techniques for defining the pattern of active faulting. Such studies can “paint” the three-dimensional time-space relation and show how strain is propagated. 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.
Only when an infrequent paroxysm of the Earth’s crust occurs in the form of an earthquake, volcanic eruption, or landslide is the dynamic nature of the Earth’s crust brought clearly to public attention. Largely unnoticed are slower movements of the Earth’s crust, which, nevertheless, are costly in terms of engineering countermeasures required or the constraints that are placed on land uses. The demands for safety and continuity of service required of modern-day critical facilities, such as nuclear reactors, large dams, and structures for defense, require a knowledge of the Earth’s crust well beyond the current state of geophysical and geologic information. As a result, costly mistakes have been made, and high-priority programs have been delayed or canceled. Without a doubt the future will call for more such uses in projects having even greater sophistication, greater potential hazard, and more complex interactions with the environment.
For the want of information about active tectonics, numerous mistakes and extremely costly delays or cancellations of major engineering projects have resulted. A few case histories will serve as examples.
In the early 1960s, the Pacific Gas and Electric Company began to develop a site for a nuclear reactor at Bodega Bay in northern California. The first site selected was astride the part of the San Andreas Fault that caused the great San Francisco earthquake of 1906. Soon, however, it became recognized that the fault might pose problems, and the site was moved a few kilometers west, but still near the fault zone. During the excavation of the giant pit (Figure 2) that was to contain the reactor, faults were found and concern was raised that movement on these secondary faults might rupture the reactor and cause a major disaster.
At that time, and to a major extent even today, the geologic and geophysical communities simply had no scientifically based answers to such fundamental questions as: Will the next rupture recur on the 1906 break of the San Andreas Fault, or will it occur on some other fault branch or strand within the kilometer-wide fault zone? How much displacement can be expected on branch faults and parallel faults at distances of several kilometers from the main break? How does a history of one displacement in 10,000 or 35,000 yr on a branch fault factor into a calculation of probability of a future displacement on that branch fault?
In 1969 the Bodega Bay nuclear reactor project was canceled after several years of acrimonious debate, because no one had the needed scientific information with which to assess the stability of the site. As a result millions of dollars were spent unproductively. The lack of understanding of active tectonics has caused problems in the development of similar facilities.
As an additional example, in 1979 the building of a large thin-arch dam at Auburn, California, in the foothills of the Sierra Nevada was stopped. Although much had been learned about earthquake hazards in the ensuing years since the Bodega Bay nuclear