Answer:
Crustal deformation refers to the shifting of the earth's surface as a result of tectonic forces accumulating in the crust and causing earthquakes. Determining which faults are most likely to cause the next earthquake requires a thorough understanding of deformation and its impacts on faults. There are a few theories regarding how this works, but more evidence is needed to establish which is the best.
Crustal deformation is a field that relies substantially on data. The USGS uses a range of techniques to measure the motions of the earth's surface, including LIDAR, the Global Positioning System, Interferometric Synthetic Aperture Radar, creepmeters, and alinement arrays. In parts of the U.S. with few or no historically-recorded major earthquakes or where background seismicity is sparse, geodetic data may provide the only insight into present-day seismic hazard.
