State of stress in central and eastern North American seismic zones

We use a Bayesian analysis to determine the state of stress from focal mechanisms in ten seismic zones in central and eastern North America and compare it with regional stress inferred from borehole measurements. Comparisons of the seismologically determined azimuth of the maximum horizontal compressive stress (S-HS) with that determined from boreholes (S-HB) exhibit a bimodal pattern: In four zones, the S-HS and regional S-HB orientations are closely parallel, whereas in the Charlevoix, Lower St. Lawrence, and Central Virginia zones, the S-HS azimuth shows a statistically significant 30 degrees-50 degrees clockwise rotation relative to the regional S-HB azimuth. This pattern is exemplified by the northwest and southeast seismicity clusters in Charlevoix, which yield S-HS orientations strictly parallel and strongly oblique, respectively, to the regional S-HB trend. Similar similar to 30 degrees clockwise rotations are found for the North Appalachian zone and for the 2003 Bardwell earthquake sequence north of the New Madrid zone. The S-HB/S-HS rotations occur over 20-100 km in each seismic zone, but they are observed in zones separated by distances of up to 1500 km. A possible mechanism for the stress rotations may be the interaction between a long-wavelength stress perturbation source, such as postglacial rebound, and local stress concentrators, such as low-friction faults. The latter would allow low-magnitude (<10 MPa) postglacial rebound stresses to locally perturb the preexisting stress field in some seismic zones, whereas postglacial rebound stresses have little effect on the intraplate state of stress in general.