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Active Tectonics: Studies in Geophysics 8 Investigation of Active Tectonics: Use of Surficial Earth Processes EDWARD A.KELLER University of California, Santa Barbara ABSTRACT Evaluation of landforms, soils, and deposits formed by active tectonics is providing basic data necessary for long-term earthquake prediction, seismic-hazard evaluation, and probabilistic seismic-risk assessment. Investigation of active tectonics based on geomorphic techniques varies from regional reconnaissance work to detailed, site-specific, process-response study. Geomorphic indices and landform assemblages are useful in regional evaluation to identify relative tectonic activity and sites where rates of active-tectonic processes may be evaluated. Process-response studies involve coupling of geologic and geomorphic processes with responses of the landscape. Such an approach involves study of faulted Holocene (less than 10,000-yr-old) deposits; faulted landforms such as offset streams, alluvial fans, marine terraces, river terraces, and glacial moraines; and change in fault-scarp morphology with time. Rates of active-tectonic processes may be calculated from geomorphic evaluation, provided deformation of a specific landscape feature is measured and chronology of the deformed feature is established. It is usually easier to identify and measure deformation of features than establish chronology, and even if rates of tectonic deformation can be established, evaluation of their significance may be difficult because they often vary in time and space owing to geologic constraints. For examples, slip rates may vary along different segments of the same fault owing to changes in tectonic framework, and rates of uplift of terraces or other landforms may change through time as a function of mechanics of deformation. INTRODUCTION Understanding and long-term prediction of earthquakes associated with active tectonics has experienced remarkable progress in recent years through the study of near-surface processes (geomorphology). Studies of geologically young (less than 10,000-yr-old) and slightly older (less than 125,000-yr-old) landforms, soils, and deposits are providing basic data necessary for long-term earthquake prediction, seismic-hazard evaluation, and probabilistic seismic-risk assessment. A large number of faults capable of producing damaging earthquakes have been identified and evaluated to determine rates of movement and potential earthquake hazard. For a few faults, including the San Andreas Fault north of Los Angeles, California, and the Wasatch Fault near Salt Lake City, Utah, recurrence intervals of recent (prehistoric) earthquakes have been determined (Allen, 1983). Information concerning rates of movement along
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Active Tectonics: Studies in Geophysics faults, potential earthquake hazard, and recurrence of large earthquakes obtained from quantitative field studies in geomorphology is extremely useful in long-term (tens to hundreds of years) land-use planning, a goal of which is earthquake-hazard reduction. Specifically, studies of active faults are providing critical information necessary for residential and commercial zoning near active faults; establishing building codes; and planning for large dams, nuclear power plants, liquified natural gas facilities, and other critical facilities. Geomorphic evaluation of active tectonics has taken two approaches depending on whether reconnaissance information or detailed evaluation is desired. Reconnaissance work to identify areas where active tectonics is particularly significant generally involves the use of geomorphic indices (sensitive to rock resistance, climatic change, or tectonic processes) or assemblages of landforms produced or modified by active-tectonic processes. Detailed, site-specific study of active tectonics often involves evaluation of process-response models that attempt to explore relations between landforms, earth materials, geomorphic processes, and active tectonics integrated through time. The concept of time or chronology is introduced here because without establishment of a reliable chronology, process-response models will not yield rates of faulting and recurrence intervals of damaging earthquakes that are necessary in evaluating seismic risk. Much of the remainder of this paper will emphasize these points: use of geomorphic indices in reconnaissance studies of active tectonism; landform assemblages as indicators of active tectonism; and use of process-response models in establishing relations between landforms, earth materials, geomorphic processes, and tectonic processes for devising rates of active tectonics. Figure 8.1 summarizes the two main ap FIGURE 8.1 Active tectonics and geomorphology: data input, output, and use to