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Active Tectonics: Studies in Geophysics
active tectonic processes are not addressed; they are in the domain of other specialists and other reports.
In the earth sciences “tectonics” refers to the deformational structures and architecture of the outer parts of the Earth and to the evolution of these features through time. Examples include folds, warpings and tiltings of crustal blocks, and displacement on faults. In this study we define “active tectonics” as “tectonic movements that are expected to occur within a future time span of concern to society.”
This definition of active tectonics has evolved from the many definitions of active fault that have been used in the past. The lack of agreement on a single definition of an active fault has caused confusion and attendant engineering, social, and legal difficulties. Many of the definitions inappropriately have mixed elements including criteria for identifying faults, criteria for estimating degree of activity, and value judgments about the level of activity that constitutes acceptable risk to mankind.
The terms active faults and active tectonics imply that events are currently happening, but here the term currently is ambiguous. The present is a moving instant that progresses ever forward in time. To most people currently very likely is thought of in minutes, or at most in years, whereas to a geologist a time sample that appropriately represents currently or present might span many thousands of years. Focusing the definition of active tectonics on the future avoids selecting a single period of time to represent the present and emphasizes the prediction of future tectonic events, which has the greatest potential benefit in guiding hazard-reduction actions.
The frequency of occurrence of specific faulting events has been incorporated in some definitions of active faults. Thus, various public agencies define an active fault as having had displacements (a) in 10,000 yr, (b) in 35,000 yr, (c) in 150,000 yr, or (d) twice in 500,000 yr. Such definitions have come to carry legal significance, and the great range in time frame has caused confusion. The differences reflected in these definitions relate principally to the degree of activity and to the levels of risk that are acceptable to various agencies. A far less confusing approach is to simplify the definition itself, to describe the rates of processes separately from the processes themselves, and to judge what risk is acceptable to society separately from the description of processes and their rates.
Geologists, geophysicists, and geodesists can identify the geologic structures and can describe and evaluate the degree of activity and the patterns of activity. It is the role of engineers to try to accommodate and minimize the deleterious effects of tectonic activity; policymakers must decide whether the rates of processes and engineering accommodations of those processes result in an acceptable situation or level of risk.
Tectonic deformation may occur as broad warping of the Earth’s surface, termed epeirogeny, to produce or reshape the larger features of continents and ocean basins, or it may be orogenic, that is, in more localized regions and belts to form mountain chains. The vertical movements of epeirogeny may result in plateaus and basins, whereas the more complex deformational processes of orogeny, or mountain building, include, for example, folding, faulting, plastic deformation, plutonism, and volcanism.
The styles and rates of tectonic processes range widely; the most active and complex tend to occur along the margins of lithospheric plates. The continental margin of western North America, where the Pacific, North American, and Juan de Fuca-Gorda plates join, exemplifies the complexity that can exist at such boundaries. Strike slip predominates where the Pacific and North American plates are in contact in Califor-