Since the early 1990s, our understanding of plate boundary zone crustal deformation has been revolutionized by advances in global positioning system (GPS) techniques. These allow us to track directly the movement of the ground in real time, quantify the rates of crustal deformation within plate boundary zones and determine the displacement of the Earth's surface during earthquakes. The GPS measurements are taken at survey points permanently attached to the ground either by intermittent (survey-style) or continuous (daily, automated) collection of phase and pseudorange data from the constellation of GPS satellites that orbit the Earth. The GPS measurements spanning some period of time (usually longer than one year) can accurately track the movement of one point on the Earth's surface relative to others (to within a few mma−1 uncertainty). Such measurements have allowed scientists to determine where and how much tectonic strain is currently accumulating within plate boundary zones (e.g. Kreemer et al., 2000; McClusky et al., 2000; Sagiya et al., 2000; Beavan and Haines, 2001).
One of the major issues facing siting of nuclear facilities is the possibility of rapid seismic or slow aseismic strain at or near the facility. Elevated strain within a site (possibly due to a seismic event) could perturb a nuclear facility and/or jeopardize the long-term isolation of a high-level waste (HLW) repository in numerous ways, including activation/formation of faults, enhanced creep deformation of engineered barriers, flexural folding of the host rock or enhanced groundwater flow. Geological and seismological data are commonly used to assess future seismic shaking and rock deformation hazards for nuclear facilities (Stepp et al., 2001).