State of Stress in the Earth's Crust

Measurements ofthe stress field within the crust can provide perhaps the most useful information concerning the forces responsible for various tectonic processes, such as earthquakes. Advances in knowledge of the state of stress at mid-crustal depths are essential if further progress is to be made toward solving a broad class of problems in geodynamics. Most stress measurements have been, and will continue to be, motivated by engineering needs rather than the needs of geologists engaged in fundamental research. Knowledge of the state of stress is critical to the design of underground excavations for mining and for nuclear waste disposal (e.g. Jaeger & Cook 1969, pp. 435-64). The massive hydraulic fracturing of formations in oil and gas fields to stimulate production is another application for which knowledge of the stress field at depth is very important and, in fact, many of the deeper stress determinations have been by-products of these hydrofrac operations (e.g. Howard & Fast 1970). A recent and exciting application of hydraulic fracturing is the Hot-Dry-Rock Geothermal Energy Program (Aamodt 1977). Heat is extracted from the rock by circulating fluid down a pipe into hot rock and then up through a second pipe. A large fracture connecting the two pipes serves as the heat exchanger. Knowing the state of stress is critical in the solution to the problem of creating and maintaining such a crack. There is little argument about the applicability of information on the state of stress to these and many other engineering problems. The application of stress measurements to the solution of problems in tectonics is not so straightforward as in engineering design. Whereas the engineer is concerned with the stress field affecting the rock, the geologist attempts to deduce the processes that might have caused the stresses. Before the measured stress field can be related