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OBSERVATIONS OF CRUSTAL DEFORMATION [c7, p190-191]

The focus here is on the spatial and temporal patterns of interearthquake horizontal crustal movements in California that owe their origins to relative motion between the Pacific and North American plates, movements that supply the strain energy se models and their predicted deformation patterns are discussed within the context of the entire earthquake deformation cycle in the next section. Readers interested in the coseismic movements observed for specific San Andreas earthquakes are referred to the reports by Lawson (1908) and Thatcher (1975) (1906 San Francisco earthquake), Zhang and others (1988) (1940 El Centro earthquake), and Segall and Harris (1987) (1966 Parkfield earthquake).

Vertical crustal movements can locally be substantial, at least when averaged over recent geologic time (see Yeats, 1977; Lajoie, 1986). Deformation from reverse-faulting earthquakes has also been well documented in several events (for example, 1952 Kern County earthquake by Stein and Thatcher, 1981; 1971 San Fernando earthquake by Castle and others, 1975; 1983 Coalinga earthquake by Stein, 1983). Nonetheless, vertical movements are second-order features along most of the San Andreas fault system, and so they are not considered further in this chapter.

Furthermore, in this chapter there is no review of measurement techniques, methods of analyzing and reducing data, or the mathematical and computational tools used in modeling deformation processes. Interested readers are referred to the reports by Bomford (1980) and Savage and Prescott (1973) for descriptions of horizontal-deformation-surveying methods and their precision, to those by Prescott (1976, 1981), Thatcher (1979), and Segall and Harris (1987) for discussions of data-analysis methods, and to the references cited below in the section entitled "Mechanics of Deformation" for details of the mathematical techniques used in model formulation.

Horizontal interearthquake deformation is summarized below in rates of both displacement and shear strain. For both of these parameters, the components parallel to major active faults are the most significant and best illustrate the dominant pattern of present-day tectonic movements, and so in this chapter these components are commonly shown exclusively. For example, although three independent tensor components are needed to completely characterize the horizontal-deformation field, in California the only significantly nonzero strain-rate component is commonly the shear strain parallel to the local trend of faults in the San Andreas system. Here, I consider only the component of maximum horizontal shear-strain rate, which, within observational uncertainty, almost invariably parallels the San Andreas fault or one of its major strands.