Crustal Unloading as a Possible Source of Seismicity at Plainfield, Connecticut

Abstract In 2014–2015, an extended swarm of small earthquakes occurred at Wauregan in the town of Plainfield, Connecticut, with epicenters around the Wauregan Tilcon bedrock quarry, which was closed about four years before the swarm occurred. Dozens of earthquakes were felt or heard by the residents near the quarry. Following the MLg 3.1 thrust mainshock, which occurred very early in the swarm, four portable seismic instruments were installed in the epicentral area. More than 180 microearthquakes were confirmed with the portable instrument data, and as many as 200 more possible events were detected by the portable instruments. The mainshock and most of the swarm events immediately following the mainshock appear to be spatially associated with the west-dipping Lake Char fault. Later events in the swarm occurred within about 500 m of a projected steeply dipping NW–SE-striking unnamed fault to the NW of the quarry. Modeling of the perturbation of the local stress field due to the removal of rock from the quarry suggests that most of the events took place where the local Coulomb failure stress was increased on thrust faults throughout the area and on left-lateral, vertical strike-slip faults west of the quarry. The delay of about 4 yr between the end of quarrying and the earthquake swarm, perhaps can be explained if static fatigue due to the past quarrying may have led to the earthquake swarm. Because of some previous studies at other quarries, it might be possible that quarrying operations can trigger seismic activity in settings with a favorable stress regime, fault orientations, and rock characteristics. However, this study does not indicate that quarrying can trigger an earthquake large enough to be damaging.