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DETAILED DISPLACEMENT-RATE PATTERNS [c7, p193-195]

Considerable detail on the distribution of deformation in the San Andreas boundary zone is provided by the rather complete geodetic coverage available in the San Francisco Bay region and southern California. In the method used to reduce these data, geodetic-line-length changes are used to determine station-displacement rates relative to a point at the center of gravity of the network. Fault-normal displacements are permitted by this method, but their values are minimized in the inversion process (see Prescott, 1981). Gross departures from this constraint would be revealed by notable disagreements between observed and predicted line-length changes, but no such discrepancies were found for the results presented here. Because the fault-normal displacement rates are small and show no consistent trends, they are not plotted on the profiles presented here.

Displacement rates in the San Francisco Bay region are plotted in Figure 7.6. The distribution of deformation varies considerably across the San Andreas boundary zone from north to south of the San Francisco Bay. In the north bay, the integrated right-lateral-displacement rate across the network of 27+/-3 mm/yr ( Fig. 7.6B) indicates that not all of the boundary zone has been captured within its 110-km aperture. Within about 5 km of the San Andreas fault, rapid change in the gradient of deformation rate indicates that inter-earthquake strain is concentrated close to the fault. Outside this near-fault region, deformation southwest of the fault appears to be negligible. Northeast of the fault, however, the persistence of significant movements right to the edge of the profile suggests that the 5-to 10-mm/yr deficit in boundary-zone deformation across this profile is being accommodated to the east of the Green Valley fault. Across the central and south bay ( Fig. 7.6C), movements are more evenly distributed through the network, and the integrated displacement rate of 37+/-3 mm/yr across the south bay suggests that the entire boundary zone has been spanned. Closer examination of the profile, however, reveals several zones of locally high deformation gradient, one across the San Andreas fault, where it resembles that observed near the fault in the north bay. In addition, rapid changes in the profile across the Hayward and Calaveras faults reflect aseismic slip at rates of 3 to 6 mm/yr on these faults (see Figure 7.5).

In southern California (Fig. 7.7), deformation across the San Andreas boundary zone notably broadens from the Salton Sea, in the south, northwestward to the Big Bend region of the San Andreas fault north of Los Angeles (see Fig. 7.5). At the south end of the Salton Sea, all of the boundary-zone deformation, 35+/-1 mm/yr, occurs within an area about 50 km wide (profile S, Fig. 7.7C) that rapidly broadens to more than 100 km wide north and west of the Salton Sea (profile N, Fig. 7.7C) and, possibly, broader still by about 50 km farther northwest (Fig. 7.7B). North and west of Los Angeles, networks of 100-km aperture capture only 18+/-2 mm/yr of the total right-lateral-displacement rate (Figure 7.5). The profiles in Figures 7.7B and 7.7C also show that in contrast with the northern section of the San Andreas fault, deformation gradients across the fault are smoother, and deformation is not so closely concentrated near the fault.