One of the lessons of geology, and now one of the tenets of plate tectonics theory, is that most of the Earth's crust does not deform very much. Most of the deformation takes place along the relatively narrow plate boundaries. Working out the details of how this deformation takes place, and the physics of the Earth revealed by it, is a major research enterprise, particularly because the slow deformation between the plates creates the elastic energy released in earthquakes.
Recent developments in measurement and laboratory studies are beginning to bring our picture of crustal deformation into better focus. Since the motion along most plate boundaries is less than 10 cm/year over a width (on the continents) of 100 km or more, the rates of strain to be measured rarely approach 10−6/yr, and are often much less (except when earthquakes occur). Conventional surveying techniques have precisions of 105, so that any motion can only be measured over many decades. Space geodesy, most notably measurements of signals from Global Positioning System (GPS) satellites operated by the U.S. Department of Defense, represent a significant advance over conventional techniques. The precision of GPS appears to be within a few millimeters over distances of hundreds of kilometers, so that rates of deformation can now be measured in a few years.