On 17 May 2013 at 1343 hours UTC, a magnitude mN = 5.2 earthquake occurred ∼4 km from the town of Ladysmith, Quebec, within the Western Quebec Seismic Zone (WQSZ). This event produced limited damage but represents the largest earthquake to occur along the southwest margin of the WQSZ since 1935, and provides important seismotectonic constraints of the intra-cratonic Ottawa–Bonnechere Graben (OBG). We performed a number of analyses of the main shock and aftershocks to determine source properties and obtain structural information. First, a moment tensor inversion of the main shock revealed a moment magnitude of MW = 4.7, with a reverse-sense rupture on a high dip fault, typical of earthquakes within the WQSZ. Dispersion curves for well-recorded Rayleigh waves were then used to estimate a crustal velocity model, which was used to relocate the four largest aftershocks. Hypocenters of relocated aftershocks indicate that the main rupture occurred on a northeast-dipping fault, antithetic to the main graben faults. A comparison of source parameters and aftershock statistics between the 2010 (MW = 5.0) Val-des-Bois earthquake and the 2013 (MW = 4.7) Ladysmith earthquake reveals a larger aftershock moment released by the 2013 event. Together, these results suggest that high-angle faults of the OBG can be reactivated in the current compressive stress regime despite their unfavourable orientation.

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