Seismicity and Rheology of the Charlevoix Seismic Zone, Quebec, Canada

G. RANALLI and M. Lamontagne

Abstract

The Charlevoix Seismic Zone (CSZ) is the most active seismic zone of Eastern Canada, with five historical earthquakes in the 6-7 magnitude range and high background activity (about 1500 shocks with magnitude 0 to 5 from 1977 to 1997). Epicentres define a 30x85 km ellipse along the St. Lawrence River 100-150 km downstream of Quebec City. The hypocentre distribution shows an increase with depth to a peak at 10-12 km, followed by a decrease: 99% of all events are shallower than 25 km. Most events occur in highly fractured volumes that are sometimes bounded by regional faults.

The temperature at a depth of 25 km is estimated to be between 220 and 360 C. Values in the upper range coincide with the brittle/ductile transition if the rheology of the lower crust is quartz-controlled. However, most interpretations of the CSZ require a more basic lower crust. Consequently, neither the peak nor the cutoff of seismicity in the CSZ appear to be related to the transition to bulk ductility.

Focal mechanism solutions show predominantly reverse faulting on steeply dipping planes. The reactivation of these fractures requires high pore fluid pressure and/or low friction coefficient. If earthquakes in the CSZ are a consequence of frictional Coulomb failure, the evidence points to the occurrence of relatively high fluid pressures (~ 60-90% of lithostatic) in the lower crust.



Last modified August 1999

Dernières modifications Août 1999