9
Seismological and Paleoseismological Techniques of Research in Active Tectonics

CLARENCE R.ALLEN

California Institute of Technology

ABSTRACT

Classical seismological techniques such as earthquake hypocentral locations and focal-mechanism studies continue to play an important role in the understanding of active-tectonic processes, but newer techniques such as seismic tomography and the determination of earthquake source parameters are being increasingly utilized. The most spectacular recent progress, however, seems to have been in the area of paleoseismicity and slip-rate studies, where documentation of the ages and displacements of various young geologic features has had great impact on both the understanding of contemporary tectonic processes and on seismic-hazard evaluation. Critical future research needs in seismological and paleoseismological areas include (1) improved local seismic networks, (2) implementation of new worldwide networks utilizing broadband digital recording, (3) increased numbers of strong-motion seismometers in earthquake-prone areas, (4) better understanding of soil development and deformation, (5) improved techniques for absolute age dating of alluvial materials, (6) increased understanding of the rates and nature of modification of surficial neotectonic features such as fault scarps, and (7) continued vigorous field studies of active tectonic processes associated with contemporary large earthquakes.

The study of contemporary and recent earthquakes represents perhaps the major contribution to the understanding of tectonic processes active in the world today. This is not to belittle studies of active folding and warping or of active volcanism, but significant tectonic changes occur so rapidly, dramatically, and over such wide areas during large earthquakes that they seem to represent the most rewarding laboratory for the study of active-tectonic processes. And large earthquakes are of relatively frequent occurrence on a worldwide basis, so that abundant research opportunities exist.

Many of the classical seismological techniques have been—and continue to be—so fundamental to studies of active tectonics that they hardly need discussion. Among these are (1) hypocentral locations of earthquakes, (2) earthquake focal mechanisms, (3) statistical studies of earthquake occurrences, and (4) studies of crustal structure. Indeed, many of the most important ideas of active plate-tectonic processes, such as the transform-fault and subduction-zone concepts, have stemmed directly from studies of earthquake locations and focal mechanisms (e.g., Isacks et al., 1968). Detailed studies of aftershock patterns of major earth-

California Institute of Technology, Division of Geological and Planetary Sciences Contribution No. 4077.



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Active Tectonics: Studies in Geophysics 9 Seismological and Paleoseismological Techniques of Research in Active Tectonics CLARENCE R.ALLEN California Institute of Technology ABSTRACT Classical seismological techniques such as earthquake hypocentral locations and focal-mechanism studies continue to play an important role in the understanding of active-tectonic processes, but newer techniques such as seismic tomography and the determination of earthquake source parameters are being increasingly utilized. The most spectacular recent progress, however, seems to have been in the area of paleoseismicity and slip-rate studies, where documentation of the ages and displacements of various young geologic features has had great impact on both the understanding of contemporary tectonic processes and on seismic-hazard evaluation. Critical future research needs in seismological and paleoseismological areas include (1) improved local seismic networks, (2) implementation of new worldwide networks utilizing broadband digital recording, (3) increased numbers of strong-motion seismometers in earthquake-prone areas, (4) better understanding of soil development and deformation, (5) improved techniques for absolute age dating of alluvial materials, (6) increased understanding of the rates and nature of modification of surficial neotectonic features such as fault scarps, and (7) continued vigorous field studies of active tectonic processes associated with contemporary large earthquakes. The study of contemporary and recent earthquakes represents perhaps the major contribution to the understanding of tectonic processes active in the world today. This is not to belittle studies of active folding and warping or of active volcanism, but significant tectonic changes occur so rapidly, dramatically, and over such wide areas during large earthquakes that they seem to represent the most rewarding laboratory for the study of active-tectonic processes. And large earthquakes are of relatively frequent occurrence on a worldwide basis, so that abundant research opportunities exist. Many of the classical seismological techniques have been—and continue to be—so fundamental to studies of active tectonics that they hardly need discussion. Among these are (1) hypocentral locations of earthquakes, (2) earthquake focal mechanisms, (3) statistical studies of earthquake occurrences, and (4) studies of crustal structure. Indeed, many of the most important ideas of active plate-tectonic processes, such as the transform-fault and subduction-zone concepts, have stemmed directly from studies of earthquake locations and focal mechanisms (e.g., Isacks et al., 1968). Detailed studies of aftershock patterns of major earth- California Institute of Technology, Division of Geological and Planetary Sciences Contribution No. 4077.

