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SHEAR STRAIN ON THE SAN ANDREAS FAULT SYSTEM
[c7, p191]

Rates of contemporary shear strain are displayed in several complementary ways in Figures 7.2 through 7.4; details of each rate determination are summarized in Table 7.1. Although only the magnitude of the maximum shear-strain rate is shown in each figure, the orientation of the maximum-horizontal-contraction axis is listed in Table 7.1. Note that for each of the strain rates shown in Figures 7.2 through 7.4, aseismic fault slip contributes only negligibly, if at all, to the measured deformation. Further details on each strain field determination can be found in the references cited in Table 7.1.

Shear-strain rates peak at 0.4 to 0.6 mu rad/yr (Figure 7.2) across the currently locked northern and southern sections of the San Andreas fault. Significant but slightly lower strain rates of 0.3 to 0.4 mu rad/yr are observed across right-lateral strike-slip faults in the northern California Coast Ranges (north of lat 38� N.) east of the San Andreas fault, as well as across the San Jacinto fault in southern California. Shear-strain rates resolvably greater than zero are observed as far as about 80 km from the San Andreas fault itself.

In addition, significant deformation is occurring across active faults in east-central California. In the White Mountains, along the southern California-Nevada State line, small but resolvable strain rates (0.06 +/- 0.02 mu rad/ yr) have been measured, and the orientation of the strain field indicates crustal extension perpendicular to north-south-striking normal faults in the area. Somewhat higher deformation rates are observed farther south, where right-lateral strain is occurring parallel to the Owens Valley fault, site of the M~~=8 earthquake of 1872 (see chap. 6).

Shear-strain rate is plotted as a function of perpendicular distance from the San Andreas fault in Figure 7.3. Deformation rates peak at the fault and decrease to half their maximums at a distance of about 30 km from the fault. Most of the deformation is encompassed within a zone about 100 km wide centered on the fault ("San Andreas boundary deformation zone"), as discussed below. However, the reader may confirm that this total lies in the range of about 30-40 mm/yr by drawing a smooth curve through the data plotted in Figure 7.3 and integrating this curve (that is, measuring and summing the total area underneath the curve) from -60 to +60 km.

Maximum shear-strain rates at the San Andreas fault tend to be higher across the 1906 earthquake rupture in northern California (approx 0.6 mu rad/yr) than in southern California (0.4 mu rad/yr), although the Carrizo Plain data violate this generalization. Rather high deformation rates are also observed 20 to 60 km east of the San Andreas fault in the northern California Coast Ranges.

Shear-strain rates at various locations on the two currently locked sections of the San Andreas fault are plotted versus time since the most recent great earthquake at each locality in Figure 7.4. Most of these data are derived from triangulation measurements, many of which were first made in the late 19th or early 20th century. Thus, these determinations are much less precise than those listed in Table 7.1 and plotted in Figure 7.2 and Figure 7.3, most of which are from the post-1970 period. Nonetheless, it is clear from Figure 7.4 that deformation rates on the fault are much higher in the years to tens of years Immediately after a great earthquake than they are later. Although it may be questionable to lump values from northern and southern California together on a single plot, the temporal decline in shear-strain rate shown in Figure 7.4 depends only on about the first 70 years of data plotted, all of which come from the 1906 rupture on the northern section of the San Andreas fault.