ON THE MODERN AND POSSIBLE PREHISTORIC EARTHQUAKE ACTIVITY NEAR WALDEN POND, MASSACHUSETTS

An area of eastern Massachusetts centered on the town of Acton has experienced regular small earthquakes over the past several decades, with a small felt earthquake on average about once every 2.8 years. The largest recent earthquake in this local seismic zone had magnitude 3.1 in 1985. This has been the most active local seismic zone within the greater Boston metropolitan area since 1975. Precise hypocenters of 4 microearthquakes less than magnitude 1.0 since 2017 suggest that the earthquakes in this area are taking place within the Nashoba terrane between the Clinton Newbury and Bloody Bluff faults at depths between 2 and 7 km and have a roughly NNW-SSE spatial trend. A focal mechanism computed for the magnitude 3.1 1985 earthquake shows thrust faulting on a NW-SE fault plane. Thus, the earthquakes around Acton do not appear to be associated with the major mapped faults of the area but rather show an orientation similar to that of minor, NW-SE cross faults. The paleoseismicity model of Ebel et al. (SRL, 2000) inferred that the modern earthquake activity centered at Acton may be aftershocks of an earthquake of about magnitude 6.3-6.7 sometime between 399 and 867 years ago. Nashoba Hill, located in the town of Westford just north of Acton, is named after the Indian word “Nashoba” meaning “hill that shakes”, which suggests that this was a locality with much earthquake activity in prehistoric times. The inference of possible strong earthquake ground shaking at Walden Pond roughly 800 years ago can be explained by the occurrence of an earthquake of about magnitude 6.5 somewhere in or near Acton, Massachusetts area at that time.