THE 2015 PLAINFIELD, CT EARTHQUAKE SWARM

From January until July 2015 an active earthquake swarm took place near the town of Plainfield, CT. The swarm was preceded by three small earthquakes in October and November of 2014 (MLg 0.9, 0.6 and 0.9), and it started in earnest on 8 January 2015 with an earthquake of MLg 2.0. The largest event with MLg 3.1 took place on 12 January 2015, and it was felt throughout eastern Connecticut and all of Rhode Island. Many of the earthquakes of the swarm were felt or heard by local residents. On 13 January 2015 Weston Observatory, in conjunction with the Connecticut Geological Survey, installed four portable seismographs in the epicentral area to record the swarm events. Those instruments were operated until September 2015. Using data from the regional and local seismic stations, at least 180 earthquakes have been confirmed, and the portable instruments contain signals from another 200 detections that may also be very small events. Absolute event locations computed using data from the portable seismic stations fall beneath an abandoned shallow quarry at focal depths ranging from about 1.6 km depth to just below the earth’s surface. The focal mechanism of the MLg 3.1 event is pure thrust on 45°-dipping planes with strikes of 20° and 200°. The absolute locations of the swarm events suggest a very shallow fault dipping toward the west with perhaps a deeper fault dipping toward the east. On the shallow fault the events appear to have migrated updip with time. The earthquakes took place near the mylonitic Lake Char Fault zone, a structure that runs from southern Connecticut into Massachusetts. The relationship of the Plainfield swarm with the local geologic structure and the quarry is still being investigated.