Stress Channelling and Partitioning of Seismicity in the Charlevoix Seismic Zone, Canada

The Charlevoix seismic zone in the St. Lawrence valley of Québec is historically the most active in eastern Canada. The structurally complex region comprises rift faults formed during the opening of the Iapetus Ocean, superimposed by a 350 Ma meteorite impact structure, resulting in a circular highly fractured zone. Although seismicity is localized along two steeply dipping planar rift-parallel zones, previous work indicates that most of the large-scale rift faults appear to bound seismicity rather than generate earthquakes themselves. In order to gain insight into the mechanics of the partitioning of this seismicity, a simple two-dimensional model of the Charlevoix seismic zone was built using the finite difference code FLAC. The rift-related faults are represented by discontinuities, which are assigned various frictional strength parameters. The heavily fractured impact structure is represented by an elastic continuum of reduced modulus. Boundary displacements are used to generate a regional stress field in the direction of tectonic loading. Given a high strength, the rift faults have little effect on the stress patterns. Stress trajectories naturally flow around the region of reduced elastic modulus, leaving the fractured area with lower stresses than the background level. However, when the rift faults have a low strength, they are unable to support stress trajectories inclined to them, due to the resolved shear stress exceeding their strength. This prevents trajectories from diverting out of the rift, effectively channelling higher magnitude stresses into the region of the impact structure between the faults than would naturally occur. Low- strength bounding faults can thus explain the localization of seismicity into linear bands, rather than distributed seismicity throughout the impact structure. It also explains how the rift faults act as boundaries to seismicity. These results indicate that the interplay between faults of varying strength and zones of differing elastic modulus can give rise to complicated stress patterns, and can explain many of the seismicity patterns observed in the Charlevoix seismic zone.