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Active Tectonics: Studies in Geophysics 11 Near-Field Tectonic Geodesy ARTHUR G.SYLVESTER University of California, Santa Barbara ABSTRACT Fault movements may be monitored by precise surveying of closely spaced arrays of permanent bench marks within 1 km of a fault. The geodetic data may complement data from tiltmeters and creepmeters as well as data from large aperature trilateration or triangulation arrays. If resurveys are temporally fortuitous, preseismic, coseismic, and postseismic movement data are obtained. Closely spaced, linear arrays of nails in pavement across active faults provide a quick, simple, and inexpensive way to locate and measure horizontal displacement across narrow, well-defined zones of faulting. Small-aperature trilateration and alignment arrays document horizontal creep across faults of the San Andreas system, especially in central California, where as much as 32 mm/yr right slip has taken place for more than two decades. Small-aperature triangulation arrays have also identified horizontal creep on faults elsewhere in California at rates of up to 5 mm/yr. Short level lines may detect height changes of 0.5 mm. Crustal tilt may be measured to about 0.5 µrad 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. Spirit-level optical tilt of triangular arrays of bench marks (dry-tilt) yields tilt data to a precision of about 1 µrad sufficient to document major movement, especially near volcanoes. From 10s to 1000s of microradians of tilt occurred over a few days or weeks prior to eruptions of the Kilauea and Mount St. Helens volcanoes. INTRODUCTION Only a short while ago tectonic movements of the Earth’s crust were generally considered to be too slow to be observed in a lifetime, and long periods of inactivity were believed to be punctuated only at the most inconvenient times and places for man by surficial fault ruptures accompanying large, infrequent earthquakes. To be sure, Reid (1910) estimated a steady movement rate of the Farallan Islands relative to Mount Hamilton, California, at about 5 cm/yr from 1850 to 1905, and in 1938 Icelanders commenced geodetic measurements to observe tectonic movements in the neovolcanic zone of north Iceland (Niemczyk, 1943). However, the discov-
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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 Movement Rate References Tectonic creep 1–30 mm/yr Steinbrugge et al. (1960) preseismic slip 1-? mm/yr Allen and Smith (1966) coseismic slip 1 to thousands of mm Many authors dynamically triggered slip 1–30 mm Allen et al. (1972) afterslip 1–300 mm/yr Allen and Smith (1966) Nontectonic subsidence 1–35 mm/yr Many authors 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 FIGURE 11.1 Types of tectonic creep.
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Active Tectonics: Studies in Geophysics 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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Active Tectonics: Studies in Geophysics lieved to be directly related to the fault that caused the main earthquake (Hart and McJunkin, 1983; Stein, 1983). Some of the myriad of surface ruptures produced in the 1971 San Fernando (ML=6.4) 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. If a surveying array covers a large area, then accumulated survey errors may yield changes of position that are nearly equal in magnitude to the actual movement. Thus the advantage of small fault-crossing networks is that they may yield more accurate displacement data; and because of their small size, they may be resurveyed more quickly and frequently, providing thereby, more nearly continuous sampling of movement (Table 11.2). In the appraisal of fault movements, we wish to know where the movement occurs, which way the fault blocks move relative to each other, how much the offset is, and when it happens. Geologic rates of crustal movement and empirical determination of historic rates of fault displacements from progressive offset of cultural features show that a precision of at least 1 part per million is required to document ongoing tectonic movements in near-field geodetic work (NRC Panel on Recent Crustal Movements, 1981). That means reproducible resolution of at least 1 mm is generally necessary. At these high levels of precision, the question of bench-mark stability (Karcz et al., 1976; Savage et al., 1979b; Sylvester, 