Abstract

Seismicity in eastern Canada hos tendency to occur in regions where ancient zones of weakness are reactivated by neotectonic processes. Seismicity along the St. Lawrence Valley occurs along steeply dipping Precambrian normal faults that were reactivated when the proto-Atlantic Ocean opened (600–700 MYA) and again when the Atlantic Ocean opened (150–200 MYA), subjecting the region along the valley to a horizontal compressive stress. Satellite imagery along the St. Lawrence and interconnected grabens delineates numerous faults (and linears) that partition the grabens and adjacent shield areas into megablocks that could be undergoing different vertical motion, thus providing a rationale as to why seismicity is not continuous along and adjacent to these grabens. The Charlevoix region, historically the most seismically active in eastern Canada, is unique in the sense that seismicity is occurring in a region where a crater impact area overlaps the St. Lawrence River. Shock waves from this meteor impact, which occurred about 350 MYA, likely reactivated and increased the permeability of these ancient zones of weakness along the St. Lawrence River; thus water from this river seeping down these permeable faults could contribute to current seismicity by effecting changes in pore pressure and fault gouge characteristics (frictional resistance). The Miramichi earthquake sequence, which commenced in January of 1982, is confined primarily to a granitic intrusion, and consequently it is hypothesized that seismicity is occurring along weakened linears that were generated during the formation (about 380 MYA) of the intrusion. The high horizontal stress field in the epicentral area indicates the possibility of stress enhancement of the tectonic stress field within the intrusion. A notable feature of the aftershock activity is the delineation of a conjugate, V-shaped pattern along a W-E vertical profile. Differences in focal depth, which range between 3–9 km for Miramichi hypocenters and 5 to 22 km for Charlevoix hypocenters, can be attributed to differences in the depth where brittle failure ends and viscoelastic processes start; different temperature-depth profiles are a major contributor as the heat flow is moderate (1.5 HFU) in the Miramichi region, and low (1.0 HFU) in the Charlevoix region. The correlation between postglacial rebound stresses and earthquake activity has been documented along the Boothia - Bell Arch system and Baffin Island - Baffin Bay region in northern and northeastern Canada respectively. However, whether postglacial rebound stresses can actually dictate the mode of failure or simply act as a triggering mechanism for earthquakes is a contentious issue. In addition, there is a dichotomy in terms of whether the neutral stress state exists with the ice bad on or off; the reason for the former view is that the ice bad has been on more than it has been off during the past 100,000 years so that transient stresses generated during loading would dissipate over this time span. Until these differences can be resolved, and vertical motion doe to postglacial rebound can be differentiated from tectonic movement and processes, the cause - effect relation between earthquake activity in eastern Canada and postglacial rebound remains uncertain.

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