Near-surface in situ stress: 2. A comparison with stress directions inferred from earthquakes, joints, and topography near Blue Mountain Lake, New York

At 11 outcrops within 100 km of Blue Mountain Lake, New York, we measured strain relaxation during overcoring of “surface,” “doorstopper,” and Bureau of Mines borehole deformation gauges. The majority of measurements showed a maximum expansion e1 parallel with the contemporary tectonic stress field. To confirm further the orientation of in situ stress, at two sites, vertical fractures were induced at borehole walls using a packer-fracturing technique. Several cores from each site were then tested for mechanical anisotropy using ultrasonic, compressibility, and thin section analyses. The orientations of mechanical anisotropy had a poor correlation with the preferred orientation of microcracks observed in thin section. The various techniques for measuring in situ stress orientations gave internally consistent results where e1 generally aligned with topographic contours and often the mechanically stiff direction of the core. Furthermore, et aligned with the known contemporary tectonic stress, local p axes of earthquakes, Precambrian structures, and local joints. We interpret the alignment of e1 and other structures to be the result of a feedback between the contemporary tectonic stress (ENE in the northeastern United States) and the process of jointing during the development of local topography. Hence e1 is controlled by local structures and is a reflection of the contemporary tectonic stress but not a direct measure of it.