1983, 1984) also clouds interpretations of tectonic movements, because any small shift of a point that is assumed to be fixed or stable will yield systematic changes in other points that are not necessarily real. All near-field geodetic arrays must be resurveyed periodically to establish bench mark and background and secular noise. Soviet scientists have found that earth background noise may be very unstable for a variety of known and unknown reasons; therefore, many repeated surveys are needed to characterize background noise (Nersesov, 1984). Out of several bench-mark stations, only a few may be good and reliable, but which ones can only be determined by observing them. Both the Soviet and Chinese experiences show that networks of instruments, especially a wide range of observations, together with good communication among participating scientists, are absolutely essential if earthquakes are ever to be predicted (Mei, 1984; Nersesov, 1984). FIGURE 11.3 Alignment array across Nadeau branch of San Andreas Fault near Palmdale, California. E and W are wing stations, IP is the instrument point for the theodolite. Horizontal Movements Alignment Arrays Perhaps the simplest and cheapest measurement technique for horizontal movements is to establish a line of nails in street pavement across a fault (Figure 11.2) and to measure their deflection over time relative to one or several arbitrarily fixed points (Rogers and Nason, 1971). If fault slip is discovered, then more expensive and rigorous techniques may be employed for more complete documentation of the spatial and temporal character of the movement. Typically nail lines, or “nail files” as they have come to be called (Louie et al., 1985), contain from 10 to 50 nails in a line as long as 100 m established perpendicular to the fault strike. The position of each nail can be measured to within 1 mm with precision calipers relative to a straight line of sight provided by a theodolite. Alignment arrays are also measured by triangulation. Using a theodolite, angles are turned from reference targets in directions parallel to the fault to nails driven into a line of utility poles, fences, or trees with a precision of 1 mm (Keller et al., 1978; Aytun, 1980; Louie et al., 1985). These kinds of arrays range in length from 100 to 500 m, are nearly perpendicular to the fault, and contain at least three points on each side of the fault (Figure 11.3). Their chief disadvantage is that their substandard bench marks will inevitably yield substandard results and thus limit their eventual utility for both coseismic and long-term changes. Alignment arrays established by the U.S. Geological Survey (USGS) for more rigorous and precise determination of creep involve triangulation with a theodolite to specially designed targets placed above class B rod
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Active Tectonics: Studies in Geophysics FIGURE 11.4 Doubly braced quadrilateral across the San Andreas Fault at Pallett Creek near Palmdale, California. marks (Floyd, 1978) and yield a precision of 0.5 mm (Burford and Harsh, 1980). The USGS arrays were established in 1967–1974 along the creeping segment of the San Andreas Fault in central California to define the width of the creep zone more clearly as well as the variability of creep along strike. The arrays have about 15 bench marks in a line from 30 to 220 m long across the strike of the fault. Trilateration The position of bench marks in closed arrays across faults (Figure 11.4) may be precisely determined by triangulation (e.g., Meade, 1971; Lensen and Otway, 1971; Henneberg, 1978, 1983) or trilateration. The former is tedious and time-consuming and still requires that a length be directly measured with high precision if displacements are to be resolved. More commonly, an array consisting of four bench marks—a quadrilateral—is established across a fault (Figure 11.4), and the lengths of each side and both diagonals are measured with an Invar tape or an electronic distance meter (EDM) with which a precision of 1 ppm is routinely achieved. Line lengths for EDM arrays range from 50 to 3000 m, depending on the topography and FIGURE 11.5 Locations of University of California, Santa Barbara, leveling arrays in central and southern California relative to major faults. Key to Figure 11.5: Locations of UCSB Level Lines (1) San Juan Bautista (2) McGee Creek (3) Fish Lake Valley (4) Triangle Spring (5) Sewage Plant (6) Artist’s Drive (7) Shorty’s Well (8) SNORT (9) Airfield (10) Duravan Ranch (11) Wallace Creek (12) Camp Dix (13) Caballo (14) Grapevine (15) Mesa Valley Farm (16) JM Quarry (17) Santa Barbara (18) Jameson Lake (19) Santa Cruz Island (20) San Fernando (21) Una Lake (22) Llano (23) Big Rock Springs (24) Pallett Creek (25) Dalton Canyon (26) Anza (27) Pinyon Flat (28) Painted Canyon (29) Arroyo Seco (30) Santa Anita Canyon (31) Parkfield (32) Koehn Lake
