[S01-P] Poster

[S01-P-21] The McAdam, New Brunswick Earthquake Swarms of 2012 and 2015-16: Extremely Shallow, Natural Events

Allison Bent¹, Stephen Halchuk¹, Veronika Peci¹, Karl Butler², Kenneth Burke², ^○John Adams¹, Nawa Dahal³, Sylvia Hayek¹ (1.Geological Survey of Canada, Ottawa, Canada, 2.Department of Earth Sciences, University of New Brunswick,Fredericton, Canada, 3.Weston Observatory, Boston College, Weston, United States of America)

Beginning in March 2012 people in McAdam, New Brunswick felt and/or heard many earthquakes within a 1x2 km area of the village. The largest events (MN < 2.6) were recorded by regional seismographs (closest was 65 km away). Public concern combined with the large number of events and their localization led the Geological Survey of Canada (GSC) to deploy 3 temporary instruments in McAdam. The University of New Brunswick provided a fourth. The instruments remained in place for several months and were then removed, as activity had largely died off by June 2012. There was a short resurgence late in 2015 with 7 felt events occurring 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 4 instruments which remained in place for several months until the activity again subsided. 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 WNW-ESE trending ellipse. Focal mechanisms determined for a small number of events are largely consistent with NE-SW compression and with regional earthquakes. Sixty-eight of the events were reported as felt, with several of the felt events being of magnitude less than 1.0. A strong motion recorder recorded a PGA 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 NW. No faults are mapped close to McAdam, but the events might have occurred on a NW-SE splay of the Fredericton Fault. Swarms like this are probably common in eastern Canada, but are seldom this well reported or recorded.