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Active Tectonics: Studies in Geophysics quakes have been critical in developing an understanding the fracture process, and dense seismographic coverage has allowed locations of microearthquakes, sometimes to within a few meters, which in turn permits portrayal of minute details of fault geometry—a geometry that generally turns out to be far more complex than we had ever imagined (e.g., Johnson and Hill, 1982; Reasenberg and Ellsworth, 1982). But certainly many of the exciting new tectonic implications are coming from those types of relatively new seismological techniques that shed light on the nature and mechanics of the fracture process. An example is the now-widespread use of seismic moment and moment magnitude (e.g., Hanks and Kanamori, 1979), which have a direct tie-in to physical parameters at the earthquake source, such as stress drop, amount of slip, and area of the broken fault surface. The use of such concepts is now widespread in regional syntheses of active-tectonic processes (e.g., Wesnousky et al., 1982; Molnar and Deng, 1984). Still more recent is the introduction of tomographic techniques to seismology (e.g., Anderson and Dziewonski, 1984), in which vast amounts of seismic data are synthesized to reveal heretofore unknown details of three-dimensional crustal structure, which may be very relevant to ongoing tectonic processes (e.g., Humphreys et al., 1984). Increased use of such techniques, together with new broadband and digitally recording seismic instruments (e.g., Alexander, 1983) and dramatically improved data-analysis techniques, is literally revolutionizing the field of seismology. And strong-motion seismology, traditionally visualized as being within the exclusive area of earthquake engineering, is having a rebirth as an interdisciplinary field with surprisingly wide impact in our efforts to understand active tectonic processes close to the center of an earthquake—in the so-called “near field” (e.g., Hanks and McGuire, 1981; Aki, 1982; Hartzell and Helmberger, 1982). One of the most significant results of recent seismological and geologic studies of contemporary earthquakes is the determination that they are far more different from one to another in their mechanical parameters than we had ever thought. Although this is not particularly good news to those scientists attempting to find methods to predict earthquakes, it surely means that we are gaining a far better understanding of the varied and complex nature of contemporary tectonic processes. We now recognize, for example, that earthquake rupture and associated deformation take place at widely varying rates and that the rupture process, particularly during large earthquakes, is by no means smoothly continuous (e.g., Aki, 1979; Hartzell and Heaton, 1983). In the author’s opinion, however, the most spectacular progress in studies of active-tectonic processes in the past few years has not been in seismology, but instead in the area of paleoseismology, where, in essence, a new research field has been born. Paleoseismology is the study of prehistoric earthquakes based on interpretation of the geologic record that these earthquakes have left behind (e.g., Wallace, 1981). Critical in developing this field have been (1) the recognition that “fossil earthquakes” do indeed leave telltale signs in the geologic column and (2) improved techniques for the absolute age dating of the affected rocks. Thus, it is not now uncommon to identify the specific dates of major earthquakes along a fault over the past few thousands of years, permitting a far better quantitative understanding of the local earthquake hazard than has ever been possible before, albeit on a probabilistic basis (e.g., Tanna Fault Trenching Research Group, 1983; Sieh, 1984). Along with developments in paleoseismology, major advances in our understanding of slip rates on faults have also occurred. Although both fields involve the establishment of time intervals during which tectonic events have taken place, it is important to recognize the distinction: paleoseismology involves the establishment of dates of individual earthquakes or earthquake sequences, whereas slip-rate studies establish only average rates of deformation. Further assumptions in both are necessary to estimate seismic hazard (e.g., Wesnousky et al., 1984; Youngs and Coppersmith, 1985). Both are important in the understanding of active-tectonic processes. Slip-rate determination on faults are usually made by observing the offset of features of relatively recent and known ages. Thus the slip rate of the San Andreas Fault of California has been determined by observing the offsets of numerous geologic features such as Holocene stream channels (e.g., Wallace, 1968; Sieh and Jahns, 1984). Offsets of late Pleistocene glacial moraines have permitted the assignment of slip rates to the Bocono Fault of Venezuela (Schubert, 1982), the Tuco Fault of Peru (Yonekura et al., 1979), and the Fairweather Fault of Alaska (Plafker et al., 1978). Offset and deformed river terrace deposits have been used extensively along the Alpine Fault system of New Zealand (e.g., Lensen and Vella, 1971; Adams, 1980), the Median Tectonic Line of Japan (e.g., Okada, 1980), and faults of the Transverse Ranges in California (e.g., Rockwell et al., 1984). Other youthful geologic features that are often offset and can sometimes be dated include soils (e.g., Machette, 1978; Borchardt et al., 1980; Schlemon, 1985), young volcanic rocks (e.g., Roquemore, 1980), offset beach deposits (e.g., Carver, 1970), and offset

