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Active Tectonics: Studies in Geophysics
ery in 1960 of aseismic slip on the central segment of the San Andreas Fault, manifested by offset buildings, irrigation ditches, and vineyard rows (Steinbrugge et al., 1960), changed many of our notions about fault mechanics, because here was an example of a geologic process occurring at rates measureable on a human time scale.
Because the movements seemed to be limited to a zone only a few meters wide, Tocher and his colleagues (Tocher et al., 1968; Nason and Tocher, 1970) initiated a variety of small-scale geodetic and instrumental studies to measure these minor, but significant movements close to the fault. Monitoring of several faults, chiefly in California but also in New Zealand (Lensen and Suggate, 1969) led to the equally surprising discoveries of minor fault movements that preceded earthquakes (Allen and Smith, 1966), that followed earthquakes (Smith and Wyss, 1968; Wallace and Roth, 1968), and that were triggered by earthquakes (Allen et al., 1972). In all cases the surficial displacements were confined to narrow zones less than 100 m wide along the fault. Although the U.S. Coast and Geodetic Survey (now the National Geodetic Survey) has conducted near-field triangulation monitoring of faults since about 1900 (Meade, 1971), several investigators including C.R.Allen, R.O.Burford, G.J.Lensen, R.D.Nason, J.C.Savage, A.G. Sylvester, and D.Tocher devised and initiated a variety of new near-field geodetic and instrumental techniques in the late 1960s and 1970s to determine the extent and rate of creep, whether creep may occur on other faults or on other kinds of faults, about the timing and magnitude of preseismic slip, the amount and duration of postseismic slip, and the significance of dynamically triggered slip.
These small movements, now found to measure from 1 to 30 mm/yr, have an impact on society as can be demonstrated by damage to buildings, streets, and subsurface pipelines in the town of Hollister, California, insofar as preseismic slip may provide information leading to the prediction of earthquakes and to the extent that earthquakes on a known active fault may trigger equal or greater movement on other faults presumed to be inactive. At the very least, understanding these movements may provide greater insight into earthquake mechanisms, knowledge of which will be requisite for eventual prediction of earthquakes.
CREEP, AFTERSLIP, AND DYNAMICALLY TRIGGERED SLIP
It is useful to discuss briefly a preferred nomenclature for minor fault slip of very different origin, because am
TABLE 11.1 Earthquake Mechanics of Minor Movements
Steinbrugge et al. (1960)
Allen and Smith (1966)
1 to thousands of mm
dynamically triggered slip
Allen et al. (1972)
Allen and Smith (1966)
biguities arise when the term “creep” is simply used for all these kinds of fault slip (Table 11.1).
The great difference among these terms is illustrated in Figure 11.1, which shows the magnitude of slip as a function of time. Creep is aseismic fault slip: it may be stable and continuous or temporally and spatially episodic (Yamashita and Burford, 1973; King et al., 1973; Nason et al., 1974; Evans et al., 1981), and the long-term rate may vary before or after earthquakes along the creeping fault segment (Nason and Tocher, 1971; Burford et al., 1973). Fault creep precedes some earthquakes, as is summarized by Mjachkin et al. (1972), Scholz et al. (1973), and Whitcomb et al. (1973), and offers the hope that near-field geodetic observations