dred established as a geodetic backbone for the study of plate boundary tectonics. The remaining GPS stations would be deployed in denser clusters to provide detailed data on active faults and volcanic systems within the most active zones.
The broad coverage and high precision of GPS geodesy permit individual faults to be studied as components in strongly interacting systems rather than as isolated elements. A disadvantage of GPS is that motion can be measured only at points on the ground where receivers are located. Although the cost of GPS receivers is decreasing steadily, it is impossible to measure the deformation field densely enough to answer some key questions of earthquake science.
Interferometric Synthetic Aperture Radar. The most recent innovation in tectonic geodesy is InSAR, which has imaged earthquake deformations at a level of detail unanticipated only 10 years ago (45). InSAR measures deformation by comparing reflected radar waves recorded on successive passes of a satellite from nearly identical positions. The simplest InSAR measurements are sensitive to just one component of displacement (toward the satellite), but stereoscopic measurements (pairs of images from multiple locations) allow measurement of vector displacements (Figure 4.13). InSAR is subject to errors from changes in reflective properties such as those caused by seasonal vegetation changes and snowfall, and it lacks the temporal resolution of GPS. However, the ability to map essentially continuous displacements over large swaths of active plate boundaries offers an enormous advantage.
Because they can map centimeter-level deformations with a spatial resolution on the order of 100 meters, InSAR systems are useful for determining co-seismic and interseismic slip on faults. This is particularly important in remote areas lacking GPS stations. InSAR has shown an ability to measure co-seismic slip on subsidiary faults, variations in slip distribution along strike, and slip on previously unknown faults. With its continuous coverage, InSAR systems can map surface displacements before, during, and after earthquakes or volcanic eruptions, providing time-dependent data on the mechanics of fault loading, earthquake rupture, and earthquake interaction, and they can image strain accumulation across broad tectonic zones (Figure 4.14), as well as regional subsidence induced by petroleum production and groundwater withdrawal. For example, InSAR data have been used to disentangle the latter types of motion from tectonic strains observed by GPS networks in the Los Angeles basin (Figure 4.15). Finally, InSAR has been used to detect postseismic poroelastic effects induced by fault movements (46). An important component of the proposed EarthScope project is a dedicated InSAR satellite, the Earth Change and Hazard Observatory (ECHO) (47).