Catastrophic events such as major earthquakes and volcanic eruptions result when the earth's crust fails in response to accumulated deformation. The accumulation of deformation results from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motions, or the ascent of magma through a volcanic plumbing system. Geodetic measurements document the crustal deformation leading to these failures and the deformation resulting from them, providing unique insight into the physical processes involved. By virtue of the space and time domains of ground deformation detected with geodetic measurements, geodesy is a useful tool in monitoring active tectonic and volcanic regimes. For a wide range of natural events including earthquakes, aseismic fault motions, and volcanic events, geodetic measurements constrain physical models of the processes that cause such events. Because these geodetic studies probe the underlying processes leading to and culminating in catastrophic natural events, many such studies are directly applicable to earthquake and volcano hazard assessment.
The occurrence of several well-recorded earthquakes and volcanic events have fostered progress during the past 4 years in hazard monitoring capabilities. Among the most exciting new developments, Interferometric Synthetic Aperture Radar (ISAR) techniques were dramatically brought to the fore in landmark papers on the 1992 Landers, California earthquake sequence. Advances in Global Positioning System (GPS) measurement and data analysis technology, and the increasing number and improving quality of GPS receivers deployed in active tectonic areas, have contributed steadily towards understanding volcanic and earthquake sources and interseismic deformation, and hence towards improving hazard evaluation. Studies of interseismic deformation are now being explicitly included in earthquake hazard probability estimates. In particular, geodetic data provide strong constraints on seismic hazard in areas where geological slip rate estimates are difficult to obtain, as is the case in broad shear zones with widely distributed faulting and in shallowly dipping systems of thrust faults, where active faults do not necessarily reach the ground surface. Furthermore, rapidly determined displacement fields after earthquakes have been particularly useful to the civil engineering and surveying communities during recent disaster response efforts. The series of important seismic and volcanic events that have occurred in recent years provides a framework for discussion of a variety of geodetic studies, and new contributions to fundamental process and applied science, as well as implementation.