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ASEISMIC SLIP, INTEGRATED DISPLACEMENT RATES, AND PACIFIC-NORTH AMERICAN PLATE MOTION [c7, p191-193]

Rates of surface aseismic slip (fault creep) at representative points on the San Andreas fault system are listed in Table 7.2 and plotted in Figure 7.5. All fault segments displaying measurable aseismic slip are represented, but the detailed distribution along each segment is not shown; interested readers are referred to the references cited in Table 7.2 for more details. Figure 7.5 also displays the rates of relative right-lateral displacement integrated across geodetic networks of 50- to 140-km aperture that span the San Andreas and related faults in seven areas of California, for several of which the detailed displacement-rate pattern is shown in Figure 7.6 and Figure 7.7.

With the notable exception of the central, creeping section of the San Andreas fault, aseismic slip at the surface represents only a very small proportion of the total right-lateral displacement across the San Andreas fault system. On the 160-km-long central section of the San Andreas fault, maximum fault-creep rates average 30 mm/yr, close to the geodetically derived displacement rate of 33 +/- 1 mm/yr obtained over a 60-km aperture that spans the fault and the California Coast Ranges to the southwest. These data are the strongest evidence that no significant strain is accumulating in the crustal blocks adjacent to the fault in this region, and so all the relative plate motion taken up by the San Andreas system is here being accommodated by rigid-block translation across the fault. Just north of this segment, on the southern section of the Calaveras fault, a significant amount of rightlateral slip at a rate of about 13 mm/yr, occurs as fault creep. Elsewhere in California, however, measured aseismic-slip rates range from 2 to 6 mm/yr, and, creep commonly occurs only on restricted segments of otherwise-locked faults (for example, the Garlock and San Jacinto faults).

The integrated right-lateral displacement rates shown in Figure 7.2 firmly constrain the proportion of Pacific-North American relative plate motion accommodated across the San Andreas fault system in California. In northern, central, and southern California, maximum rates range from 33 to 37 mm/yr. Global reconstructions of the motions of the major tectonic plates over the past 3 Ma, as well as analyses of magnetic-anomaly lineations at the mouth of the Gulf of California, point to a relative Pacific-North American plate-motion rate of 49 +/- 3 mm/yr (DeMets and others, 1987). The San Andreas fault system thus accounts for 70 to 80 percent of the relative plate motion, although the San Andreas fault itself does not everywhere take up all of this motion, and deformation is typically distributed across a boundary zone about 100 km wide.

Precisely how much additional relative plate motion is accommodated across other faults in California is uncertain, although the amount is probably very little. According to Minster and Jordan (1987), very long baseline interferometric (VLBI) surveying results indicate that oblique extension of the Basin and Range province, directly east of California, is occurring at a rate of 10 +/- 2 mm/yr with an orientation of N. 56� +/- 10� W. Depending on the exact rate and orientation of this extension, as well as on the precise direction of relative Pacific-North American plate motion, the residual "missing" plate motion being accommodated in California on faults other than those of the San Andreas system ranges from negligibly small to possibly as much as 10 mm/yr. Thus, although the geodetic coverage in California is far from complete (see (Figure 7.2, and (Figure 7.5, all or most of the zone of significant plate-boundary deformation apparently has been encompassed.