Crustal Velocity Variations and Constraints on Material Properties in the Charlevoix Seismic Zone, Eastern Canada

Abstract

AbstractCrustal velocity variation within impact‐related seismic zones is commonly attributed to factors such as the presence of dense fracture networks, fluid pressure change, and compositional variations. In this study, we combine seismic tomography, rock physics analysis, and gravity anomaly modeling to quantitatively investigate the mechanisms that influence crustal velocity heterogeneities in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Earthquakes in the CSZ align in two broad NE‐SW trending clusters, possibly due to the reactivation of St. Lawrence paleorift faults. Within the meteorite impact structure, earthquakes are diffusely distributed, and ubiquitous lower velocity volumes have been attributed to crustal damage from tectonic inheritance exacerbated by the meteorite impact. The Bouguer gravity anomaly contrast across the St. Lawrence River is due to density disparities between the Grenville Province on the north shore and the Appalachians on the south shore. We find a higher velocity region northeast of the impact structure that does not exhibit an observable gravity anomaly, which suggests the co‐existence of rock types of comparable densities but different elastic moduli (e.g., anorthosite and charnockite). Outside the impact structure, compositional variations control velocity changes, whereas inside the impact structure, velocity variations can be explained by porosity enhancement of up to 10% by low (0.1) aspect‐ratio cracks. Our results suggest that intense fracturing and compositional alteration, rather than pore pressure change, have the primary control on seismic velocity variations, and consequently, earthquake processes in the CSZ.