The tectonic stress field is both the cause and result of active geologic processes. At the largest scale, stresses in the earth's lithosphere arise from such processes as mantle convection and lithospheric density imbalances. The forces generated by these processes are ultimately responsible for driving the plates. Examples include the ridge-push force at spreading centers and the trench-pull force associated with negative buoyancy of subducting plates (utilizing the terms of Forsyth and Uyeda, 1975). A variety of important processes act at more regional scales. These include stresses generated by relative plate motion, magmatic and thermal processes, regional crust and lithospheric thickness and density variations, and topography (e.g., Jeffreys, 1976; Bott and Dean, 1972; Artyushkov, 1973; Fleitout and Froideveaux, 1983), and lithospheric flexure (e.g., McNutt, 1984). Improved knowledge of the tectonic stress field is intimately tied to improved understanding of the mechanisms that drive plate motion, the dynamics of faulting along both plate boundaries and in intraplate areas, and the overall mechanical, thermal, and rheological constraints on volcanism, mountain building, basin formation, and other active geologic processes.

We have been accumulating data on the orientation and relative magnitude of the tectonic stress field for over a decade. The study described here is based upon integration of our previous analyses of regional stress patterns in the conterminous United States (Zoback and Zoback, 1980, 1981, 1989; Zoback and others, 1986, 1987) and the studies of Canada (Adams and Bell, this volume), Mexico and Central America (Suter, this volume), and Alaska (Eastabrook and Jacob