A- Located some 150 km downstream from Quebec City, the Charlevoix Seismic Zone is the most active earthquake zone of Eastern Canada. Five earthquakes in the magnitude 6-7 range have occurred there in historical times.
B- In addition, frequent micro-earthquake activity is recorded over an area of about 30 km X 80 km elongated along the St. Lawrence River (representing some 1800 events of magnitude -0.5 to 5.0 between 1977 and 1996). The focal characteristics of the earthquakes vary geographically, possibly due to the level of fracturing created by the Devonian meteor impact. The largest events (mbLg >= 4.0) appear to concentrate at both ends of the seismic zone, slightly outside the impact crater rim. On the other hand, the central part shows a higher level of activity but with smaller events.
C- A high level of fracturation is apparent from the seismic traces of the events as well as from the complex focal mechanism patterns. While some earthquakes show highly correlated seismic traces indicative of repeated slips on the same fracture plane, most appear to be isolated events. In a small area towards the NE end of the zone chosen for a special study, a systematic comparison of seismic traces revealed only two groups of multiplets out of 37 events.
D- Similarly during the summer 1996 field survey, a few swarms of events with similar seismic traces were recorded.
E- Focal mechanisms indicate a complex pattern of fault reactivation where local stress/strength variations exist. These highly variable focal mechanisms for small events may be explained by pervasive high pore-fluid pressures in a highly fractured environment. The exact source of these high pore-fluid pressures remains unknown.