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The orientation and relative magnitudes of in situ tectonic stress in the continental United States have been inferred from a variety of indicators, including earthquake focal mechanisms, stress-induced elliptical borehole enlargement (“breakouts”), hydraulic fracturing stress measurements, and young fault slip and volcanic alignments. The data come from a wide range of depths (0.1 to 20 km) and have been assigned a quality ranking according to their reliability as tectonic stress indicators. The data show that regionally consistent orientations persist throughout the seismic “brittle” upper crust. Stress provinces are defined on the basis of uniform stress orientations and relative stress magnitudes (style of faulting).

The sources of stress for all the major stress provinces are believed to be linked either directly or indirectly to plate-tectonic processes. Most of the central and eastern United States is part of a broad “midplate” stress province (which includes most of Canada and possibly the western Atlantic basin) characterized by NE– to ENE–oriented maximum horizontal compression. This orientation range coincides with both absolute plate motion and ridge push directions for North America. With the exception of the San Andreas region and most of the Pacific Northwest area, the remainder of the western United States (generally, the thermally elevated region from the high Great Plains to the west and including the Basin and Range province) is characterized by extensional tectonism. Within areas of “classic” basin-range structure, both those currently active and those largely quiescent, the least horizontal stress is oriented approximately E-W (between WNW and ENE). In the Colorado Plateau interior and the southern Great Plains, the least horizontal stress is NNE, roughly perpendicular to that in surrounding areas. The state of stress in these two regions may be related to pronounced lateral variations in lithosphere thickness beneath them.

New focal-mechanism data in western Washington and northern Oregon define a region of NE compression apparently associated with subduction of the Juan de Fuca plate. In the coastal region of central California there appears to be a zone approximately 100 to 125 km wide on both sides of the San Andreas fault in which the maximum horizontal stress is oriented NE, nearly perpendicular to the fault itself. While compression orthogonal to the San Andreas fault explains late Tertiary-Quaternary folding and reverse faulting subparallel to the San Andreas, this NE compression seems incompatible with right-lateral slip on the N40°W-trending San Andreas fault. The observed stress field, however, is consistent with current estimates of the direction of relative plate motions, provided that the shear strength of the San Andreas is appreciably lower than the level of far-field shear stress in the crust.

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