Relationship between structures, stress and seismicity in the Charlevoix seismic zone revealed by 3-D geomechanical models: Implications for the seismotectonics of continental interiors

The Charlevoix seismic zone in the St. Lawrence valley of Québec is the most active in eastern Canada. The structurally complex region comprises a series of subparallel steeply dipping Iapetan rift faults, superimposed by a 350 Ma meteorite impact structure, resulting in a heavily faulted volume. The elongate seismic zone runs through the crater parallel to the rift. Most large events localize outside the crater and are consistent with slip along the rift faults, whereas background seismicity primarily occurs within the volume of rock bounded by the rift faults within and beneath the crater. The interaction between rift and crater faults is explored using the three-dimensional stress analysis code FLAC3D. The rift faults are represented by frictional discontinuities, and the crater is represented by a bowl-shaped elastic volume of reduced modulus. Differential stresses are slowly built up from boundary displacements similar to tectonic loading. Results indicate that weakening the rift faults produces a stress increase in the region of the crater bounded by the faults. This causes a decrease in stability of optimally oriented faults and may explain the localization of low-level seismicity. Additionally, slip distribution along the rift faults shows that large events localize at the perimeter of the crater and produce focal mechanisms with P axes oblique to the applied stress field, consistent with historic large earthquakes. It is speculated that similar systematic rotation of focal mechanism P axes may be expected along other intraplate rift zones, raising a potential caveat for the use of focal mechanisms for stress estimation in continental interiors.