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Active Tectonics: Studies in Geophysics
ing to these satellites from the Earth’s surface it is anticipated that relative horizontal and vertical positions can be determined within a few centimeters (NRC Panel on Crustal Movement Measurements, 1981). Furthermore, this precision can be obtained at large station separations (hundreds of kilometers), adjacent stations need not themselves be intervisible, and measurements can be made even in overcast conditions.
The precision of GPS measurements degrades for intercontinental station separations (~1000–10,000 km). However, other extraterrestrial surveying methods such as Very-Long-Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) are expected to be capable of measuring relative positions over these longer ranges to 3 cm or better (NRC Panel on Crustal Movement Measurements, 1981).
Relative to conventional geodetic methods, the most important feature of the new space techniques is the capability for measuring long ranges with high precision. Thus, for station separations greater than about 100 km, GPS methods are expected to become more accurate than land-based surveying. VLBI or SLR measurements over intercontinental baselines will then be capable of resolving relative movements of the Earth’s major tectonic plates, and GPS networks with station separations of about 100 km can be used to outline the broad-scale deformation patterns in intracontinental active regions like the western United States and central Asia.
Depending on their ultimately achievable accuracy and measurement costs, extraterrestrial methods may also be competitive with land-based surveying over shorter ranges as well. In remote and inhospitable environments, where clear sighting conditions are rare and station intervisibility is difficult to obtain, GPS methods may also prove to be more feasible and cost-effective than conventional techniques.
Geodetic observations provide a direct measure of strain changes occurring at seismogenic depths, and as a result they will play an important role in determining the degree to which large, destructive earthquakes are predictable. In recent years geodetic monitoring has been intensified in areas of identified high seismic potential both in the United States and elsewhere. Further detailed monitoring can be anticipated in the future. Most of this work comprises annual surveys, but in several parts of California monthly and weekly surveys are now being carried out as well (Langbein et al., 1982; Prescott and Savage, 1984). With a long record of frequent measurements, the patterns and rates of interseismic movement will be outlined in considerable detail. The typical variability of these patterns and rates as a function of time will be determined as well. Eventually, large earthquakes will occur in these closely studied regions. When these events take place, the accumulated data should be sufficient to determine precisely whether diagnostic crustal movement anomalies preceded their occurrence.
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