Abstract

The Charlevoix Seismic Zone (CSZ) is the most active area of Eastern Canada with more than 1,500 earthquakes (magnitudes from about 0.0 ML to 5.0 mbLg) recorded between 1978 and 1995. Earthquakes in a sub-area of the CSZ were analyzed to find evidence of event clustering and of correlations with geological features. Out of 37 events recorded between 1988 and 1995, two triplets had highly-correlated seismic traces and may represent repeated slip on the same fractures. Focal mechanisms of the two largest earthquakes of the sub-area (magnitude mN 4.0 and 3.3) are consistent with the region's predominant reverse faulting regime. For these two events, nodal planes trend WNW-ESE, parallel to lineaments reactivated by the Devonian Charlevoix meteor impact. Thus, these earthquakes do not follow the generally assumed association with the NE-striking paleorift faults. For smaller events, discrete and composite focal mechanisms show variable trends and complex faulting styles including normal faulting and some part of strike-slip motion. This complexity suggests that small magnitude earthquakes might be related to local stress/strength variations, possibly in areas where high pore-fluid pressures are present.

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