Combining GPS and Classical Geodetic Surveys for Crustal Deformation in the Imperial Valley, California

Summary

In 1986 the National Geodetic Survey (NGS) performed a GPS survey of a 38-station network in California’s Imperial Valley. Here relative motion between the Pacific and North American plates has spawned several moderate earthquakes such as the M=7.1 event of 1940 and the M=6.6 event of 1979. Most stations occupied in 1986 had been positioned beforehand during one or more of the previously observed classical surveys. Triangulation was observed in 1934, 1939, 1941, and 1954–55; and triangulation/trilateration in 1967, 1975, 1979, and 1980. We supplemented the combined data set with prior information to estimate hew individual stations have moved from survey epoch to survey epoch. The prior information has the form of quasi-observations that damp the amount of displacement that can occur at individual stations in the direction perpendicular to the plate boundary. The quasi-observations serve not only to remove datum defects encountered when estimating displacements from geodetic data; they also serve to better resolve attributes of the deformation that are hidden within the noise of the reed observations. Estimated displacement vectors agree at the 0.1-m level with displacement vectors produced by relatively simple dislocation models.