society. proaches to studying geomorphologic indicators of active tectonics and use to society. GEOMORPHIC INDICES AND ACTIVE TECTONICS Geomorphic indices are useful tools in evaluating active tectonics because they quickly provide insight concerning specific areas or sites in a region that is adjusting to relatively rapid rates of active-tectonic deformation. Indices that have been most successful are related to erosional and depositional processes associated with fluvial (river) systems. The best known of these are the stream-gradient index (SL index) developed by Hack (1973), the mountain-front sinuosity (Smf index) developed by Bull (1977a, 1978), and the ratio of valley-floor width to valley height (Vf index) also developed by Bull (1977a, 1978). Stream-Gradient Index The stream-gradient index (Hack, 1973), later applied to the San Gabriel Mountains in southern California by Keller (1977), is defined as (8.1) where SL is the stream-gradient index, ΔH/ΔL is the local gradient of the stream reach where the index is computed (ΔH is the drop in elevation of the reach and ΔL is the length of the reach), and L is the total channel length from the drainage divide to the center of the reach, measured along the channel. The SL index is crudely related to the available stream power, defined as the product of water discharge and water-surface slope, and thus reflects the ability of the stream to transport its load. The index is a surrogate for stream power because the upstream channel length is proportional to bankfull discharge and the slope of the water surface is approximated by the slope of the channel bed. The stream-gradient index is particularly sensitive to changes in slope and thus is a valuable tool in evaluating active tectonics with a strong vertical component of deformation. However, the index is also sensitive to rock resistance (resistant rock produces a steep channel slope), and differentiating between effects of tectonics and rock resistance may be difficult. That is, values of the index are high in areas where the rocks are particularly resistant or where active tectonics has resulted in vertical deformation at the Earth’s surface. Therefore, anomalously high SL indices in rocks of low or uniform resistance is a possible indicator of active tectonics. Figure 8.2 shows stream-gradient indices for the San Ga-
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Active Tectonics: Studies in Geophysics FIGURE 8.2 Stream gradient (SL) indices for the San Gabriel Mountains, California. See text for explanation. briel Mountains in southern California. Areas of anomalously high indices are located along the southern and eastern fronts of the range. Although it was previously known that rates of uplift were relatively high in these areas, the indices verified this and also delineated an area of unusually high indices near the location of the 1971 San Fernando earthquake. Thus a regional evaluation of the San Gabriel Mountains suggests that detailed studies along the southern and eastern fronts of the range as well as near San Fernando have the best chance of yielding rates of vertical tectonics (uplift), slip rates along active faults, and recurrence intervals of damaging earthquakes. Areas where stream-gradient indices are relatively low are associated with two general conditions: areas where soft sedimentary rocks are abundant and along major strike-slip faults (the San Andreas and San Gabriel Faults) where horizontal movement has crushed the rocks producing zones low in resistance to erosion. The SL index over a region can be computed from small-scale topographic maps. The index could also be computed from analyses of elevational data stored in computer systems. Therefore, in theory, large regions may be evaluated quickly, although interpretation of the index will remain crude because it may be difficult to separate effects of rock resistance from active tectonics. Nevertheless, the SL index is a valuable reconnaissance tool useful in isolating smaller areas for detailed work. Mountain-Front Sinuosity Mountain-front sinuosity (Smf) is defined as (8.2) where Lmf is the length of mountain front along the mountain-piedmont (foot of mountain) junction and Ls is the straight-line length of the front. The Smf index reflects a balance between the tendency of streams and slope processes to produce an irregular (sinuous) mountain front and vertical active tectonics that tends to produce a prominent straight front (Bull and McFadden, 1977). Thus, mountain fronts associated with active uplift are relatively straight, but if the rate of uplift is reduced or ceases, erosional processes will begin to form a sinuous front that becomes more irregular with time. Mountain-front sinuosity was used by Bull and McFadden (1977) to evaluate the marked contrast in tectonic activity north and south of the Garlock Fault in California. North of the fault the values of Smf are low, suggesting active tectonics, whereas south of the fault the Smf values suggest relative tectonic stability. Rockwell and Keller (in press) used mountain-front sinuosity in the Ventura Basin, southern California, where Smf values vary from about 1 to 3. Low sinuosity (1.01 to 1.14), characteristic of technically active fronts, can be maintained in the Ventura area if a threshold rate of uplift greater than 0.4 mm/yr is maintained. Mountain-front sinuosity, like the stream-gradient
