The Charlevoix zone is the most active seismic area in eastern Canada, with more than 2000 earthquakes of magnitude ML>= 0.5 between 1978 and 1997. The epicentres occupy an approximately ellipical area of dimensions 30 X 85 km and long axis parallel to the St. Lawrence River, which was subject to geological faulting and intense fracturing during the St.Lawrence rift episode and a Devonian meteor impact.The earthquake depth distribution exhibits an approximately linear increase down to 10 km, followed by a nearly exponential decrease to 30 km. About 99% of earthquakes occur at depths less than 25 km. The temperature at this depth is estimated to be in the 215 to 355 oC.
Most earthquakes reactivate pre-existing faults in a thrust to oblique-thrust regime as inferred from focal mechanisms. On the basis of the Coulomb-Navier shear fracture criterion (the so-called Byerlee law), thrust faulting at depth larger than about 15 km requires unrealistically high stress differences (in excess of 200-300 MPa) for an average value of the friction coefficient mu=0.75 and low-to-moderate pore fluid pressures (less than hydrostatic, i.e. lambda<=0.40). It appears therefore that faulting (especially thrust, which requires the largest stress difference) at mid-to-lower crustal depths is associated not only with sufficiently low temperatures (a condition satisfied in the Charlevoix zone), but also with shear strengths less than the values implied by the usual frictional law.
Assuming the validity of a Coulomb-type law, a decrease in shear strength can be achieved by a decrease in the friction coefficient and/or an increase in pore fluid pressure. Taking 200 MPa as a realistic upper limit for stress difference and a depth of 20 km as an example, thrust reactivation can occur, for zero pore-fluid pressure, if the friction coefficient is low; if on the other hand, the friction coefficient is comparable to low-pressure laboratory values, a high pore-fluid pressure (lambda >= 0.7) is nessary. Independently of the occurrence of soft fault material (clay, graphite), a decrease in friction coefficient with increasing pressure has been experimentally observed in most rocks. This, coupled with the intense fracturing of the rock and with moderate-to-high pore fluid pressures, may provide an explanation for the lower crustal seismicity in the Charlevoix zone and in other continental intraplate environments.