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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