Depth-Dependent Crustal Stress Rotation and Strength Variation in the Charlevoix Seismic Zone (CSZ), Quebec, Canada

Intraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new data set of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that S-Hmax gradually rotates clockwise from approximately St. Lawrence River-parallel (similar to 54 degrees) near the surface to river-perpendicular (similar to 120 degrees) in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between similar to 13 and similar to 26 km depth, where glacial rebound induced stress perturbation is further amplified by a "weaker" middle crust. Finally, depth-dependent b-values confirm the rheological difference between upper and middle crust in the CSZ.