SEARCH FOR THE FAULT OF THE 1727 M5.6 NEWBURYPORT EARTHQUAKE

In 1727 a strong earthquake of estimated magnitude 5.6 was experienced in southern New England. This earthquake was centered in northeastern Massachusetts at Newburyport and nearby communities, where it damaged chimneys, collapsed some cellar walls, and knocked down stone fences. The strong ground shaking from the earthquake caused a number of sandblows to form in lowland areas from Hampton Falls, NH to Newbury, MA. The earthquake was followed by a protracted aftershock sequence with notable seismicity for several years after the mainshock. The fault on which the earthquake took place remains something of an enigma. The major bedrock faults that have been mapped in the Newburyport area trend from southwest to northeast across the area, including the terrane-bounding Clinton-Newbury Fault. However, the focal mechanism of a small earthquake centered in Amesbury, MA in 1999 indicates thrust faulting on a northwest-southeast oriented fault plane for the modern seismicity, including the 1727 earthquake. This fault orientation is strongly at odds with the mapped bedrock faults. The regional gravity data show a decrease in the Bouguer-corrected gravity from northeast to southwest across northeastern Massachusetts, a trend that could be consistent with a bedrock fault that is oriented northwest-southeast across the Newburyport area. We propose that a detailed regional gravity survey in the Newburyport area can help resolve the questions of whether or not there is a NW-SE oriented bedrock fault that could explain the occurrence of the 1727 earthquake. If this fault exists and is seismically active, it could pose a significant seismic hazard to eastern Massachusetts and southeastern New Hampshire.