Recent advances in space geodesy provide unprecedented opportunities for measuring and understanding processes related to earthquake occurrence and volcanic eruptions in the United States and elsewhere. The Global Positioning System (GPS) uses Earth-orbiting satellites to obtain relative movements of ground points accurate to a few millimeters, either through periodically repeated surveys or by continuous measurements at permanent sites [Segall and Davis, 1997]. Satellite interferometric synthetic aperture radar (InSAR) uses repeat-pass radar backscatter images of Earth's surface to obtain complete spatial mappings of surface deformation over 100 km × 100 km scenes to centimeter precision [Massonnet and Feigl, 1998].
During the past 5 years GPS and InSAR increasingly have been applied to local studies of active fault zones and volcanic systems, imaging the sources of deformation buried in Earth's crust and quantifying the hazards they pose to society. The potential now exists to deploy these tools to map and monitor all of the actively deforming western United States, and a new scientific initiative with these goals is under active discussion [Silver et al, 1998].