Rheological and geological constraints on the earthquake distribution in the Charlevoix Seismic Zone, Quebec, Canada

Located an 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 5 historical earthquakes in the magnitude 6 to 7 range and continuous microearthquake activity. Between October 1977 and December 1997, some 1500 earthquakes with magnitudes between --1.0 and 5.0 were recorded by a local seismograph network. Epicentres define a 30 by 85 km ellipse with the long axis parallel to the river, with 99% of hypocentres shallower than 25 km depth (D99%). Earthquakes are not distributed uniformly across the seismic zone, but concentrate in groups separated by less active areas.;On the basis of the heat conduction equation and Grenvillian surface heat flow, the temperature at D99% has been estimated to be between 215 and 355°C. These temperatures and the inferred quartz-depleted mid- and lower-crustal compositions imply a brittle-ductile transition deeper than 25 km. The quartz-depleted mid- to lower crust is supported by the high P-velocities revealed by seismic refraction surveys. This depth may represent the passage from velocity weakening to velocity strengthening if the onset of flow in hydrated feldspars occurs at temperatures in the 300--350°C range. With an assumed maximum crustal stress difference of about 200 MPa, fault reactivation at mid-crustal depth can occur with a low friction coefficient and/or with a high pore fluid pressure.;Remote sensing, magnetic, gravimetric and seismic reflection data are used to define the positions of geological faults. Most regional normal faults correspond to lineaments or geophysical trends generally parallel to the St. Lawrence River. Most regional faults bound active volumes while one correlates with earthquake hypocentres, including some magnitude >4 events.;The orientations of the local stress and of the reactivated faults vary across the CSZ. The comparison of some 20 focal mechanisms with earthquake groups and multiplets suggest reactivated faults with various orientations, not necessarily parallel to the regional paleo-rift faults. Most microearthquakes appear to occur in highly fractured volumes, partly related to the impact structure. No surface rupture is found on the seismic reflection lines acquired on the St. Lawrence River.;Using local earthquake data, a pseudo-2D layered velocity model is proposed. The main upper crustal velocity perturbations are imaged. From Vp/Vs ratio information, the CSZ velocity structure differs from that of the neighbouring Laurentides Park region, where upper crustal basic rocks are found.;Even with the addition of all local stress contributors, the CSZ is not subject to substantially higher stress difference levels than the rest of Eastern Canada. Consequently, the anomalous CSZ earthquake activity must be due to inherent crustal weakness and/or high pore fluid pressure. Although the presence of a gouge may weaken some faults, the existence of pervasive high pore-fluid pressure coupled with a high degree of fracturing is the favoured explanation for the weak crust that gives rise to earthquake activity. A qualitative model is proposed where some rift faults act as conduits to crustal fluids under pressure, triggering earthquakes on these faults and in neighbouring fractured volumes. (Abstract shortened by UMI.).