Crustal velocity variations and constraints on material properties in
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, and compositional variation. In this study, we combine
seismic tomography, rock physics analysis, and potential field modeling
to quantitatively investigate the mechanisms that influence crustal
velocity variation in the Charlevoix Seismic Zone (CSZ), a meteorite
impact-related seismic zone in eastern Canada. Earthquakes in the CSZ
align along two broad NE-SW trending clusters related to reactivated
paleo-rift faults. Within the impact structure, the earthquakes are
diffusely distributed and lower velocity bodies are ubiquitous which can
be attributed to crustal damage from tectonic inheritance exacerbated by
the meteorite impact. The Bouguer gravity anomaly decreases
southeastward across the St. Lawrence River due to density disparity
between rocks in the Grenville Province and the Appalachians. We find a
higher velocity body northeast of the impact structure that does not
exhibit an observable gravity anomaly, which suggests the presence of a
rock (e.g. anorthosite) of comparable density but a higher elastic
moduli within another rock (e.g. 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, control velocity variations, hence earthquake
processes in the CSZ.