A DETAILED GRAVITY SURVEY IN THE AREA OF CENTRAL NEW HAMPSHIRE

This study describes a detailed gravity survey of the central New Hampshire area, more specifically, north of Canterbury and south of Sanbornton. The study has been conducted to discover evidence of a possible bedrock fault located in the area on which an earthquake of estimated magnitude 6.5 is thought to have taken place in 1638. The area of our study is referred to as the Central New Hampshire Seismic Zone, describing an area of a north-to-south trend of seismicity. The gravity survey consisted of approximately sixty sites where relative gravity readings were taken with a Lacoste-Romberg gravimeter with precision of less than 10 microgals. Elevation and position data were determined at each site with the use of a Leica GS08 GPS system. Site selection was split into five general east-west routes across central New Hampshire with approximately 1.6 km of spacing between each designated site. The gravity data taken at each site was corrected to the absolute gravity value of a base station in Farmington, NH. The corrections applied to the gravity data are as follows: drift correction, latitude correction, free-air correction, and simple Bouguer correction. With the data we currently have, we plan on mapping the corrected gravity measurements for each site using the Leica GPS system positions. Based on the spatial change of the corrected gravity from site to site, if consistent geographically, the existence of a fault will be determined. The mapping and subsequent modeling of the gravity data is in progress.