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Active Tectonics: Studies in Geophysics index, is a valuable reconnaissance tool when evaluating effects of active vertical tectonics. The Smf index is particularly attractive because it can be quickly and easily measured from aerial photographs, satellite or other high-altitude imagery, or topographic maps. Ratio of Valley-Floor Width to Valley Height The ratio of the width of valley floor to valley height Vf may be expressed by (8.3) where Vfw is the width of valley floor, Eld and Erd are the respective elevations of the left and right valley divides, and Esc is the elevation of the valley floor (Bull and McFadden, 1977). In determining Vf, the data are measured at a given distance up from the mountain front. The index reflects differences between broad-floored canyons with relatively high values of Vf and V-shaped canyons with relatively low values. Comparison of Vf values measured from valleys emerging from different mountain fronts or different parts of the same front provides an indication of whether the streams are actively downcutting (forming V-shaped valleys with low Vf) in response to active tectonics or are being eroded laterally (forming broad valleys with high Vf) in response to relative stability of the front. The Vf index was tested by Bull and McFadden (1977) for mountain fronts north and south of the Garlock Fault. They found that values of the index varied from 0.05 to 4.7, with the lower values being derived from valleys north of the fault where mountain fronts are tectonically active. The index was also tested by Rockwell and Keller (in press) for mountain fronts near Ventura, California, where Vf ratios show similar trends to that established by Bull and McFadden—being lower for relatively active fronts than for fronts with lesser rates of uplift. TECTONIC GEOMORPHOLOGY AND LANDFORM ASSEMBLAGE A genetic classification of landforms is possible because different geomorphic processes tend to produce a characteristic assemblage of landforms. For example, the discussion of geomorphic indices suggested that active vertical tectonics tends to produce straight mountain fronts with V-shaped canyons and streams with relatively steep gradients for a particular rock type. On a more local scale, as for example along a specific mountain front or fault zone, active tectonics often modifies or produces characteristic landform assemblages. For example, alluvial fans have a variable morphology somewhat dependent on tectonic processes, and active strike-slip faulting produces a specific set of tectonic landforms. A tacit assumption in the evaluation of landform assemblages produced by active tectonics is that the more pristine or fresh appearing the landforms are, the younger the tectonics is assumed to be. Discussion of alluvial-fan morphology and tectonic activity as well as the assemblage of landforms associated with strike-slip faulting will illustrate the above concepts. Alluvial Fans An alluvial fan is the end point of an erosional-depositional system in which sediment eroded from a mountain source is transported to the mountain front. There it is deposited as a cone or fan-shaped body of fluvial and/or debris-flow deposits (Bull, 1977b). The stream is the connecting link between the erosional and depositional parts of the system (Bull, 1977b) and therefore has a significant influence on the morphology of the alluvial fan. Radial profiles for most fans are composed of several segments, which together are gently concave. Breaks in slope mark boundaries between segments, and younger segments may be identified from older ones based on relative soil profile development, weathering of alluvial clasts, dissection of the surface by small streams, and development of desert varnish [see Bull (1964, 1977b) for a more detailed discussion of segmented alluvial fans]. Alluvial-fan morphology is an indicator of active tectonics because the fan form reflects varying rates of tectonic processes such as uplift of the source mountain along a range-bounding fault or tilting of the fan surface. When the rate of uplift of the mountain front is high relative to rate of stream-channel downcutting in the mountain and to fan deposition, then fanhead deposition tends to occur, and the