ABSTRACT

Beginning in March 2012, inhabitants of McAdam, New Brunswick, reported feeling and/or hearing many earthquakes within a 12  km2 area of the village. The largest events (MN<2.6) were recorded by regional seismographs, the closest at that time being 65 km away. Public concern combined with the large number of events and their localization led the Geological Survey of Canada (GSC) to deploy three temporary instruments in McAdam. The University of New Brunswick provided a fourth. The instruments remained in place for several months and were then removed because activity had largely died off by June 2012. There was a short resurgence in activity late in 2015 with seven felt events occurring during 7–9 December. In February 2016, the swarm activity picked up considerably. In light of the increased activity, which included the largest earthquake of the sequence (MN 3.3 on 9 February 2016), the GSC redeployed four instruments which remained in place for several months until the activity subsided again. As of 1 November 2016, 164 earthquakes were located, some by enough stations to allow precise depth determination by one or more methods. All the earthquakes were extremely shallow, 0.0–1.2 km. The majority of epicenters lie in a west‐northwest–east‐southeast‐trending ellipse. Focal mechanisms determined for a small number of events are largely consistent with northeast–southwest compression and with those of earthquakes in neighboring regions. Sixty‐eight of the events were reported as felt, with several of the felt events being of magnitude (MN or ML) less than 1.0. A strong‐motion recorder recorded a peak ground acceleration of 9%g from an MN 1.7 earthquake at about 0.8 km hypocentral distance. There was no human activity that could have induced or triggered the swarm, and the cause remains unexplained. McAdam sits on Silurian metasediments intruded by the granitic Pokiok Batholith exposed just to the northwest. No faults are mapped close to McAdam, but the events might have occurred on a northwest–southeast splay of the Fredericton fault.

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