Velocity models and hypocenter relocations for the Charlevoix Seismic Zone

Abstract

AbstractThree‐dimensional P and S wave velocity (Vp and Vs) models and hypocenter relocations are determined for the Charlevoix Seismic Zone (CSZ) using local arrival time tomography. The CSZ is located along the St. Lawrence River about 100 km downstream from Quebec City, Canada, and is one of the most active seismic zones in eastern North America. Earthquakes occur in Grenvillian basement rocks affected by Iapetan Ocean rifting and a large Devonian meteor impact. Low Vp and Vs are associated with the impact structure to a depth of 12 km. Relocated hypocenters form a semicircle delineating the eastern margin of the impact structure. Northeast of the impact, hypocenters cluster into two groups separated by a region of high Vp and Vs velocity. Hypocenters within each group align along planar surfaces, suggesting the presence of distinct seismogenic faults. The planes trend NE, in the same direction as the Iapetan rift faults, and dip to the SE. One plane is located beneath the north shore of the St. Lawrence River and extends to a depth of at least 30 km. Two other planes with shallower dips are located beneath the river and extend to depths of 12 and 15 km. All three planes are disrupted within the impact zone but can be followed, suggesting partial preservation of rift‐parallel fault fabric.