youngest fan segment is near the apex of the fan. If the rate of uplift of the mountain front is less than or equal to the rate of downcutting of the stream in the mountain, then fanhead trenching occurs and deposition is shifted downfan. Younger fan segments will then be found well away from the mountain front (Figure 8.3 shows these two conditions). Change in sediment or water yield may also cause fanhead trenching, but this tends to be temporary if mountain-front uplift persists (Bull, 1964). The above model of segmented alluvial fans relative to active tectonics has been successfully tested for alluvial fans in Death Valley, California (Hooke, 1972). Hooke found that eastward tilting and normal faulting produced segmented alluvial fans. On the east side of
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Active Tectonics: Studies in Geophysics FIGURE 8.3 Alluvial fan morphology: (A) deposition adjacent to mountain front and (B) deposition shifted downfan as a result of fanhead entrenchment. See text for further explanation. From Bull (1977b). the valley alluvial fans are relatively small and steep, and active normal faulting produces a straight mountain front with young fan segments being deposited on fanhead areas. On the west side of the valley alluvial fans are larger, not so steep, and not so influenced by mountain-front uplift. Lesser uplift and eastward tilting of the fans has shifted to the locus of fan deposition downfan; fanhead trenching occurs, and the younger fan segments are located well away from the fan apex. A similar tendency was noticed for alluvial fans in the Ventura, California, area that are being tilted basinward by active tectonics (Rockwell and Keller, in press). As with the geomorphic indices, the study of alluvial fans provides reconnaissance information concerning relative rates of active tectonics. Landform Assemblage: Strike-Slip Faulting Active strike-slip faulting produces a characteristic assemblage of landforms including linear valleys, offset or deflected streams, shutter ridges, sags, pressure ridges, benches, scarps, and small horst and grabens known as microtopography (see Figure 8.4, opposite page). Figure 8.5 shows fault-related landforms associated with an offset alluvial fan located along the San Andreas Fault in the Indio Hills of southern California. The fan is offset about 700 m, and making an assumption concerning the age of the fan from soils (Keller et al., 1982a) the slip rate for this part of the San Andreas Fault is at least 1 cm/yr and probably closer to 3 cm/yr. Small streams at the upper part of the fan are offset several meters, suggesting that tectonic creep or moderate to large earthquakes have occurred in the past few thousand years. There have been no large earthquakes along the southern part of the fault in historical time (several hundred years), but the geomorphology in the fault FIGURE 8.5 Sketch map of offset alluvial fan along the San Andreas Fault in the Indio Hills, California. Modified from Keller et al. (1982a).
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Active Tectonics: Studies in Geophysics FIGURE 8.4 Assemblage of landforms associated with active strike-slip faulting. Modified from Wesson et al. (1975). zone suggests that earthquakes are likely to be generated there in the future. Active strike-slip faulting in the offshore southern California continental borderland also has distinctive geomorphic expression characterized by linear troughs, sags, tectonic benches, fault scarps, and offset or deflected channels associated with submarine fans. A recent study of the San Clemente Fault zone (Legg and Luyendyk, 1982), utilizing topographic data from Seabeam surveys, demonstrated that the data base and resolution is now sufficient to begin studying submarine-tectonic geomorphology to improve mapping of active faults and to evaluate long-term earthquake hazard. Figure 8.6 shows topography and tectonic landforms associated with the San Clemente Fault zone (M.R.Legg, University of California, San Diego, personal communication, 1983). Many of the topographic features associated with active strike-slip faulting such as sags, pressure ridges, and fault scarps can be explained by simple shear that produces contraction and extension as illustrated on Figure 8.7 (Wilcox et al., 1973; Sylvester and Smith, 1976). Others are better explained by extension or contraction associated with releasing or constraining bends or steps of fault traces as illustrated on Figure 8.8 (Crowell, 1974; Dibblee, 1977). PROCESS-RESPONSE MODELS: RATES OF ACTIVE TECTONICS Process-response models in active tectonics are broadly defined to include the investigation of earth ma- FIGURE 8.6 Sketch map of part of the San Clemente Fault zone. Data are from Seabeam survey. Courtesy of Mark R.Legg, University of California, San Diego.
