shows an excellent spatial correlation with plate-tectonic movements. Most of the historical or geologically young fault ruptures are located on or near boundaries between plates and microplates.

Importance to Society

Most evaluations of active faults are conducted at or near plate boundaries, where consideration of design, siting, zoning, communication, and response to earthquake hazards is necessary for all types of major engineering structures in order to reduce potential loss of life, injury, or property damage. The seismic motion or deformation effects on facilities such as nuclear generators, dams, communication centers, and other lifelines is critical because of great potential harm to society.

The importance of earthquake evaluation in intraplate regions has been recognized recently. In the intraplate region east of the Rocky Mountains, earthquakes affect larger areas than in the western United States. Although intraplate earthquakes are more infrequent and unexpected, the impact of earthquakes on society can be greater than is generally perceived because of the large affected areas and greater population density in the eastern United States.

Difficulties in Making Evaluations

The evaluation of faults, particularly assessment of their seismic potential, is often difficult because of the following factors: poor conditions of surface exposure (concealment by bodies of water or young sediments); plastic deformation of near-surface materials; transitional or branching rupture character; detachment, décollement, or listric faulting of shallow materials; conflicting or incomplete geologic, seismologic, or geophysical observations; incomplete bases for analysis; and basic assumptions about activity or nonactivity of faults. These factors have led to smaller, shorter, more discontinuous expression of surface faulting parameters in almost one-fourth of the historical examples of surface faulting in North America.

Approach for Earthquake Evaluation

Two approaches for earthquake-hazard assessment have been used in the United States. The western United States is dominated by active-tectonic processes and many active faults. Those faults with proper orientations or connections to plate boundaries may be active and can be evaluated by methods that are discussed in this paper. Faults within this region are commonly assumed to be active unless there are contrary data.

In the central and eastern United States—in intraplate regions east of the Rocky Mountains—most faults are inactive and rarely have the potential for being sources of damaging and hazardous earthquakes. Earthquakes of these zones may not show characteristics typical of active faults, with lower magnitudes (commonly less than 5.75 or 6), and may be assumed to have random or “floating” epicenters within a province. Man’s structures may need to be designed conservatively for the largest historical earthquake of the region or province or may be designed conservatively for a higher magnitude or intensity. Most faults for such regions are commonly assumed, or may appear to be inactive, although the Meers Fault (Oklahoma) and the scarp at Reelfoot Lake (near New Madrid, Missouri) suggest that some faults of central United States may be active, and deterministic assessments should be used.

Many different disciplines are used for studying active faults. Some of these are shown in Figure 3.1 in a time perspective.

FIGURE 3.1 Time relations of different disciplines associated with active faulting. (From Kasahara, 1981.)

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