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Active Tectonics: Studies in Geophysics
lier measurements are less precise, deformation rates of this order, averaged over periods of a decade or longer, are readily obtainable from the earlier historical data.
Because of their high precision and generally wide areal extent (~10 km aperture or greater), geodetic observations made at the Earth’s surface provide a measure of the deformation actually occurring at the depths where damaging earthquakes originate (about 20 km or less). Regions of tectonic deformation are invariably seismically active, and it is convenient to characterize the alternating periods of slow aseismic deformation and abrupt earthquake strain release in terms of a simple, repetitive sequence—the seismic deformation cycle. Figure 10.1 shows both the idealized model of the cycle first suggested by Reid (1910) and the more refined one accepted today. Both are considerably simplified, showing the time history of cumulative deformation of a single point or localized region, ignoring spatial variations in movement history, and smoothing out temporal fluctuations in deformation rate.
Reid’s elastic rebound theory, based on his studies of geodetic measurements related to the great 1906 San Francisco earthquake, postulates that earthquakes represent the release of accumulated elastic strains, and Reid assumed that a major earthquake would not recur until all strains released by the preceding event had reaccumulated [Figure 10.1(a)]. However, geologic field observations certainly demonstrate that not all crustal deformation is elastic and recoverable; indeed, in some seismically active regions inelastic processes such as folding and metamorphic deformation may pre-
FIGURE 10.1 Simplified forms of the earthquake deformation cycle. Cumulative deformation (e.g., strain, tilt, ground displacement) measured at the Earth’s surface is plotted as a function of time. Step offsets correspond to the occurrence times of major earthquakes. Dashed lines give failure level, constant in the idealized cycle (a), and (b) varying with time when the effects of permanent inelastic deformation are included.
dominate. As Figure 10.1(b) shows, the existence of a significant component of permanent deformation notably modifies the cycle. Rapid postearthquake deformation, which can persist from years to decades following major events, introduces additional complexity into the simple cycle visualized by Reid.
Thus, in the modern view the complete cycle consists of the coseismic deformation that accompanies the earthquake itself, the postseismic transient movements that follow it, and the relatively steady interseismic motions that comprise the majority of the cycle. Permanent deformation results if the interearthquake strain buildup is not exactly balanced at all points by the coseismic strain release. Where permanent movements have been documented, it has been shown that the coseismic offset can either locally exceed the accumulated interearthquake straining or be less than this amount; both cases are illustrated in Figure 10.1(b).
Geodetic measurements are then capable of delineating major features of the earthquake deformation cycle and closely monitoring current movement patterns. Historical surveys, which typically have repeat times of decades or longer, sample long portions of the cycle, record coseismic and postseismic movements related to past great earthquakes, and provide estimates of the permanent deformation component of the cycle. Modern observations have been most useful in determining interseismic movement rates with high accuracy and refined temporal resolution and are beginning to provide precise estimates of present-day fault slip rates and evidence for hitherto unsuspected short-term irregularities in deformation rate. The purpose of this chapter is to illustrate these capabilities with examples drawn from recent work, especially emphasizing the relation between the geodetic results and those obtained using the geologic measures of deformation and deformation rate discussed elsewhere in this volume.
PRESENT-DAY DEFORMATION RATES
Rates of deformation have been obtained for much of the seismically active western United States and parts of Alaska; these results have recently been summarized by Savage (1983). In addition, extensive geodetic surveys in active regions elsewhere in the world, notably Japan and New Zealand, have been used to determine patterns and rates of contemporary deformation in tectonic environments similar to those found in this country.
Because of California’s high seismicity and population density, intensive measurement efforts are concentrated there. Some typical results, from a laser-ranging (trilateration) network in the southern San Francisco Bay area, are illustrated in Figure 10.2. The network