Crustal velocity variation and constraint on material properties of the Charlevoix seismic zone, eastern Canada

Abstract:

Crustal velocity variation within impact-related seismic zones is commonly attributed to mechanisms such as pore pressure changes, dense fracture network formation, and compositional alteration. For a meteorite impact-related seismic zone such as the Charlevoix Seismic Zone (CSZ) in eastern Canada, the relative influence of pore pressure, intense fracturing, and compositional alteration on seismic velocity structure and earthquake distribution is yet to be quantified.

In this study, we combine seismic tomography, rock physics analysis, and potential field modeling to constrain mechanisms that influence crustal velocity variation in the CSZ. Firstly, we determine 3D Vp and Vs model with body wave travel times of earthquakes reported in the CSZ between January 1988 and March 2019. We combine the 3D velocity model with effective media analysis of the most prevalent basement rocks in the study area (gneiss, charnockite, and anorthosite) to infer a 3D density model, assuming that the observed velocity variations are due to intense fracturing and/or rock compositional alteration. We use a Multi-Layer Perceptron regressor (a neural network) to predict density values at nodes where poor ray coverage hinders seismic velocity inversion. Finally, we compare the Bouguer gravity anomaly predicted with the full 3D density model to observations.

Our results show that lower velocity bodies are ubiquitous within the impact structure, which is attributed to distributed damage as a result of the meteorite impact. We infer a gravity anomaly decrease southeastward across the St. Lawrence River due to prevalence of higher density rocks in the Grenville Province in comparison to the Appalachians. A higher velocity body northeast of the impact structure does not exhibit an observable gravity anomaly, which suggests the presence of a rock (e.g. anorthosite) of comparable density but higher elastic moduli within another rock (e.g. charnockite). Within the Charlevoix impact structure, spheroidal cracks/fractures of aspect ratio corresponding to 0.1 control velocity changes, whereas compositional alteration dominates velocity variations outside the impact structure. Overall, intense fracturing and compositional alteration, rather than pore pressure, control velocity variations in the CSZ.