Velocity models and Hypocenter Relocations for the Charlevoix Seismic Zone

Abstract:

We present 3-D P- and S-wave velocity (Vp and Vs) models and new hypocenter locations for the Charlevoix seismic zone (CSZ) based upon local travel time tomography. Prominent velocity anomalies and the distribution of earthquakes are discussed in relation to known structural features produced by Iapetan rifting and a large Devonian meteor impact. The CSZ is located along the St. Lawrence River about 100 km downstream from Quebec City, Canada. A 7 station permanent network, augmented by temporary stations, records more than 200 earthquakes annually. The inversion dataset consists of 1,329 earthquakes providing 8,540 P-wave and 8,304 S-wave arrival times. Velocity model resolution is adequate to a depth of at least16 km as indicated by recovery of synthetic checkerboard models. Low Vp and Vs are associated with the impact structure to a depth of 12 km. A prominent high Vp feature is present north of the impact structure and high Vp and Vs extend below the impact at depths exceeding 12 km. Following inversion, hypocenter location errors are less than 0.2 km horizontally and 0.4 km vertically. Hypocenters form a semicircle delineating the eastern margin of the impact structure. Northeast of the impact, hypocenters cluster into planes in several locations, suggesting the presence of distinct, seismogenic faults. The planes trend NE, in the same direction as the Iapitan rift faults, and dip to the SE. One steeply dipping plane is located below 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 below the river and extend to depths of 12 and 15 km. All three planes are disrupted when they encounter the impact but the north shore plane appears to continue through and below the impact zone. The presence of through-going Iapetan faults within the impact structure is an important constraint for 3-D stress models developed to explain the spatial distribution of seismicity in the CSZ. We identify individual fault segments and their relationship to the impact structure using a pattern recognition method called optimal anisotropic dynamic clustering that reconstructs the 3D structure of a fault network using the spatial locations of earthquakes.