A DETAILED GRAVITY SURVEY OF THE NEWBURYPORT, MASSACHUSETTS AREA LOOKING FOR THE 1727 EARTHQUAKE FAULT

A detailed gravity survey was carried out in the area of Newburyport, Newbury and Amesbury, Massachusetts to look for a possible fault to explain the 1727 M5.6 earthquake, which almost certainly was centered in this area. Seismological data have been used to infer that the 1727 earthquake involved thrust faulting on a NW-SE oriented fault with moderate dip. The survey entailed six lines along major roads of relative gravity measurements, with a nominal spacing of 500 m for the measurement sites. Site locations and elevations were determined using TKS GPS measurements with a precision of about 10 cm. The relative gravity measurements, which are estimated to have a precision of 0.1 milligals, were reduced to simple Bouguer readings with a sea-level datum. A subsurface density model was constructed for the area, where the model blocks are cubes that are 500 m on a side. The densities in the model were perturbed by trial and error until an optimal match of the predicted and observed gravity readings was found to within about 1 milligal for each measurement site. North and northeast of the Clinton-Newbury the area of the Newburyport quartz diorite has very high relative density, but the density decreases significantly further to the west in the area of Merrimac. Between the Clinton-Newbury and Parker River faults, the high metamorphic grade Nashoba Terrane rocks also have very high relative densities. In contrast, south of the Parker River Fault the Newbury Volcanic Complex has a locally low relative density, although this low density region terminates at the NW-SE fault that was inferred by Shride (1976) along the lowest stretch of the Parker River. Relative to the Newbury Volcanic Complex the adjacent Boxford Amphibolite to the west and the Topsfield Diorite to the east have somewhat higher densities. A fault on-strike with the inferred NW-SE fault along the lowest stretch of the Parker River is necessary to explain the spatial density variations north of the Clinton-Newbury Fault, and this proposed fault could have been the structure on which the 1727 earthquake took place.