ABSTRACT

The 17 May 2013 M 4.5 Ladysmith, Quebec, earthquake produced a rich instrumental and felt ground‐motion database. We use instrument‐corrected response spectra and Fourier amplitude data from 189 stations, at distances from 10 to 700 km, to examine the attenuation and source characteristics of this event. An interesting observation is that amplitudes were elevated throughout southern Ontario, probably due to site effects, possibly coupled with source radiation effects. Using attenuation‐corrected observations, in combination with the known seismic moment to constrain the absolute amplitude level at the source, we obtain an apparent source spectrum for the Ladysmith earthquake. The Ladysmith source spectrum is well described by a Brune model spectrum for an event of M 4.5 with a stress drop of ∼500 bars. Overall, the source and attenuation characteristics of the Ladysmith event were typical for regional events. The earthquake occurred within the boundaries of the Ottawa–Bonnechere graben, which suggests a reactivation of one of its faults, though it is unclear which fault was involved. The focal mechanism does not match the orientation of the most conspicuous lineaments of the area. The earthquake may have reactivated a northwest–southeast brittle fault of local extent or a ductile thrust fault of the Grenville Geological Province.

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