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Active Tectonics: Studies in Geophysics FIGURE 8.7 Simple shear associated with strike-slip faulting produces preferred orientation of fractures, faults and folds (A) as well as extensional and contractional landforms (B). Figure modified from Wilcox et al. (1973), Sylvester and Smith (1976), and Keller et al. (1982a). terials, landforms, and late Pleistocene-Holocene chronology, the purpose of which is to derive rates of active tectonics (slip rates on faults, rates of uplift or subsidence, and recurrence intervals of damaging earthquakes). Such investigations have been very successful in recent years and are providing basic data necessary for long-term (tens to hundreds of years) earthquake prediction. Examples include (1) paleoseismic construetion (occurrence and recurrence intervals of prehistoric earthquakes) obtained from studying faulted Holocene-alluvial sequences of stream, marsh, lake, or landslide deposits (Clark et al., 1972; Sieh, 1978; Davis, 1981; Rust, 1982); (2) paleoseismic construction based on fault-scarp morphology change with time (Wallace, 1977; Bucknam and Anderson, 1979; Nash, Chapter 11, this volume); and (3) rates of uplift, slip rates on active faults, and/or recurrence intervals of assumed earthquakes based on chronology and offset of landforms such as alluvial fans (Keller et al., 1982a), marine terraces (Matsuda et al, 1978; Lajoie et al., 1979, 1982; Keller et al., 1982b), offset streams (Sharp, 1981; Sieh, 1981), and glacial deposits (Schubert, 1982). Fault-Scarp Morphology Slope morphology of scarps produced by faulting is a successful geomorphic indicator of active tectonics. Figure 8.9 shows generalized slope elements associated with a fault scarp. All the elements shown need not be present for a particular scarp, and because slopes are dynamic changing landforms, the dominance of one element relative to others changes with time. Table 8.1 and Figure 8.10 summarize form-process relationships for fault-scarp morphology change with time as discussed by Wallace (1977) for the Great Basin area in Nevada. Wallace was able to develop the chronology shown on Figure 8.10 and Table 8.1 by studying fault scarps that truncate 14C-dated shorelines of Pleistocene Lake Lahontan, are associated with volcanic ash of known age, were produced by known earthquakes, or can be dated by tree rings (dendrochronology). Recurrent displacement along the same fault line produces a composite fault scarp. Wallace (1977) stated that multiple displacements on a compound fault scarp FIGURE 8.8 Pressure ridges and sags associated with restraining and releasing bends and/or steps along strike-slip faults. Modified after Crowell (1974) and Dibblee (1977).
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Active Tectonics: Studies in Geophysics FIGURE 8.9 Basic slope elements that may be present on a fault scarp. After Wallace (1977). FIGURE 8.10 Change in slope elements (fault-scarp morphology) with time for fault-scarp degradation in the Great Basin area of Nevada. After Wallace (1977). can be recognized by sharp breaks in slope on the scarp, benches or terraces associated with small channels that have eroded through the fault scarp, knickpoints (short vertical or steep sections) in channels that cross the scarp, scarp height that exceeds that likely produced by a single event, and progressive displacement (older material has been displaced more than younger material). Change in fault-scarp morphology with time is being treated quantitatively. Bucknam and Anderson (1979) developed relations between scarp height and scarp-slope angle for fault scarps in Utah with estimated ages ranging from 1000 to 100,000 yr (Figure 8.11). Their studies verify Wallace’s (1977) conclusion that with TABLE 8.1 Fault Scarp-Slope Morphologya Slope Element Morphology Process (Formation and/or Modification Comments and General Chronology Crest Top of fault scarp (break in slope); initially sharp, becomes rounded with time Produced by faulting; modified by weathering, mass wasting Becomes rounded after free face disappears; usually rounded after about 10,000 yr Free Face Straight segment; initially 45° to overhanging Produced by faulting; modified by weathering, gullying, mass wasting; eventually buried from below by accumulation of debris Dominant element for 100 year or so; disappears after about 1000–2000 yr Debris Slope Straight segment; at angle of repose of material usually 30° to 38° Accumulation of material that has fallen down from the free face Is dominant element after about 100 yr, remains dominant until about 100,000 yr, disappears at about 1,000,000 yr Wash Slope Straight to gently concave segment; overlaps the debris slope; slope angle generally 3° to 15° Fluvial erosion and deposition; deposition of wedge or fan of alluvium near toe of the slope; some gullying Is developed by 100 yr, significant by 1000 yr, and dominant by 100,000 yr Toe Base of fault scarp (break) in slope) slope; may be initially sharp, but with time may become indeterminate as grades into original slope Fluvial erosion and deposition; owing to change in process/form from upslope element (free face, debris slope, or wash slope) to original surface below the fault-scarp slope More prominent in young fault scarps or where wash slope is not present; on scarps older than about 12,000 yr. the basal slope break is sharper than the crestal slope break aAfter Wallace (1977) for fault scarps in the Great Basin.
