In order to test dynamic models of the Earth, crustal deformation measurements should have the greatest possible temporal and spatial distribution. GPS can provide fairly good temporal resolution, with continuous networks currently estimating deformation on a daily basis. Higher temporal resolution requires more detailed modelling [ Genrich and Bock, 1992]. Even so, based on past records of crustal deformation measurements, fluctuations in deformation rate do not appear to vary over periods of several years.
GPS gives the displacement, and thus deformation, at a single point. This was also the case with SLR and VLBI. The greatest advance in spatial resolution for crustal deformation studies is the SAR (Synthetic Aperture Radar) technique. Hudnut [this volume] discusses this technique in the context of the spectacular results presented by Massonet et al. [1993] for the 1992 Landers Earthquake. Nonetheless, SAR has limitations, in particular the restriction of only one vector component of the deformation field. Large coseismic deformation in an arid, non-populated region was visible for the Landers event, but it has not been tested in populated regions. Interseismic deformation has yet to be observed with SAR, although there are ongoing studies, most recently the Long Valley Caldera [ Webb et al., 1994]