Seismic activity and structural features in the Charlevoix region, Quebec

Lamontagne, M., 1987. Seismic activity and structural features in the Charlevoix region, Québec. Canadian Journal of Earth Sciences, vol. 24, p. 2118-2129.

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 hypocenters. 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 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 nodal 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 earthquake and the north-south faults mapped outside the crater. The magnitude mb (Lg) 5.0 earthquake of August 19, 1979, the largest event to have occurred 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 crater were found to be produced along more variable fault orientations, with an average trend similar to 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 discernable 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.



Last modified August 1999

Dernières modifications Août 1999