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 Earth Sciences Sector > Priorities > Canadian Hazards Information Service > Earthquakes Canada |
in the Charlevoix, Canada, intraplate seismic zone
Lamontagne, M. and Ranalli, G. 1996. Thermal and rheological constraints on the earthquake depth distribution in the Charlevoix, Canada, intraplate seismic zone. Tectonophysics, 257, 55-69.
The Charlevoix Zone is the most active seismic area of eastern Canada. For the period 1978 to 1993, 99% of earthquakes occurred at less than 25 km depth and 80% at less than 15 km. This depth distribution is compared with the estimated brittle-ductile and velocity weakening-velocity strengthening transitions. Using realistic ranges of thermal parameters and a 41 +/- 10 mW/m2 surface heat flow, one-dimensional thermal models show that 90% of 22,000 computed geotherms fall between 215 and 355oC at 25 km. For the central value of heat flow, this range is reduced to 280 and 340oC. These temperatures and the inferred mafic mid- and lower-crustal composition imply a brittle-ductile transition deeper than 25 km. With a higher than average geotherm, the maximum depth of seismicity could correspond to the velocity weakening-velocity strengthening boundary. The basic lower crust of the area precludes a correlation of this depth with the onset of ductility of quartz at around 300oC. However, it may correspond to the onset of ductility for hydrated feldspar at about 350oC if the geotherm is relatively high. Since the maximum possible crustal stress difference is unlikely to be larger than 200 MPa, high pore-fluid pressures and/or low static coefficient of friction are required for the occurrence of lower-crustal earthquakes.