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Active Tectonics: Studies in Geophysics FIGURE 8.11 Relationship between fault-scarp height and scarp-slope angle from scarps ranging in age from 103 and 105 yr. After Bucknam and Anderson (1979). time the angle of a scarp degrades to a lower-slope angle. For example, for a 3-m-high scarp (shown on Figure 8.11), as the scarp-slope angle decreases from 28° to 10° the scarp age increases from 1000 to 100,000 yr. In theory, once fault-scarp degradation curves, such as those on Figure 8.11, are derived for an area, the investigator can assign estimated ages to fault scarps of known height and slope angle and estimate the earthquake hazard and history. However, care must be taken in making paleoseismic statements because (1) the initial scarp from a single event may vary with materials composing the scarp and local change in pattern of fault displacement, e.g., multiple, composite-overlapping rupture versus single rupture scarps (A.J.Crone, U.S. Geological Survey, personal communication); and (2) temporal and spatial variations in climate may produce a variable fault-scarp morphology. Faulted Holocene Deposits Erosional and depositional processes produce stream, marsh, lake, and landslide deposits that, when faulted, may produce valuable information concerning slip rates, rates of uplift or subsidence, and paleoseismicity. Two examples from the San Andreas Fault system in southern California are the Coyote Creek Fault—part of the San Jacinto Fault zone (a major branch of the San Andreas Fault)—and the San Andreas Fault at Pallett Creek (Sieh, 1978). Holocene paleoseismicity on the Coyote Creek Fault was determined by evaluating progressive vertical displacement of lake deposits. Clark et al. (1972) and Sharp (1981) estimated that the recurrence interval for earthquakes similar to the April 9, 1968, Borrego Mountain earthquake (M=6.7) varies from 50 to several hundred years. The slip rate for the fault is also variable, being 1 to 5 mm/yr for Holocene (less than 10,000 yr) offsets and as high as 8 to 12 mm/yr for a mid-Pleistocene (about 700,000 yr) offset. Geomorphic investigation of the San Andreas Fault north of Los Angeles has helped answer an important question for understanding the earthquake hazard—how often do large earthquakes occur? Data from Pallett Creek, 55 km northeast of Los Angeles (Sieh, 1978) and two other sites up to 125 km northwest of Pallett Creek (Davis, 1981; Rust, 1982) suggest that three large prehistoric earthquakes since the sixteenth century may be correlated over a long (125 km) segment of the fault. Sieh (1978) believed that evidence from faulted peat deposits at Pallett Creek (dated by 14C) suggest that there may have been 12 large earthquakes in the past 1700 yr. One of these was historical (1857) and 11 were prehistoric, suggesting an average recurrence interval of 145 yr. However, the length of time between such events may vary from as short as 50 yr to as long as 250 yr (Sieh, 1978). Using the most recent five events, which are well dated and established, the average recurrence interval is about 200 yr (K.E.Sieh, California Institute of Technology, personal communication, 1984). Paleoseismic construction of this segment of the San Andreas Fault, known as the “Big Bend,” is providing data useful in long-term earthquake prediction. Faulted Landforms Evaluation of faulted landforms (especially those with multiple displacements) including stream channels, river terraces, marine terraces, and glacial moraines is helping answer fundamental questions concerning active tectonics. Some of these questions are: (1) Are rates of faulting constant through