Active Tectonics Impact on Society (1986) / Chapter Skim
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Overview and Recommendations
Overview and Recommendations
Pages 3-19
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From page 3... ...
Thus, the evaluation of active tectonic processes is critical to many of mankind's activities, so that hazards can be minimized and structures can be sited and constructed in ways that serve their functions most effectively, economically, and safely. To fully evaluate ongoing tectonic activity and its associated hazards requires knowledge of the rates, styles, and patterns of tectonic processes.
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From page 4... ...
of earthquakes, volcanic eruptions, and landslides are to be made successfully. In summary, this study addresses tectonic processes, their rates, and methods of identifying and evaluating active tectonics by analysis of events, especially in the time frame from the present to about 500,000 yr ago.
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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 PROCESSES 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 erogenic, that is, in more localized regions and belts to form mountain chains.
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Active erogenic movements are well characterized by active faulting and active folding. Major strike-slip faults, such as the San Andreas Fault system in California, disrupt the landscape by offsetting streams, sedimentary basins, and mountain masses.
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Explosive rates are represented by such dramatic volcanic eruptions of historical times as Tambora (1815) and Krakatau (1883)
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For example, fault-generated range fronts on which displacement rates average tenths of a millimeter per year for a million or more years are characterized by low sinuosity of the topographic boundary between the range and basin and by faceted spurs. The range margin sinuosity increases with slower rates, and the pattern of faceted spurs gives way to more highly dissected and less planar landforms along range fronts.
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From page 9... ...
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.
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From page 10... ...
Stratigraphic relations along active faults may reveal a sequence of successive offsets and deposition of sediments that permit the reconstruction of the history of faulting, the recurrence intervals, and size of faulting events. Sedimentary structures such as sand blows, elastic dikes, and deformed beds related to liquefaction may reveal the history of strong ground shaking and add to the evidence of prehistoric earthquakes.
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Studies of marine terraces, river geomorphology, fault scarps, and eruptive volcanic features have demonstrated a rich and readily available source of information about active tectonics recorded in landforms. Coseismically uplifted marine terraces may provide evidence of prehistoric earthquakes.
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From page 12... ...
Additionally, digitally recorded data from broadband, wide dynamic-range seismometers have permitted greatly improved analysis of the faulting process. · Broad vertical changes in elevation that occur on an historical time scale, within what are generally thought of as stable intracontinental regions, have been found to affect dramatically the regimes of major rivers, bank stability and silting, and navigation and flood control.
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From page 13... ...
The San Andreas Fault lies under Bodega Bay in the background, and branch faults were found in the excavation. Concern over faulting events that might rupture the reactor caused cancellation of the project.
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From page 14... ...
, and Holocene displacement had been identified. As a result of correctly anticipating the location and size of the 1983 earthquake, automatic shut-off systems functioned effectively at INEL and closed down nuclear reactors, and a potentially hazardous situation was countered.
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As a result of the studies, nuclear reactors at the nearby Idaho National Engineering Laboratory were equipped with automatic shut-off systems that functioned successfully during the earthquake (photograph by R
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Engineering design and construction of structures to resist earthquake shaking, offset by faulting, and differential settlement can greatly reduce the hazard of structural collapse. Planning the use of land so that structures, especially those for dense occupancy, are not built astride active faults or in the potential paths of volcanic lava flows or lahars is prudent and effective, and yet such obvious methods of reducing hazards commonly have been ignored.
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From page 17... ...
Other National Research Council reports have already addressed many of these topics. Research Priorities · Dating Techniques: The greatest need is for data and models concerning the rates of active-tectonic processes.
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The observation of volcanic eruptions, earthquakes, or landslides while they are occurring can reveal facts about Earth processes unavailable at any other time. For example, recording of strong ground motion during an earthquake, measurement of changes in pore pressure during a landslide, or the changes in composition of gases during volcanic eruptions can be carried out only at specific critical moments.
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From page 19... ...
New, sophisticated technology, such as used in the airborne profiling of terrain (APT) system, may provide previously unobtainable data on geo detic and geophysical changes that are a direct reflection of active tectonics.
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