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Active Tectonics: Studies in Geophysics the apparent width of the fault zone. The dimensions of taped quadrilaterals are smaller, from 1 to 50 m. Horizontal displacements of bench marks are measured by resurveys of the array, and single bench-mark displacement-rate vectors and their standard deviations are computed from a variation of coordinates adjustment using all line-length change-rates and their standard deviations (Lisowski and Prescott, 1981). The small-aperature trilateration studies complement creepmeter, strainmeter, alignment array, and broad-scale geodolite measurements (e.g., Savage et al., 1979a) to show that (1) the creep rate in the central segment of the San Andreas Fault varies stepwise from less than 1 mm/yr at each end of the fault segment to 30 mm/ yr at the center (Lisowski and Prescott, 1981); (2) the width of the main zone of slip is generally less than 70 m (Burford and Harsh, 1980); (3) episodic creep occurs in the Salton Trough at rates ranging from 1 to 10 mm/yr (Keller et al., 1978; Louie et al., 1985), although the infrequency of surveys there does not clearly show whether the fault slip is dynamically triggered or is truly tectonic creep (Goulty et al., 1978); and (4) creep has been observed outside of California only on the North Anatolian Fault in eastern Turkey, where dextral creep of 10 mm/yr has taken place during the 10 years of geodetic monitoring (Aytun, 1980). Vertical Movements Precise leveling is the most common method used to detect and document vertical crustal movements over periods of days to decades, because when compared with currently available alternatives, leveling is more stable over longer periods of time and long distances and is less costly and more accurate over short and moderate distances (Brown and Reilinger, 1980). If repeated sufficiently frequently, precise leveling may aid other geophysical techniques for earthquake prediction, as has been demonstrated in China (Tanaka, 1978; Mei, 1984; Zhu et al., 1984). Preseismic fault offsets of 1- to 4-mm amplitude and from 50 to 200 km from the epicentral area were observed in the year prior to each of four M= 7+ earthquakes in China (Zhang and Fu, 1981). Tryggvason (1968) 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. A few lines, with only three or four bench marks in 1 or 2 km, were also established across the Atwater and Wairau Faults in New Zealand in 1930 (Mackie, 1971). Because so few investigators are involved in this kind of work, it is instructive to outline the procedures and techniques employed in our studies of near-field tectonic strain. Following Tryggvason’s (1968) example and since 1970, my students and I have established a variety of fault-crossing leveling arrays across strike-slip, normal, reverse, and thrust faults throughout southern and central California (Figure 11.5) to document in time and space the vertical movements that occur near different kinds of faults and to understand tectonic processes leading to, and following, fault rupture. For example, our straight-line array at San Juan Bautista (Figure 11.6) measures vertical separation across the San Andreas Fault (Figure 11.7), where horizontal creep is monitored by creepmeters (Sylvester et al., 1980); our W-shaped array in the Garlock area northeast of the Mojave Desert (Figure 11.8) monitors nontectonic fault slip caused by groundwater withdrawal (Sylvester, 1982; Holzer, 1984) and in 9 years shows a constant height change across the fault of 35 mm/yr (Figure 11.9); straight lines across the Punchbowl Fault at Pallett Creek and across the Pleito Fault at Grapevine (Figure 11.10) measure movement across reverse and thrust faults (Figure 11.11); an irregular- FIGURE 11.6 Site map for level line across San Andreas Fault at San Juan Bautista. SJN1 and XSJ2 are locations of the U.S. Geological Survey creepmeters for measurement of horizontal movement.
