ABSTRACT

A swarm of 21 small earthquakes, with the largest being MLg 1.7, was recorded by regional seismic network monitoring from near Searsport, Maine, in April and May 2011. An additional five events were detected by two portable seismic instruments that were installed in the Searsport area for the later part of the swarm. Relative locations of the larger events of the swarm, computed in relation to a selected master event, showed that the swarm events extended for a distance of about 2.5 km and migrated from northeast to southwest. The events also became shallower toward the southwest. If the area of the swarm had ruptured in a single earthquake, the magnitude of the event would have been about M 5.1–5.5. The SP time of only about 0.34 s at one of the portable seismic stations for the detected events from the swarm indicates that the station was located about 2.7 km from the hypocenters, thus constraining the location of the southwest end of the swarm. The events took place within the Devonian Mount Waldo pluton, a granitic body that locally cuts northeast–couthwest‐oriented thrust faults that parallel the Norumbega fault zone. The trend of the swarm events is parallel to and on‐strike with the trend of a thrust fault mapped to the southwest of the Mount Waldo pluton. The seismic data suggest that the fault might be seismically active, although the modern seismotectonic relationship of the fault and the pluton is far from clear.

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