State of stress in the conterminous United States
Authors: Zoback Mary Lou , Zoback Mark 
Published: February 2008

Inferring principal stress directions from geologic data, focal mechanisms, and in situ stress measurements, we have prepared a map of principal horizontal stress orientations for the conterminous United States. Stress provinces with linear dimensions which range between 100 and 2000 km were defined on the basis of the directions and relative magnitude of principal stresses. Within a given province, stress orientations appear quite uniform (usually within the estimated range of accuracy of the different methods used to determine stress). Available data on the transition in stress direction between the different stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modern pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE‐SW compression in the cold, thick lithosphere of the craton, although drag‐induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counter‐flow models do not apply in this region, as predicted stress orientations are about 90° off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW‐SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental‐oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.