Four extensive microearthquake swarms, in 1981, 1982, 1986 and 1987–1988, have occurred near Moodus, Connecticut since the installation of a dense local monitoring network in 1979. All of the swarms have been accompanied by at least one event of M≥2.0, have had depths of less than 2.4 km, and have originated from one small primary source volume. A few of the events from 1981 and 1982 were located about 1.5 km west to southwest of this primary source locality. The time histories and energy releases have varied from swarm to swarm, with the 1981 and 1982 swarms having some foreshocks, a main shock, and a decaying aftershock pattern. The 1986 and 1987–1988 swarms were more distributed in time and totaled fewer events than the earlier swarms. The b-value for the 1987–1988 swarm is closer to that of the foreshocks from 1981 and 1982 than that of the aftershocks from those swarms. The 1980’s have been an unusually active period at Moodus, but the geologic and seismological evidence are ambiguous as to the cause the Moodus earthquakes and how large an event can take place at this locality.

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