Present-day plate-tectonic motions have been estimated from the space-based geodetic techniques of Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) between sites located on seven of the major tectonic plates. The observational history for the two techniques extends back to the late 1960s, but data taken during the last decade is particularly robust and of sufficient quality to resolve relative motions of the observing stations at the 2–4 mm/year uncertainty level. With the continuation of these measurements and the expansion of tracking networks, more detailed interpretations of the surface motions can be made, further deepening our understanding of plate-driving processes and the lithospheric response to tectonic stress.
Shortly after the Loma Prieta earthquake in the Santa Cruz Mountains of California on October 18, 1989, mobile VLBI systems visited three sites within 150 km of the epicenter and made measurements jointly with other sites in an intensive observational campaign to measure co- and postseismic deformation. At Fort Ord (50 km south of the epicenter), the earthquake-related horizontal displacement was estimated at 4914 mm, oriented in a northerly direction. At the Presidio (in San Francisco, 100 km northwest of the epicenter), the horizontal displacement was estimated at 1215 mm oriented in a southeasterly direction, and further north, at Point Reyes (140 km northwest of the epicenter), no horizontal displacement was detected beyond the noise level of the observations. A coseismic slip model in which slip on the rupture zone's southern segment is shallower than that on the more northern segment predicts static displacements that are consistent with these results [Clark et al., 1990].