time? (2) Which faults produce the greatest earthquake hazard? (3) Are historical rates of faulting, based on first-order leveling, verified in the recent geologic record? (4) What is the potential for seismic shaking or ground rupture at a particular site? (5) What is the likely displacement per event and recurrence interval of earthquakes for specific active faults? Study of a series of marine terraces (Matsuda et al., 1978) south of Tokyo, Japan—believed to have been produced by a series of sudden uplifts during large earthquakes—suggest that there have been four great
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Active Tectonics: Studies in Geophysics FIGURE 8.12 Faulted terraces of the Ventura River near Oak View, California. (M greater than 8) earthquakes in the last 6000 yr with an average recurrence interval of 1500 yr. The investigators also located an area where they believe a large earthquake is likely in the relatively near future. They base their long-term prediction on the fact that the area near the expected epicenter may be a seismic gap, identified by a local uplift rate that is less than the regional average during the last several thousand years. Investigation of faulted terraces in New Zealand, Japan, and the United States are yielding estimates of slip rates and recurrence intervals of potential earthquakes. A flight of seven terraces of the Waiohine River, New Zealand, have been progressively offset during the late Pleistocene and Holocene along the Wairarapa Fault (Lensen and Vella, 1971). Making assumptions concerning the chronology the investigators concluded that the horizontal slip rate for the fault is 3.4 to 6 mm/yr, and assuming a 3-m horizontal displacement per earthquake event, a recurrence interval of 500 to 900 yr is obtained. A similar study of a flight of nine terraces of the Kiso River, Japan—progressively displaced (left-lateral) by the Alter Fault—is presented by Yoshikawa et al. (1981). A 14C date of about 27,000 yr for a terrace with measured horizontal and vertical displacement of 140 and 28 m, respectively, provides a slip rate of about 5 and 1 mm/yr, respectively. Assuming a 8-m displacement per event (based on an M=8.4 earthquake in 1891 on a similar fault 60 km to the east) yields a recurrence interval of 1600 yr for a similar event on the Alter Fault. As a final example of river terraces, investigation of several late Pleistocene-Holocene terraces of the Ventura River near Oak View, California (Keller et al., 1982b), displaced by flexural-slip faulting (Figure 8.12), yields slip rates that vary from about 0.3 to 1.1 mm/yr. The important aspect of the study was the recognition that the faults produce a ground-rupture hazard rather than seismic-shaking hazard (Yeats et al., 1981; Yeats, Chapter 4, this volume). Assuming a slip event of 25 cm (similar to a flexural-slip event near Lompoc, California, in 1981 that produced a 570-m rupture surface and an M=2.5 earthquake (Yerkes et al., 1981), the recurrence interval would vary from 250 to 750 yr. Offset stream channels and glacial moraines are also yielding slip rates for active faults. Sieh (1981) and Sieh and Jahns (1984) estimated from offset stream channels of Wallace Creek along the south-central part of the San Andreas Fault (Figure 8.13) that the slip rate during the latest Pleistocene and Holocene has been about 30 to 40 mm/yr. Finally, Schubert (1982) reported an estimated range of slip rate (3 to 14 mm/yr) for the right-lateral Bocono Fault in western Venezuela. His estimate was based on several measured offsets (60 to 260 m) of FIGURE 8.13 Channel offset along the San Andreas Fault. See text for explanation. After Sieh (1981).
