Geodetic measurements of crustal deformation provide direct tests of geophysical models which are used to describe the dynamics of the Earth. Although geodetic observations have been made throughout history, only in the last several hundred years have they been sufficiently precise for geophysical studies. In the 19th century, these techniques included leveling and triangulation. Approximately 25 years ago, trilateration measurements were initiated by the USGS (United States Geological Survey) to monitor active faults in the United States. Several years later, NASA (National Aeronautics and Space Administration) begin an effort to measure plate tectonic motions on a global scale, using space geodetic techniques, VLBI (Very Long Baseline Interferometry) and SLR (Satellite Laser Ranging). The period covered by this report to the IUGG, 1991-1994, was a transition period in the field of crustal deformation. Trilateration measurements (previously the backbone of measurements across plate boundaries in the western United States and Alaska) have been abandoned. This system was labor-intensive, involved highly trained crews to carry out the observations, and only measured the length between sites. In addition, NASA drastically cut the budgets for VLBI and SLR during this period. Fixed site VLBI systems are still operational, but mobile VLBI measurements in North America have ceased. SLR measurements continue on a global scale, but the remaining crustal deformation measurements are now being made with the Global Positioning System (GPS). Nonetheless, because of the time scales involved, older geodetic data (including leveling, triangulation, and trilateration) continue to be important for many geophysical studies.
GPS has greatly expanded the number of scientists involved in crustal deformation measurements. Trilateration, SLR, and VLBI were sufficiently expensive and complicated that for the most part government agencies collected the measurements. Logistical concerns also reduced the effectiveness and application of these techniques. With GPS, scientists are both collecting and interpreting the observations. GPS has great flexibility and can be used both to study the details of plate boundaries and to measure global plate motions. Even with this great potential, GPS is only beginning to make a major impact on crustal deformation research. Only in the last few years have there been a significant number of GPS publications including geophysically relevant results. Since crustal deformation rates are slow, and all geodetic measurements are prone to systematic errors, interpretation of GPS derived crustal deformation rates must be done carefully.
By making precise measurements of deformation of the Earth's crust, one can infer the accumulation of strain energy and its release during earthquakes. The signal associated with deformation between earthquakes, known as interseismic deformation, is quite different than the instantaneous displacement associated with coseismic deformation, although it is necessary to measure both modes of deformation to understand faulting and plate boundary dynamics. While there is no scientific justification for separating a discussion of interseismic and coseismic deformation, as a practical matter for the purposes of the IUGG review, Hudnut [this volume] will review the literature for co-seismic deformation and measurements of volcanic activity in his discussion of hazard monitoring and earthquake geodesy. Thus in this review, I concentrate on geodetic measurements of interseismic crustal deformation published by American authors between 1991 and 1994. For discussion of the geodetic techniques described herein, the reader is directed to Yunck [this volume], Blewitt [1993], or Herring [this volume]. Nearly all the results reported here can be described as fundamental process science.