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Active Tectonics: Studies in Geophysics FIGURE 11.7 Height change data across San Andreas Fault at San Juan Bautista, 1975–1983. Bench mark 7328 is arbitrarily held fixed. shaped array at Anza (Figure 11.12) detects minor vertical movements across a narrow, well-defined rift zone of the San Jacinto Fault (Figure 11.13); a closed quadrilateral across the San Andreas Fault at Wallace Creek (Figure 11.14) provides us two measurements of vertical offset across the fault as well as of the tilt of the fault blocks; lines in Death Valley and Long Valley are across youthful, normal faults; and the quadrilateral at the Pinyon Flat Geophysical Observatory (Figure 11.15) provides geodetic control for arrays of long-base fluid tiltmeters and other strain-measurement devices (Sylvester and Jackson, 1982; Sylvester, 1984). We choose our surveying sites where geomorphic evidence shows that significant vertical movements have occurred in the recent geologic past, and we rely on other investigators to provide medium-range precursory information that identifies faults that may be in the preparation stages for a major earthquake. Then we establish arrays in the target area and resurvey them as frequently as possible and practical. Like other investigators (e.g., Sharp and Lienkaemper, 1982), 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. All our leveling arrays are relatively short—line lengths range from 200 to 2600 m and contain as many as 70 bench marks. Geometry of arrays is generally dictated by the terrain and property access and includes L-, Z-, W-, and closed, quadrilateral-shaped arrays (Sylvester, 1982). Some of the straight-line segments across faults are also aligned with a theodolite to document horizontal movement. Many of the fault-crossing arrays are quadrilateral-shaped to determine the tilt of each fault block independently by analyzing L-shaped subsets of bench marks (Figure 11.14). Following Tryggvason (1968) and our accumulated FIGURE 11.8 Site map of W-shaped leveling array across subsidence fault in Fremont Valley, California.
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Active Tectonics: Studies in Geophysics FIGURE 11.9 Height change data for subsidence fault at Duravan Range, Mojave Desert. Bench mark RRT is arbitrarily held constant. 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. Our leveling is always double-run using a shaded precision automatic level and strut-supported Invar leveling rods. The same rod is always placed on the same bench mark in each successive survey, and our rods are annually calibrated every 10 cm along their entire length. Perhaps because only a few investigators have been leveling in the near field for a short time in limited parts of the world, few reports of significant results are available in contrast to those from broad-scale leveling. A review of vertical preseismic, coseismic, and postseismic slip associated with U.S. earthquakes is given by Reilinger and Brown (1981). Vertical afterslip was indicated but not authenticated in two New Zealand earthquakes—the Murchison earthquake of 1929 (Henderson, 1937) and the Napier earthquake of 1931 (Henderson, 1933). Otherwise vertical afterslip has been documented in only four earthquakes. Sylvester and Pollard (1975) found that afterslip amounted to less than 1 percent of the coseismic vertical separation in the year following the 1971 San Fernando (M=6.4) earthquake; Sharp and Lienkaemper (1982) found that 14-cm afterslip, nearly equal to the coseismic slip of 16 cm, occurred in the 10 weeks following the 1979 Imperial Valley (M=6.5) earthquake; afterslip following the Alaskan (M=8.3) earthquake of 1964 reached 0.55 m over 10 years (Brown et al., 1977; Prescott and Lisowski, 1977); Lensen and Suggate (1968) measured 12 mm of vertical afterslip in 2 months across the Inangahua Fault (New Zealand) after about 1 m of vertical separation in the 1968 Inangahua (M=7) earthquake. Near-field leveling revealed 54 µrad of crustal tilt 6 months before the Imperial Valley earthquake (Sharp and Lienkaemper, 1982), and significant tilt has also occurred before some Chinese earthquakes (Mei, 1984; Zhu et al., 1984). Previous reports of regional tilt before Japanese earthquakes have been attributed to refraction errors in the leveling (Mogi, 1984). Vertical movement across normal faults of the Asal-Ghoubbet rift, Dijoubti, East Africa, is regarded as aseismic creep by Ruegg et al. (1984), but vertical creep has not been observed elsewhere in spite of specific searches for it, especially in California (Sylvester, 1982). CRUSTAL TILT Tilt of the Earth’s surface has been observed prior to earthquakes by broad-scale leveling (e.g., Bendefy,
