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Active Tectonics: Studies in Geophysics
of Reclamation, the U.S. Geological Survey, Woodward-Clyde Consultants, and five additional independent consultants to the Bureau of Reclamation. Estimates of the magnitude of the maximum earthquake on a fault in the vicinity of the dam ranged from 6.0 to 7.0; the closest approach of the source of the maximum earthquake ranged from less than 0.8 to 8 km; estimates of the focal depth of the maximum event varied from 5 to 10 km; the amount of the surface displacement expected during the maximum event varied from 25 cm to 3 m; and estimates of the recurrence interval of the maximum earthquake ranged from 10,000 to 85,000 yr. Characteristics of expectable faulting within the dam foundation similarly had a wide range of estimated values: the maximum earthquake was 5.0 to 7.0; displacement per event was less than 2.5 cm to 1 m; and the recurrence interval of an event in the foundation was 260,000 to about 1,000,000 yr. This clearly illustrates the differences in perception among the various consultants or groups regarding both the physical basis for quantifying a particular fault parameter and the general understanding of fault behavior.
During the past 10 yr the integration of geologic, seismologic, and geophysical information has led to a much better, though still far from complete, understanding of the relationships between faults and earthquakes in space and time. Geologic studies, especially a few highly focused fault-specific studies, have shown that individual past large-magnitude earthquakes can be recognized in the geologic record and that the timing between events can be measured. Such investigations of prehistoric earthquakes have developed into a formal discipline called paleoseismology (Wallace, 1981). Additionally, they have yielded information on fault slip rate, the amount of displacement during individual events, and the elapsed time since the most recent event. These data can be used in a number of different ways and have led to the development of new approaches to quantifying seismic hazards. Specifically, they have allowed us to begin to develop models of fault zone segmentation, which can be used to evaluate both the size and potential location of future earthquakes on a fault zone, and also earthquake recurrence models, which provide information on the frequency of different size earthquakes on a fault. At the same time, significant advances have been made in developing earthquake hazard models that use probabilistic approaches. These are particularly suited to incorporating the uncertainties in seismic source characterization and our evolving understanding of the earthquake process.
In the present paper we discuss new trends in seismic hazard analysis using geologic data, with special emphasis on fault-zone segmentation and recurrence models and the way in which they provide a basis for evaluating long-term earthquake potential.
THE GEOLOGIC DATA BASE
Figure 14.1 is a schematic diagram showing the types of geologic data that can be obtained for individual faults and the applications of each to the evaluation of seismic hazards.
Slip rate is the net tectonic displacement on a fault during a measurable period of time. In recent years a great deal of emphasis has been placed on obtaining sliprate data, and published rates are available for many faults. Slip rates are an expression of the long term, or average, activity of a fault. In a general way, they can be used as an index to compare the relative activity of faults. Slip rates are not necessarily a direct expression of earthquake potential. Although faults with high slip rates generally generate large-magnitude earthquakes, those with low slip rates may do the same, but with longer periods of time between events. Slip rates reflect the rate of strain energy release on a fault, which can be expressed as seismic moment. Because of this they are now being used to estimate earthquake recurrence on individual faults, especially in probabilistic seismic hazard analyses.
FIGURE 14.1 Relationship between geologic data and aspects of seismic hazard evaluation.