Active Tectonics Impact on Society (1986) / Chapter Skim
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11. Near-Field Tectonic Geodesy
11. Near-Field Tectonic Geodesy
Pages 164-180
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From page 164... ...
Crustal tilt may be measured to about 0.5 grad if special attention is paid to type and stability of bench marks. Thus, Chinese precise leveling has documented several centimeters of vertical movement within a few hundred meters of faults a few days before surface rupture; 14-cm vertical afterslip was measured in the 10 weeks following the 1979 Imperial, California, earthquake; and 35 mm/yr nontectonic subsidence has occurred across a fault in Fremont Valley, California, for at least the last 9 yr.
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From page 165... ...
At the very least, understanding these move ments 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 165 TABLE 11.1 Earthquake Mechanics of Minor Movements Rate References Tectonic creep .
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From page 166... ...
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 FIGURE 11.2 Offset of line of nails across San Andreas Fault near San Juan Bautista.
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From page 167... ...
earthquake may have been dynamically triggered. METHODS, TECHNIQUES, AND RESULTS Geodesists have classically documented the direction and magnitude of small crustal movements by repeated surveys of arrays of bench marks and by comparing changes in line lengths, angles, or heights among bench marks between an initial and a subsequent survey.
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From page 168... ...
and yield a precision of 0.5 mm (Burford and Harsh, 1980~. The USGS arrays were es tablished in 1967-1974 along the creeping segment of N the San Andreas Fault in central California to define the width of the creep zone more clearly as well as the vari ability of creep along strike.
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From page 169... ...
was one of the first to establish and frequently resurvey short leveling arrays of closely spaced bench marks to study small vertical fault movements in detail, although the U.S. Coast and Geodetic Survey established lines of closely spaced bench marks across the southern San Andreas Fault in the 1930s.
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From page 170... ...
, our procedure is to repeat precise leveling surveys of arrays of permanent bench marks established across active and potentially active faults. Comparison of surveys reveals height changes that then may be related spatially to surface faults and temporally to occurrences of earthquakes.
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From page 171... ...
experience, most turning points in our level are permanent bench marks no more than 40 m apart throughout the line, and permanently marked instrument points are exactly midway between bench marks. The relatively short, balanced sights minimize systematic errors from refraction and collimation, respectively, and assure that the rod images are sufficiently large to be read accurately.
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From page 172... ...
The dry-tilt method determines tilt of a plane defined by three or more bench marks by measuring height differences among the bench marks between two separate surveys (Sylvester, 1978; Yamashita, 1981~. Both in our work and in Hawaii a shaded precision level is erected at the center of an array of at least three permanent bench marks on each of which three precise Invar leveling rods are erected simultaneously (Figure 11.16~.
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From page 173... ...
PROBLEMS Although there is a fairly voluminous literature on the origin of creep and related minor fault movements, sys `; meters Contours ore schematic FIGURE 11.12 Site map of irregular leveling array across San Jacinto Fault near Anza, California. tematic near-field geodetic measurements of these movements have proceeded only about two decades now only beginning to be a significant length of time.
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From page 174... ...
Dynamically Triggered Slip (1) Does dynamically triggered slip represent release of a fraction of stored elastic strain energy along a given fault, or is it just a manifestation of jiggling of two fault blocks caused by shaking?
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From page 175... ...
\ WC 21 · WC22 ~\~` WC 23 ~ \6,:; ~\ / 175 / WC 03 WC04 WC05 ~6 WCO9 WC11 ~ WC08 · ·-WC10 WC. ~wc 12 5 ^/ "WC14 ~ WC 27 WC24 ,< ~\ N WC 25 WC26 · .K ,~ ~o~ ~ ~o |
From page 176... ...
from a limited base of observations? To date, dynamically triggered slip has been documented for only the Salton Trough of southern California.
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From page 177... ...
. Displacements on the Imperial, Superstition Hills and the San Andreas Faults triggered by the Borrego Mountain earthquake, U.S.
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From page 178... ...
(1978~. Geodetic bench marks, NOAA Manual NOS NAT 1, 50 pp.
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From page 179... ...
Riggs (1980~. Vertical movements and aseismic horizontal creep on the San Andreas Fault at San Juan Bautista, California, in Studies of the San Andreas Fault Zone in Northern California, R
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From page 180... ...
. Re-examination of the anomalous phenomena taken as precursory before the Haicheng earthquake in 1975, in A Collection of Papers of International Symposium on Continental Seismicity and Earthquake Prediction, Gu Gongxu and Ma Xingyuan, eds., Seismological Press, Beijing, China, pp.
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