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Active Tectonics: Studies in Geophysics FIGURE 11.10 Site map of level line across Pleito thrust fault near townsite of Grapevine, California. 1958, 1965) and by near-field leveling (Sharp and Lienkaemper, 1982; Mei, 1984; Zhu et al., 1984). Broad-scale leveling has also demonstrated recent influx of magma into the crust in the Yellowstone area (Pelton and Smith, 1982); beneath Long Valley caldera in eastern California (Savage and Clark, 1982); beneath Mount Etna (Murray and Guest, 1982); and in the rift zone of northern Iceland (Björnsson et al., 1979). Magmatic inflation of volcanoes has been geodetically monitored for many years at Kilauea and Mauna Loa Volcanoes on Hawaii (Kinoshita et al., 1974; Decker et al., 1983) and more recently at Mount St. Helens (Chadwick et al., 1983). One of the small-scale, geodetic methods for monitoring fairly large tilt is spirit-level optical tilting (Kinoshita et al., 1974) or dry-tilt as it is popularly termed. 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). We take care to choose sites for dry tilt that are reasonably flat; that have radial symmetry; and that are not near oil and water wells, landslides, or recently imposed construction loads such as bridges, buildings, and land fills. In our experience, the noisiest data are obtained from tilt sites on ridge crests and forested areas on bedrock, whereas data showing least noise are obtained from sites on wide open flats underlain by relatively thick deposits of alluvium. In Hawaii the dry-tilt measurements complement those of borehole tiltmeters, and short-base (3 m) and long-base (50 m) water-tube tiltmeters. With superior equipment in good adjustment, with careful attention to detail, and with rigorous systematic measurement procedure, a resolution of from 2 to 3 µrad has been achieved in Hawaii as shown in comparative tests with the 50-m water-tube tiltmeter. In general, however, resolution of tilt with three point arrays ranges from 5 to 10 µrad (Isacks et al., 1978; Björnsson et al., 1979; Savage et al., 1979b; Decker et al., 1983; Otway et al., 1984). Therefore, the dry-tilt method is best suited for monitoring large and fairly rapid tilts, from tens to hundreds of microradians per day which may be expected to accompany magmatic inflation of volcanoes (Dzurisin et al., 1982a,b; 1983; Fiske and Shepard, 1982; Chadwick et al., 1983). For more precise determination of tectonic tilt, long L- and T-shaped and closed arrays of bench marks having apertures of from 500 to 1000 m $
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Active Tectonics: Studies in Geophysics FIGURE 11.11 Height change data cross the Pleito thrust fault, 1980–1983. Bench mark 1 is arbitrarily held fixed. are necessary (Savage et al., 1979b). With such figures it is possible to achieve resolution approaching 0.5 µrad (Sylvester and Jackson, 1982). PROBLEMS Although there is a fairly voluminous literature on the origin of creep and related minor fault movements, sys 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. Thus it is imperative that this time base of data be lengthened to obtain empirical data to answer and refine earlier answers to the following questions. Creep Why, with but the exception of the North Anatolian Fault in Turkey (Aytun, 1980), is creep restricted to the central and southern segments of the San Andreas Fault? Is it because the fault is relatively straight and parallel to the lithospheric plate motion, that in its central segment it juxtaposes relatively weak, serpentine-bearing rocks as several authors have maintained? Why is vertical creep so uncommon? Is it simply because the search has not been sufficiently broad in scope and duration? Or are there more fundamental tectonic reasons? How does creep relate to the earthquake mechanism? Is it a safety valve that periodically releases stress, thus precluding great earthquakes as many authors believe? Or is creep a form of long-term, preseismic slip for a truly great earthquake?
