Located on the St. Lawrence River some 100 km downstream from Quebec City, the Charlevoix Seismic Zone (CSZ) is the most active seismic zone of eastern Canada with five historical earthquakes in the magnitude 6 to 7 range and continuous microearthquake activity. Contrasting with the general assumption that CSZ earthquakes occur on regional faults, hypocenters recorded between 1977 and 1999 reveal that most events concentrate in highly fractured zones bounded by regional faults.
The positions of regional geological faults are known from remote sensing, magnetic, gravimetric and seismic reflection data. Most of these faults trend parallel to the St. Lawrence River. Most earthquakes do not concentrate along these regional faults, but regroup in seismically active volumes. Within these volumes, the orientations of the reactivated faults (and possibly the local stresses) vary. This is suggested by some 20 focal mechanisms (mostly reverse to reverse-oblique faulting) and by earthquake clusters and multiplets (found to represent less than 15% of the events between 1988 and 1997). Only one cluster of earthquakes (that may include some magnitude > 4 events and possibly the 1925 M 6.2 event) may correspond to a regional fault. A shallow seismic reflection profile that crosses the surface projection of this fault does not show any evidence of recent movement. Hence, the rate of large CSZ earthquake occurrences in the Holocene may be lower than that indicated by the historical record.