Focal Mechanisms of Relocated Earthquakes and Stress Orientation in the Charlevoix Seismic Zone

Abstract:

Focal mechanisms of relocated earthquakes (M ≥ 2) in a 3D model derived from a previous tomography study for the Charlevoix Seismic Zone (CSZ) were determined using wave polarities and amplitude ratios as constraints to understand the stress state of the seismic zone. Stress inversion of all the focal mechanisms shows that the orientation is 99.4º, similar to previous studies. However, stress inversion of focal mechanisms grouped in area and in depth show that stress changes within the seismic zone in systematic ways. The results show a partitioning in the orientation between the earthquakes in the NW cluster relative to the SE cluster, but in an opposite sense of relative orientation compared to previous studies; orientation in the NW cluster is 121.1º compared to the 89.7º for the earthquakes in the SE cluster. We observed a lower orientation within the impact structure (53.9º) compared to the orientation outside the impact structure (100.2º), similar to the orientation for all the earthquakes. The results also show that the degree of clockwise stress changes increases with depth, contrary to previous numerical stress modeling of the CSZ. The orientation changes from about 72.5º at a depth slice (±2.5 km) of 5 km to 166.1º at 25-km depth slice. The increase in orientation with depth is also observed when we considered different depth ranges. The orientation changes from 87.8º between 6-12 km to 131.4º for 12-40 km depth; orientation in 0-6 km depth range is less constrained. We speculate that the combined effect of the high-velocity body imaged at mid-crustal depth from previous tomography study, the impact structure and the fault geometry may explain observed stress variations in the CSZ.