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Active Tectonics: Studies in Geophysics FIGURE 11.13 Height changes of bench marks across San Jacinto Fault near Anza, California, 1981–1983. Bench mark 1 is arbitrarily held fixed. Preseismic Slip Leveling evidence for vertical preseismic slip is generally sparse and ambiguous (Reilinger and Brown, 1981), but some fundamental questions arise. Does preseismic slip occur only on faults that are-also characterized by creep? Does discrete vertical preseismic slip occur, or are preseismic movements typified only by bending, warping, or tilt? What is the temporal relation of onset and duration of creep to the impending main earthquake? Afterslip Is afterslip truly aseismic, or is it caused incrementally by aftershocks? Continuously recording creepmeters may study this question, but the full range of the movement zone generally needs to be covered geodetically. Is significant afterslip restricted to strike-slip earthquakes? Afterslip is nearly equal to coseismic slip in strike-slip earthquakes, but less than 5 percent of coseismic movement in normal and reverse fault earthquakes. Why? Just what is the mechanism for afterslip? Some authors (e.g., Rundle and Jackson, 1977; Wahr and Wyss, 1980) maintain that it is a viscoelastic relaxation phenomenon, others regard afterslip as a quasi-static relaxation of stress changes (Matsu’ura and Iwasaki, 1983), while still others (e.g., Burford, 1972) regard afterslip as time-delayed propagation from bedrock offset through overlying cover rocks. The latter view is questioned (Sylvester and Pollard, 1975; Bucknam et al., 1978), but more observations and measurements are clearly needed. Dynamically Triggered Slip 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? A related question is how can quite small slips (1 to 30 mm) occur over such long (22 km) fault segments (Fuis, 1982; Sieh, 1982)? Why isn’t dynamically triggered slip more common and widespread? Perhaps it is, but because of general preoccupation with the main fault rupture, seldom are careful investigations made of nearby faults. Certainly attention must be paid to surrounding faults following moderate and large earthquakes just to increase the base of data from the two clear examples at hand. Does dynamically triggered slip occur along segments of faults destined to be the loci of earthquakes in the near future? Thus, is dynamically triggered slip a
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Active Tectonics: Studies in Geophysics FIGURE 11.14 Site map of closed leveling array across the San Andreas Fault near Wallace Creek, Carizzo Plain, California.
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Active Tectonics: Studies in Geophysics FIGURE 11.15 Site map of closed leveling array at Pinyon Flat Geophysical Observatory near Palm Springs, California. From Sylvester (1984) with permission of the American Geophysical Union. form of precursory slip as suggested by Sieh (1982) from a limited base of observations? To date, dynamically triggered slip has been documented for only the Salton Trough of southern California. Is its identification there a function of good exposure, many postseismic investigations, and instrumental coverage? Or is it a function of the tectonics? Here, too, the data base must be increased considerably. CONCLUSIONS Near-field tectonic geodesy has been instrumental in documentation of minor fault movements that follow earthquakes together with those that are aseismic and coseismic. Although continuously recording instruments such as creepmeters, strainmeters, and tiltmeters provide important time-history data, geodetic informa-
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Active Tectonics: Studies in Geophysics tion is required to know where to establish instruments in the first place, to provide a spatial range of observations intermediate between broad-range geodesy and the instruments, and to maintain a monitoring capability over periods of time far longer than are feasible or economic for continuous instrumental coverage. Owing to the high degree of precision required for near-field measurements, careful attention must be paid to the kind and method of emplacement of bench marks. They must be made to have a half-life of at least 50 yr and should have secular movements of no more than about 0.25 mm. In most cases, investigations seem to have required about 10 yr from the time of establishment of a given geodetic array to first publication of significant results. Such a length of time is probably necessary to quantify the noise—including bench-mark motions, survey errors, and Earth noise in the particular arrays, methods, and part of the world. Thus near-field geodetic studies are necessarily, or at least typically, long term, and agencies and institutions that provide initial support must realize the nature of the long-term commitment here to investigate a geologic phenomenon on a human time scale. It cannot be guaranteed that the Earth will cooperate and yield significant results in the course of a grant or contract period or soon enough to provide grist for a series of publications that will assist in promotion for a young assistant professor. The Earth moves, often slowly and inexorably. To study these movements carefully requires time, patience, and not a little luck. To understand these movements requires long-time series of data—more than exist today. FIGURE 11.16 Diagrammatic representation of dry-tilt array. REFERENCES Allen, C.R., and S.W.Smith (1966). 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Representative terms from entire chapter: