GRAVITY SURVEYING TO LOCATE A FAULT: USING GRAVITY ANOMALIES TO INVESTIGATE THE 5.6 MAGNITUDE EARTHQUAKE OF 1727 IN NORTHEAST ESSEX COUNTY, MA

The New England region experiences minor earthquakes every year, which have been recorded as far back as the 17th century. According to such historic records, there was an earthquake on October 29th, 1727 in northeastern Massachusetts with an estimated magnitude of around 5.6. The challenge is that the faults of these older earthquakes have yet to be geographically identified. Such faults are important to locate, as they may help identify the potential seismic hazard in New England, a densely populated region. We present results of a study that aims to advance the understanding on where the buried fault on which the 1727 earthquake may lie. In order to look for gravity anomalies that could provide evidence of faults in the region, we collected 130 data points around Newburyport/Amesbury, MA every 500 m using a gravimeter to measure gravity, and a Leica GPS unit to precisely record location. We then applied the drift, latitude, free air and simple Bouguer corrections to our data, and measured the gravity at Hampton Falls in order to calibrate our data to the absolute gravity value of Hampton Falls, as determined in December 1969 for the World Relative Gravity Reference Network. We will be using these gravity data to estimate the density of the rocks in this region to see if there is a density contrast consistent with a fault to explain the 1727 earthquake.