This study is a new analysis of the Newbury, Massachusetts earthquake of 1727, a damaging event that was widely felt in the northeastern U.S. A number of details of the 1727 main shock source parameters are inferred from a new compilation of felt aftershocks of the earthquake combined with an analysis of a series of small earthquakes that took place at Amesbury, Massachusetts in January 1999. The distribution of intensity with distance suggests that the 1727 earthquake had an mb of about 5.6, and the area of the greatest number of felt aftershocks indicates it probably had a shallow epicenter just northwest of Newbury. The main shock was followed by a very energetic and long-lasting aftershock sequence, similar to that following the 1982 earthquake at Miramichi, New Brunswick. The 1727 earthquake may have occurred on a northwest-southeast-striking thrust fault whose trend is perpendicular to most of the mapped bedrock faults in the vicinity. The 1727 main shock today still may be generating a felt earthquake every several years in its epicentral area.