Source processes of injection induced earthquakes involve complex fluid-rock interaction often elusive to regional seismic monitoring. Here we combine observations from a local seismograph array in the Montney basin, northeast British Columbia, and stress modeling to examine the spatial and temporal evolution of the 30 November 2018 M 4.5 hydraulic fracturing induced earthquake sequence. The mainshock occurred at ~ 4.5 km in the crystalline basement two days following injection at ~ 2.5 km, suggesting direct triggering by rapid fluid pressure increase via a high-permeability conduit. Most of the aftershocks are located in the top 2 km sedimentary layers, with focal mechanisms indicating discrete slip along sub-vertical surfaces in a ~ 1 km wide deformation zone. Aftershock distribution is also consistent with static stress triggering from the M 4.5 coseismic slip. Our analysis suggests complex hydraulic and stress transfer between fracture/fault networks needs to be considered in induced seismic hazard assessment.

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.

Intraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new dataset of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River-parallel near the surface to river-perpendicular in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ~13 and ~26 km depth, where glacial rebound induced stress perturbation is further amplified by a “weaker” middle crust of an estimated apparent friction coefficient of ~0.5. Finally, depth-dependent b-values confirm the rheological difference between upper and middle crust in the CSZ.