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Active Tectonics: Studies in Geophysics
processes of erosion, such as aeolian, raindrop impact, ice, and geochemical processes, interact in different climates to affect collectively the overall degradation and destruction of landforms constructed by tectonic processes.
Slopes of fault scarps or wave-cut cliffs may be modified rapidly by gravitational processes. The near-vertical free face of a few-meter-high scarp may disappear within a few hundred years, as material falls off the steep face to rest at the angle of repose in a debris slope covering the free face. The scarp later becomes modified by downslope movement of material according to a diffusion model, which operates at slower and slower rates. However in a desert climate, a meter-high fault scarp may be clearly recognizable for a hundred thousand years.
TECHNIQUES FOR EVALUATING ACTIVE TECTONICS
The entire range of geologic, geophysical, and geodetic techniques may have a bearing on evaluating active tectonics, but for studying ongoing processes some are more useful than others.
The period of past behavior of tectonic movements that is significant to predict future movements may range from days to thousands, or even millions, of years. In general, however, the predictive value of events of the past decreases with age, but the length of time analyzed must be sufficiently long to sample adequately a particular series of events, changes in rates of events, or changes in patterns of tectonics. For example, large intraplate earthquakes generally recur on a given fault at intervals of several thousands of years, and, thus, an estimate of earthquake potential based on only a century or two of recorded history may mistakenly suggest quiescence in regions that hold the most severe threat of great earthquakes. Each sample interval of time provides different insight into tectonic processes. Repeated geodetic measurements made over days or decades provide details of tectonism unidentifiable in most geologic studies, but only from the longer-term geologic record can many tectonic movements be identified and their rates determined.
In regard to active tectonics, the segment of the future of greatest concern to mankind generally is only the next few years to few decades, although for safe disposal of some radioactive waste the period is thousands or tens of thousands of years.
Events of the past 100,000 to 200,000 yr, and especially of the past 10,000 to 20,000 yr, are particularly significant as a basis for predicting future trends. Geodetic techniques can be used to identify and quantify very recent historical tectonism, and real-time geology and geophysics have already been used to provide predictions of specific future events. Some of the greatest successes have been achieved in predicting eruptions of volcanoes. The basic physical and chemical models that can be translated into geophysical, geochemical, and geologic predictions are in the process of rapid evolution, and many advances seem to be just over the horizon.
Though this emphasis is on events of late Quaternary time, we do not exclude significant evidence about tectonic history and processes obtainable from longer periods of geologic time, for example, the past few million years, or latest Neogene and Quaternary time. Neotectonics, which pertains to the tectonics of this longer period of time, is the focus of the Neotectonic Map Project under the Geological Society of America’s Decade of North American Geology (DNAG) project. For this review, neotectonics is distinct from active tectonics but is recognized as providing an important set of data.
Even the longer record cannot be ignored, because to a large extent those structures that originated in Mesozoic and Tertiary time greatly influence the patterns of Quaternary structures. Paleozoic and older structures have less influence. Stress orientations may have changed, but during any period of strain the prefractured nature of the Earth’s crust and other anisotropism affect the response of the crust. Furthermore, the tectonics of old features now inactive can cast light on those currently active.