapped outside the impact crater. The magnitude mb (Lg) 5.0 earthquake of August 19, 1979, the largest event in the selected time window, had different fault planes than some of its aftershocks. Nevertheless, the polarity distribution of the aftershocks was in agreement with the average trend for the events outside the crater. Events inside the impact crater were found to be produced along more variable fault orientations, with an average trend similar to that of the rift fault system. It is proposed that the meteor impact weakened the rift faults and introduced its own fractures. The present earthquake activity probably occurs along these weak fault surfaces. The effect of the impact crater on the type of faulting versus depth is not readily discernible from available data. In general, meteor impacts do not leave neotectonic seismic signatures: the Charlevoix impact crater might represent a different case because of the presence of weakened paleorift faults.

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