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Active Tectonics: Studies in Geophysics landslide deposits (e.g., Sieh, 1978a). Rates of erosional degradation of fault scarps have been used to put limits on slip rates (e.g., Wallace, 1977; Bucknam and Anderson, 1979), and theoretical studies of this phenomenon appear particularly promising (e.g., Nash, 1980; Colman and Watson, 1983; Hanks et al., 1984). Estimated rates of river entrenchment associated with regional uplift have also been used to put limits on slip rates on individual faults within the uplifted area (e.g., Allen et al., 1984). Other examples of slip-rate determinations and recurrence intervals between major earthquakes have been summarized by Sieh (1981). As opposed to slip-rate determinations, paleoseismological techniques must utilize geologic features associated with individual past earthquakes, which is a task that usually constitutes a greater challenge to the geologist. Furthermore, good exposures are almost always critical, which typically implies excavating trenches across the fault under investigation. Among the features that have been used to identify individual paleo-earthquakes from exposures on trench walls are the following: Identification of a fault that can be shown to break older strata but which is, in turn, erosionally truncated and buried by unbroken younger strata that had not yet been deposited at the time of the earthquake (Figure 9.1a ), thus bracketing the time interval within which the earthquake must have occurred (e.g., Clark, et al., 1972; Sieh, 1978b). Identification of buried sand-blow deposits or injected sand dikes resulting from soil liquefaction during heavy shaking (Figure 9.1b), usually close to or along the causative fault (e.g., Sieh, 1978b). Such deposits appear to be the only remaining near-surface evidence of the two great historical earthquakes in the eastern United States—the 1811–1812 events near New Madrid, Missouri (Russ, 1979) and the 1886 earthquake at Charleston, South Carolina (Talwani and Cox, 1985). These localities are particularly important to understand, inasmuch as similar deposits elsewhere may be the only surficial geologic clue to eastern U.S. paleoseismicity—and therefore to regional seismic hazard evaluation. Closely related to liquefaction is the phenomenon of intense “rumpling” of newly deposited water-laid sediments (Figure 9.1c), associated with heavy localized shaking, which has also been used as an indication of paleoseismicity (e.g., Sims, 1975; Reches and Hoexter, 1981). Identification of a fault scarp that was subsequently buried by younger unbroken deposits (Figure 9.1d) (e.g., Sieh, 1978b). Closely related to 4, identification of a buried FIGURE 9.1 Sketch diagrams of cross sections of geologic relations that might result from individual paleo-earthquakes. See text for explanation. landslide feature or a colluvial apron derived from an eroding fault scarp (Figure 9.1e) (e.g., Swan et al., 1980). Identification of a crevice associated with surficial fault movement that was later filled in by surficial materials (Figure 9.1f) (e.g., Allen et al., 1984). Although each of the above techniques applies ideally to a single paleo-earthquake, as illustrated by the examples in Figure 9.1, a number of repeated earthquakes are likely to be represented in a given exposure, so that the resulting geologic relationships can become exceedingly complex; relationships resulting from one earthquake are modified by subsequent earthquakes along the same fault. Trenches only 5 m deep across the San Andreas Fault in southern California, for example, reveal evidence of 12 individual great earthquakes on the fault within the past 2000 years (Sieh, 1978b, 1984). One might ask how exposures of fault offsets on vertical trench walls can display evidence of displacements along faults such as the San Andreas that have had predominantly horizontal displacements. It turns out that

