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Active Tectonics: Studies in Geophysics
FIGURE 5 Causative structures revealed. Until various geophysical techniques were brought to bear in the search for possible buried faults that could have caused the great 1811–1812 earthquakes, similar earthquakes were presumed to be possible over a vast area of the Mississippi Valley. As a result, concern was raised about major engineering projects. This map of the second vertical derivation of the total magnetic-field intensity reveals a northeast-trending magnetic feature that is interpreted as a completely masked graben. Historical seismicity has been concentrated within the graben, and thus the rift structure is presumed to be responsible for the generation or control of seismicity. [Figure from T.G.Hildenbrand, M.F.Kane, and J.D.Hendricks (1982), Magnetic basement in the upper Mississippi Embayment region—A preliminary report, in Investigations of the New Madrid, Missouri, Earthquake Region, U.S. Geol. Surv. Prof. Paper 1236-E, pp. 38–53.]
use of the land, event prediction, and disaster preparedness. 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.
In many states and countries, building codes and standards are followed to some extent, but in practice the craftsman or builder, through ignorance or neglect, too often fails to incorporate the specified design features in the final structure. Simple factors such as the lime content of mortar and the proper wetting of bricks before application of mortar in masonry construction greatly affect the strength of buildings. Special studies are required when building near active faults in California. Disaster plans for responding to potential hazards from volcanic eruptions have been prepared in California, Washington, Japan, and Italy, among others.
Successful prediction of at least a few dozen volcanic eruptions undoubtedly has reduced the hazard to life and property. Among damaging earthquakes for which claims of prediction have been made, only the Haicheng, China, earthquake of 1974 seems to be widely credited by the scientific community as having had a valid scientific basis. For the first time in the United States, a long-term earthquake prediction was reviewed and approved as scientifically valid by both the National Earthquake Prediction Evaluation Council and the California Earthquake Prediction Evaluation Council. The prediction was announced officially by the Director, U.S. Geological Survey, on April 5, 1985, and the announcement stated that “an earthquake of magnitude 5.5 to 6 is likely to occur in the Parkfield, California, area within the next several years (1985–1993)….”
Identification of the most stable tectonic blocks and prediction of the degree of stability, or lack of active tectonism, have been even more difficult than predicting tectonic events. Few methods of analysis and little understanding of the long-term processes are at hand.
The past two decades have seen a major acceleration in disaster preparedness. One example is the Southern California Earthquake Preparedness Project, which was