WAVEFORM AND GEODYNAMIC MODELING OF SEISMICITY ASSOCIATED WITH THE CHARLEVOIX SEISMIC ZONE

The Charlevoix seismic zone (CSZ) is the most seismically active region in eastern Canada. It is located within the Late Proterozoic-Early Cambrian St. Lawrence rift zone in southeastern Quebec. The location was also the site of a Devonian impact crater. The CSZ poses a high seismic risks due to its seismicity compared to the other seismic zones in the area and its proximity to densely populated cities (e.g., Quebec City, Ottawa, and Montreal). It is also responsible for much of the seismic hazard in New England. Previous studies in the CSZ observed a maximum compressive stress orientation of N86, which represents a 32 clockwise rotation from the regional orientation determined from nearby boreholes (i.e., 54). Dividing the CSZ into two clusters, one northwest of the SLF and one to the southeast. The stress inversion for earthquakes in the NW cluster) and for earthquakes in the SE cluster yielded orientations of N55 and N101, respectively. Previous studies also suggest two sets of geometries for the rift faults. One set has a uniform dip of 70SE for all three faults while the other has 65, 40, and 40SE, from north to south, respectively. My research focuseson the waveform and geodynamic modeling of seismicity associated with the CSZ by examining the characteristics of the earthquakes usingfocal mechanisms and wave propagation, stress inversion, clustering analyses and numerical models. Firstly, the seismological information offers an important database to be examined with geodynamic modeling, using PyLith, to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We conclude that the CSZ seismicity is due to the combined effects of the rift faults and the impact structure and that the planar faults we considered in the model can explain the observed seismicity but not the stress rotations. We also conclude that an impact structure 4 times less elastically stiff than the surrounding crust can explain the seismicity in the CSZ. Secondly, part of my research involves determining focal mechanisms ofrelocated earthquakesin the CSZ with M 2 usingwave polarities and amplitude. I perform stress inversion using the new focal mechanisms and investigate the change in maximum stress orientation laterally and with depth. Stress inversion using all focal mechanisms shows that the orientation is N103, in contrast to N86 from previous work, with reverse faulting dominating. We observed a orientation of N73 within the impact zone compared to N106 outside the zone. A distinct systematic clockwise change in orientation of 44 is observed as a function of source depth from the surface to 20 km, changing from N83 to N127. We suggest that seismicity and the observed stress rotations of the CSZ are controlled by velocity heterogeneity and rock stiffness, including the impact structure, and pre-existing fault structure within a stress field generated by both plate tectonic forces and glacial rebound. Lastly, this study also uses clustering analyses of earthquake locations to constrain the geometry of the rift fault better using a modifiedversion of the Optimal Anisotropic Dynamic Clustering (OADC) algorithm. Our fault model shows dips of 65, 39 and 47 for the three fault system in the CSZ near the impact but changes to a 65-69-64 dipping fault geometry in the NE end of the CSZ. Our model also shows the geometry of some bounding faults associated with the damage zone of the impact structure. This study presents a realistic but simplified fault geometry of the CSZ and suggests a combination of the type of the two fault geometries found in previous studies