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Active Tectonics: Studies in Geophysics TABLE 8.2 Uplift and Tilting of Bedrock and River Terraces over the Ventura Avenue Anticline Geomorphic Surface Present Height above Ventura River (m)c Potential Range Uplift Rate to Present (mm/yr)d Minimum Possible Uplift Rate to Present (mm/yr)e Designation Age (yr)a Sea Level (m)b Q5a 15,800±210 −110 30.5±10 5.5±4.2 1.9±0.7 Q5a-b 20,040±590 −90 85.3±10 6.6±2.9 4.3±0.5 Q5b 29,700±1250 −41 120±10 4.8±1.2 4.1±0.5 Q6a 38,000±1500 −38 175±10 5.1±1.0 4.7±0.4 Q6c 80,000 or 105,000 −13 625±100 7.1±2.1 7.1±2.0 Bedrock 200,000 Present 2720±200 13.6±1.0 (minimum) 13.6±1.0 aBased on 14C and amino acid racemization chronology (Lajoie et al., 1982). bAfter Lajoie et al. (1979). cProjected to the axis of the anticline. dAssumes complete range of possible adjustment of Ventura River to lower sea level. eAssumes no adjustment of Ventura River to lower sea level. lateral moraines with an estimated age of 18,000 yr based on palynological, sedimentological, and 14C methods. RATES, DATES, AND TECTONIC FRAMEWORK: SELECTED OBSERVATIONS RELATIVE TO SOCIETAL NEEDS Tectonic geomorphology, defined as the application of geomorphic principles to tectonic problems, is significant to society when rates and dates of tectonic events provide the data framework useful in long-term earthquake prediction and land-use planning to reduce the earthquake hazard. Important considerations in developing rates of active tectonics based on geomorphic evaluation are measurement of deformation associated with past tectonic events (such as offset streams, glacial moraines, alluvial deposits, or other features), development of the late Pleistocene to Holocene chronology to derive rates (defined as the ratio of measured deformation to the appropriate time interval), and interpretation of rates of active tectonics that often vary significantly owing to geologic constraints. The most reliable rates of past tectonic events are derived from well-constrained, measured deformation and chronology. However, it is often easier to measure deformation than to establish the necessary chronology. Methods of establishing chronology are discussed by Pierce (Chapter 13, this volume). The use of soil geomorphology in establishing late Pleistocene-Holocene chronology is emerging as a powerful tool in deriving rates of active tectonic deformation (see, e.g., Keller et al., 1982a,b; Dembroff, 1982; Rockwell, 1983; Rockwell et al., 1984). The basic idea is to produce a soil chronosequence, defined as a series of soils arranged from youngest to oldest for an area. Relative chronology is based on physical and chemical properties of the soils, and absolute chronology is provided by 14C dates and other methods discussed by Pierce (Chapter 13, this volume). Once the chronosequence is established, it may be applied over a large area independent of further absolute dates at sites where deformation is measured; that advantage is the real power of the soil chronosequence. Rates of active-tectonic deformation must be carefully interpreted because they may vary in time and space owing to geologic constraints such as style of faulting and mechanics of folding. For example, slip rates of flexural-slip faulting near Oak View, California, vary from about 0.3 to 1.1 mm/yr as a function of fault location in a syncline and/or mechanics of folding (Keller et al., 1982b; Rockwell, 1983; and Rockwell et al., 1984), and rate of uplift associated with folding of the Ventura Avenue anticline has decreased from over 10 mm/yr in mid-Pleistocene time to about 5 mm/yr during the late Pleistocene and Holocene (Table 8.2), as a function of mechanics of folding (Keller et al. 1982b; Dembroff, 1982; Rockwell, 1983). Thus, there is not necessarily a direct relationship between rate of fault displacement or uplift and earthquake hazard. Understanding the tectonic framework and geologic constraints is necessary to make such a determination. ACKNOWLEDGMENTS Reviews of the manuscript and suggestions for improvement by W.B.Bull and J.G.Dennis are appreciated. REFERENCES Allen, C.R. (1983). Earthquake prediction—past and future, Geology 11, 682. Bucknam, R.C., and R.E.Anderson (1979). Estimation of fault-scarp
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Representative terms from entire chapter: