THE EASTFORD FAULT - A REGIONAL LATE–STAGE FAULT ZONE IN EASTERN CONNECTICUT: IMPLICATIONS FOR THE ORIGIN OF MOODUS AREA SEISMICITY AND AUDIBLE EARTHQUAKES

Using integration of Light Detection and Ranging (LiDAR) imagery, topographic maps, geological field mapping, and geophysical data, a ~120-kilometer long regional lineament in eastern Connecticut and south-central Massachusetts was identified and named the Eastford lineament based on its coincidence with the ~50-kilometer long Eastford fault (Pease, 1989). Close examination of LiDAR imagery and topographic maps suggests that the Eastford fault with its linear extensions continues southwestward through the Willimantic dome, the Moodus seismic area, and then beyond to the village of Ponset where it coincides with a 7-kilometer fault that offsets the Jurassic Higganum diabase dike. To the northeast the Eastford lineament has been traced into south-central Massachusetts where it coincides with mapped faults that cut the Oxford anticline (Barosh, 2005).

Comparison of borehole geophysical data from the 1987 ~1.5 kilometer deep Moodus research drill hole (located 150 meters northwest of the lineament) with seismic data from the nearby 1987 earthquake swarm and nodal planes of fault plane solutions for that swarm suggests that the Moodus segment of the Eastford lineament is the surface expression of a steep, NW-dipping fault that intersects the bore hole at ~400 meters at the location of a dense zone of fractures. Seismic profile data that were acquired across the lineament near the 1987 cluster of quake epicenters revealed steep, NW dipping faults, confirming our hypothesis that the Moodus segment of the Eastford lineament is a fault and is where the Eastford fault daylights, and that it is the likely source for the 1987 Moodus earthquakes. The alignment of individual colinear faults (i.e., the Eastford fault and the new ‘Moodus fault’) and the LiDAR linears representing the Eastford lineament is not a coincidence, and likely arises from a composite structure that transgresses the Eastern Highlands of southern New England. Its estimated age of activity is Jurassic to recent. Historical reports of Moodus quakes (some of which can be audible), including those from near Cave Hill, could be related to faulting along the Eastford fault (Moodus segment).