Seismicity along St. Lawrence Paleorift Faults Overprinted by a Meteorite Impact Structure in Charlevoix, Québec, Eastern Canada

Abstract

Abstract The Charlevoix seismic zone (CSZ) in Quebec has the highest number of earthquakes in eastern Canada, with cataloged events diffusely distributed between the St. Lawrence paleorift faults. In this study, we relocate 1242 out of 1639 earthquakes in the CSZ from 1988 to 2010 using double‐difference relocations with phase arrivals from manual picks and waveform cross correlations and requiring a strict set of quality control criteria. Our results show a diffuse distribution of earthquakes beneath the St. Lawrence River within a meteorite impact structure and clear lineations of event hypocenters outside the inner boundary of the impact structure. The diffuse seismicity distribution coupled with a lower cumulative seismic moment within the impact structure, despite a higher number of events, suggests the presence of highly fractured crust and weakened pre‐existing faults. A lack of M N ≥4.0 earthquakes and a wide variation of focal mechanism P‐axis orientations also provide evidence for weakened pre‐existing faults. Relocated earthquakes beneath the north shore of the St. Lawrence River illuminate a broad southeast‐trending linear structure, suggesting the geometry of the Gouffre northwest River fault. A paucity of seismicity between the north shore and the river may correspond to fault‐controlled basins of unconsolidated sediments at the surface and volumes of high‐seismic‐velocity material at midcrustal depths. Online Material: Earthquake catalogs.