A DETAILED GRAVITY SURVEY IN THE AREA OF NEWBURYPORT, MASSACHUSETTS

This study describes a detailed gravity survey that was carried out in the area of Newburyport, Massachusetts to look for evidence of the fault on which an earthquake of estimated magnitude 5.6 took place in 1727. 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 and is the inferred fault orientation for the 1727 earthquake. Our gravity survey was designed to look for evidence in the bedrock of this fault. The survey consisted of about 150 sites at which relative gravity readings to a precision of 10 microgals were taken with a Lacoste-Romberg gravimeter. The gravity sites were taken along five roads that run from the north-northeast to the south-southwest across the Newburyport area, with a nominal station spacing of approximately 0.5 km. Elevations at each site were precisely measured using a Trimble RTK GPS Rover instrument. The relative gravity measurements for each of the lines were converted to absolute gravity values by tying the gravity measurements along the different roads to a reference absolute-gravity station at Hampton Fall, NH. The following corrections were applied to our survey gravity data: drift correction, latitude correction, free-air correction and simple Bouguer correction. The resulting Bouguer-corrected gravity field from our survey shows a decrease from northeast to southwest, similar to that seen in the regional gravity data. We intend to use our data to create a detailed model of the variations of bedrock density across the survey area, with the goal of looking for a density contrast that might be indicative of a subsurface fault. The modeling of the gravity data is currently in progress.