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Active Tectonics: Studies in Geophysics even strike-slip faults usually have small components of vertical displacements, which tend faithfully to repeat themselves from earthquake to earthquake at a given locality (Allen, 1981). Thus vertical trench walls normally display consistent offsets of strata even along predominantly strike-slip faults. Evidence for the actual amount of strike-slip displacement during individual paleo-earthquakes can sometimes be obtained from horizontal excavations that reveal stream paleochannels or other offset linear features within the displaced strata (e.g., Sieh, 1984). In all these paleoseismological techniques, optimal bracketing of the time of the earthquake requires dating of (1) the oldest unbroken postearthquake strata and (2) the youngest deformed pre-earthquake strata. Unfortunately, the probability is small of this being practical in any individual exposure. That is, the chances are slim of finding a locality where one of these unique geologic situations can be observed and where the adjacent rocks can be radiometrically or otherwise dated. Thus, it is not surprising that many, if not most, trenches excavated for paleoseismological studies have turned out to be inconclusive. But those that have been successful, such as along the San Andreas Fault of California (e.g., Sieh, 1984), the Wasatch Fault of Utah (e.g., Swan et al., 1980), and the Tanna Fault of Japan (Tanna Fault Trenching Research Group, 1983) have had profound implications in terms of seismic-hazard evaluation and the understanding of active-tectonic processes. In commencing a paleoseismological investigation, therefore, one must be aware that the chances of immediate success are not high, and numerous trenches and considerable perseverance are usually called for. It should also be pointed out that many practical difficulties face one attempting to excavate trenches across faults, such as the problems of shallow groundwater, absence of visible stratigraphy, property ownership complications, legally mandated safety precautions, access for equipment, and cost. In addition to geologic relations that might be observed in excavated trench walls, several other types of geologic relation can be related to individual paleo-earthquakes. Along a strike-slip fault, for example, if abandoned offset stream channels are spaced periodically with respect to their former headwaters (e.g., three abandoned channels laterally offset 10, 20, and 30 m from their former source across the fault) (Figure 9.2), one might conclude that each progressive offset was caused by an individual earthquake with 10 m of displacement, and dating of related alluvium or terrace deposits might permit age assignments to the individual earthquakes (e.g., Sieh and Jahns, 1984). Or in the case of raised marine wave-cut benches, arguments can often FIGURE 9.2 Sketch of map of offset stream channels that might result from repeated strike-slip displacements during individual paleo-earthquakes. be made that individual benches are related to abrupt uplifts during individual earthquakes, such as has been well documented from the long historical records in Japan (e.g., Matsuda et al., 1978) and has been important in quantifying seismic hazard in Alaska (e.g., Plafker and Rubin, 1978). In most paleoseismological investigations, it is necessarily assumed that surficial fault displacement and/or heavy shaking has been limited to infrequent large earthquakes and that moderate-sized earthquakes or continuous fault slippage have not occurred during most of the time interval between the large events. The justification for this assumption lies in the increasing evidence that a given fault at a given locality is in fact typified by a “characteristic earthquake,” so that earthquakes of comparable magnitudes tend to repeat one another faithfully and periodically (Schwartz and Coppersmith, 1984). Although relatively continuous fault slippage (or fault “creep”) is common during aftershock sequences, continuous fault creep on a long-term basis has been identified only along limited parts of the San Andreas Fault in California (Schulz et al., 1982; Louie et al., 1985) and at one locality along the North Anatolian Fault of Turkey (Aytun, 1982). It does not appear to be as widespread a phenomenon as was postulated when continuous creep was first discovered on the San Andreas Fault only 25 years ago (Steinbrugge et al., 1960), which is encouraging from the point of view of paleoseismological investigations. What are our seismological and paleoseismological needs, if researches in these fields are to continue to move vigorously forward? In the area of seismology, a number of important needs can be pointed out.

