The historic geodetic data base in the eastern United States dates back over 100 years. Although instrumentation and hence measurement precision has improved since the initial observations, the basic techniques [leveling, triangulation, water level gauging] remained unchanged until the recent introduction of space geodesy [Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Positioning System (GPS)]. Because of this, it has been possible to compare repeated observations to identify apparent changes in relative positions. Such “changes” can reflect observational errors, non-tectonic movements (water level effects, monument instability, loading, etc.) or tectonic/isostatic deformation.
A number of possible deformation features in the eastern U. S. have been reported in the literature, including uplift of the southern Appalachian and Adirondack mountains, doming of the Gulf coast inland of the Mississippi delta, subsidence of Chesapeak Bay and along the coast of Maine, horizontal deformation in New York and Connecticut, and possible fault related deformation near Charleston and in the New Madrid area. Unfortunately, it is not dear to what extent any of these features represent tectonic deformations and hence what their significance may be for the earthquake problem.
An important recent development in monitoring regional deformation has been the establishment of the eastern U. S. GPS strain network by the National Geodetic Survey. The network consists of roughly 45 sites uniformly distributed east of the Rocky Mountains. The network was established in 1987 and will be reobserved in 1989. Subsequent reobservations will be made at 2 to 5 year intervals depending on analysis of the early measurements. Given the precision of the GPS measurements (few cm in 3-D relative positions), a considerable time period will be required to detect the subtle deformations expected for this intraplate region.
Perhaps more immediate information will result from recent progress in recognizing and correcting systematic errors in the historic geodetic data base. This, together with the newly automated historic data base, and improved techniques for integrating and analyzing these extensive observations, provide the necessary basis for effectively evaluating the deformational features listed above. Given the potential importance of these data for understanding tectonic processes (and the huge expense of collecting these measurements over the pst century), such studies should be an integral part of future earthquake studies in the eastern U. S.