TABLE 11.2 Near-Field Tectonic Geodesy

Motion/Technique

Measured Changes

Typical Aperture

Required Precision

Frequency of Resurvey

Horizontal

alignment

Deflection

100 m

+1 mm

Months

triangulation

Angles

1000 m

+5 mm

Months to years

trilateration

Lengths

1000 m

+5 mm

Months to years

Vertical

precise leveling

Heights

1000 m

+1 mm

Months

Tilt

precise leveling

Heights

500 m

+1 µrad

Months

dry tilt

Heights

40 m

+10 µrad

Months to years

may be one of the effective methods for earthquake prediction. The tectonic significance of creep is still a topic of debate: some investigators believe that creep relieves sufficient buildup of stress so that large earthquakes are precluded in a creeping segment of a fault (Brown and Wallace, 1968; Prescott and Lisowski, 1983), and that notion seems to have gained support by great accumulation of strain data over the last decade (Langbein, 1981). Alternatively, creep is postulated to be the first step in progressive failure leading to a major earthquake (Nason, 1973).

Afterslip is fault slip that occurs in the days, weeks, or even months following the main earthquake. Most reported (Nakamura and Tsuneishi, 1967; Ambrayses, 1970) and documented instances of significant afterslip are for strike-slip faults. The principal characteristic of afterslip is that the slip rate decreases logarithmically (Smith and Wyss, 1968; Wallace and Roth, 1968; Sylvester and Pollard, 1975; Bucknam et al., 1978; Cohn et al., 1982; Harsh, 1982). The magnitude of displacement may equal or exceed the coseismic slip, as has been observed in strike-slip earthquakes (Smith and Wyss, 1968; Burford, 1972; Bucknam et al., 1978; Sharp and Lienkaemper, 1982), but in other kinds of earthquakes it is small relative to the coseismic slip (Lensen and Suggate, 1968; Lensen and Otway, 1971; Sylvester and Pollard, 1975; Stein and Thatcher, 1981). Whether afterslip is truly aseismic has not been clearly established, although Stein and Lisowski (1983) found that afterslip following the 1979 Homestead Valley, California, earthquake (ML=5.8) was much greater than the summed M0 of the aftershocks, and they concluded that the afterslip, which constituted about 10 percent of the seismic slip, was aseismic.

Dynamically triggered slip is coseismic slip on a fault or faults outside the epicentral area of the main shock. The phenomenon has been documented in moderate earthquakes in the Salton Trough, where up to 30 mm slip was found on faults as far as 40 km from the causative fault and epicenter (Allen et al., 1972; Fuis, 1982; Sieh, 1982). Ambiguities inevitably arise in the definition, however, such as in cases of the May 1983 Coalinga (ML=6.7) earthquake where aftershocks and surface ruptures occurred on faults distant from and not be-

FIGURE 11.2 Offset of line of nails across San Andreas Fault near San Juan Bautista. Line was originally straight in 1967.



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