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Active Tectonics: Studies in Geophysics Dense local seismographic networks, together with computerized data-analysis facilities, are absolutely essential if detailed studies of earthquake-related active tectonics are to be carried out in a given area. Ideally, the distance between seismometers should be roughly comparable to the average depth of earthquakes in the area if good hypocentral locations (including focal depth) are to be obtained. Modern techniques of seismic analysis, particularly as related to the understanding of fault mechanics, are increasingly dependent on improved instrumentation, such as wideband, digitally recording seismometers. Particularly for the studies of worldwide large earthquakes, which represent our best “window” to contemporary tectonic processes, it is essential that support be obtained and continued for the proposed new Global Seismographic Network (NRC Committee on Seismology, 1983; Incorporated Research Institutes for Seismology, 1984), which will effectively replace the 23-year-old World Wide Standardized Seismographic Network, which has served its purpose well but is now clearly outmoded. Every effort should be made to increase the number of strong-motion accelerographs in areas—anywhere in the world—where large earthquakes are most likely to occur (NRC Committee on Earthquake Enginering Research, 1982). Despite the many years of recording, engineers and seismologists have obtained very few records of the actual ground motion in the close vicinity of a truly great earthquake, and thus we are still deficient in our knowledge of active-tectonic processes in the “near field” of such an event. In the area of paleoseismology, perhaps our greatest need is simply for more trenches across active faults where significant results might be obtained. But it is also critical that we improve our basic understanding in several of the following areas: Soils are among the geologic features most often disturbed by faulting and earthquakes, but our knowledge of the ages of soils and their rates of development in different climatic environments leaves much to be desired. Furthermore, slow gravity-induced downhill movements of soils (soil “creep”), even on very gentle slopes, can sometimes cause numerous deformational features remarkably similar in appearance to those due to sudden earthquake movements; a better understanding of this phenomenon is important. The most common problem in paleoseismological investigations is that of dating the strata involved, particularly those of an alluvial nature that still defy most of the traditional methods of absolute age dating. Carbon-14 dating, for example, requires the collection of organic materials that are typically rare in alluvial deposits. Improved geochemical techniques of dating young materials are critical, as well as the further development of other promising techniques such as those based on paleomagnetism. Insofar as many prehistoric earthquakes have been associated with the formation of geomorphic features such as fault scarps that are still preserved on the landscape, an improved understanding of the erosional degradation of such features is important if we are to understand their ages of formation. Recent research work in quantitative geomorphology (e.g., Hanks et al., 1984) has been encouraging in this regard and deserves continued vigorous support. Detailed field studies of recent earthquakes, such as the 1980 Algerian disaster (e.g., Philip and Meghraoui, 1983), indicate that many types of surficial deformation other than primary fault scarps may occur in the epicentral area. These features, too, may become buried in the geologic section and be recognizable at a later date, so it is important to understand their origins and possible mechanisms of preservation. Only by studying contemporary earthquakes in the field in great detail will we learn to recognize what is important in identifying and interpreting “fossil” earthquakes. REFERENCES Adams, J. (1980). Paleoseismicity of the Alpine Fault seismic gap, New Zealand, Geology 8, 72–76. Aki, K. (1979). Characterization of barriers on an earthquake fault, J. Geophys. Res. 84, 6140–6148. Aki, K. (1982). Strong motion prediction using mathematical modeling techniques, Bull. Seismol. Soc. Am. 72, S29-S41. Alexander, S.S. (1983). Developments in digital seismology, Rev. Geophys. Space Phys. 21, 1132–1142. Allen, C.R. (1981). The modern San Andreas Fault, in The Geotectonic Development of California, W.G.Ernst, ed., Prentice-Hall, Englewood Cliffs, N.J., pp. 511–534. 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Active Tectonics: Studies in Geophysics Wallace, R.E. (1981) Active faults, paleoseismology, and earthquake hazards in the western United States, in Earthquake Prediction: An International Review, D.W.Simpson and T.G.Richards, ed., Maurice Ewing Ser. 4, American Geophysical Union, Washington, D.C., pp. 209–216. Wesnousky, S.G., C.H.Scholz, and K.Shimazaki (1982). Deformation of an island arc: Rates of moment release and crustal shortening in intraplate Japan determined from seismicity and Quaternary fault data, J. Geophys. Res. 87, 6829–6852. Wesnousky, S.G., C.H.Scholz, K.Shimazaki, and T.Matsuda (1984). Integration of geological and seismological data for the analysis of seismic hazard: A case study of Japan, Bull. Seismol. Soc. Am. 74, 687–708. Yonekura, N., T.Matsuda, N.Nogami, and S.Kaizuka (1979). An active fault along the western front of the Cordillera Blanca, Peru, J. Geogr. (Tokyo) 88, 1–19. Youngs, R.R., and K.J.Coppersmith (1985). Implications of fault slip rates and earthquake recurrence models to probabilistic hazard estimates, Bull. Seismol. Soc. Am. 75, 939–964.