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Active Tectonics: Studies in Geophysics
FIGURE 10.2 (A) Stations and lines observed in the southern San Francisco Bay region. Active faults are shown for reference. (B) Displacement rates parallel to N 35° W plotted versus distance normal to strike of San Andreas Fault. (C) Schematic interpretation of (B). Solid curve was drawn for 12 mm/yr slip below 7 km on the San Andreas Fault, 6 mm/yr rigid-block slip on the Hayward Fault, and 6 mm/yr rigid-block slip on the Calaveras Fault. Half of Calaveras fault slip was distributed over 5-km-wide zone. Dashed line is displacement field that would be observed if motion were distributed uniformly. From Prescott et al. (1981), with permission of the American Geophysical Union.
contains 43 lines whose lengths have been measured roughly annually since 1970; the precision of each measurement depends on line length but averages about 3 parts in 107. The line length changes during 1970–1980 have been analyzed by Prescott et al. (1981), who determined the average displacement rate of each station relative to a fixed center of mass of the network as a whole.
Figure 10.2(B) shows the displacement rate components parallel to the San Andreas Fault plotted versus distance from the fault, and Figure 10.2(C) is a schematic interpretation of this result. Clear offsets occur across the Hayward and Calaveras Faults, and their magnitudes agree well with observed creep rates obtained independently from small-aperture arrays and wire extensometers that span each of these faults (see Sylvester, Chapter 11, this volume, for discussion of these measurement methods).
The displacement-rate profile across the San Andreas Fault is more interesting. The absence of any discontinuity at the fault trace indicates that the San Andreas is locked at the surface; increasing movement rates away from the fault suggest that it is freely slipping below some locking depth, D. A simple calculation shows that for such a model the deep slip rate is 12±4 mm/yr and D=7 km. This same fault slipped 2 to 3 m from the surface to depths of 5 to 10 km at the time of the great 1906 San Francisco earthquake (Thatcher, 1975), and the current deformation pattern represents strain buildup leading to the repeat of a large or great earthquake like the 1906 shock. If slip rates inferred for the past decade are representative of the long-term rate, and if coseismic offsets of 2 to 3 m per event are typical of this segment of the San Andreas Fault, then the average recurrence interval for such events is 170 to 250 yr.
Geologic data independently support the geodetic results. Although direct evidence is lacking on occurrence times and offsets of past events, measures of late Holocene slip rate confirm the value obtained from geodetic measurements. Dated offsets of late Holocene geomorphic features that cross the San Andreas Fault near Crystal Springs Reservoir, 40 km northwest of the geodetic network shown in Figure 10.2(A), yield a slip rate of 12 mm/yr over the last 1130±160 yr (Hall, 1984).
Geodetic estimates of slip rate have been obtained for several other segments of the San Andreas system (see Table 10.1), and more will become available in the future. Several of those listed in Table 10.1 are only approximate and are subject to a number of caveats: often the entire deformation zone of a single fault is not spanned, subsidiary subparallel faults may contribute to observed movements, and deformation rates (see below) may vary notably over time scales of a few years or less.