yet possible in any region, some generalizations have emerged on fault-system behavior at different time scales.

Across periods of perhaps a million years, fault systems evolve as slip brings different geologic formations into juxtaposition, new faults become activated, and previously existing faults go dormant. Processes on these time scales are undoubtedly important for understanding the origins and evolution of fault-system architecture. However, for estimations of earthquake probabilities and simulations of seismic activity on shorter times scales, an assumption of fixed fault-system geometry appears to be a reasonable approximation.

On time scales of a thousand years and less, there is clear evidence that earthquake activity is not stationary in time or space. That is, some regions show episodes of high earthquake activity followed by long periods of relative inactivity. Perhaps the best known example of episodic earthquake activity on a regional scale is from the north Anatolian fault in Turkey (Figure 3.21). Similarly in China, which has a long historical record of major earthquakes, it is evident that large regions have been episodically activated for many decades followed by long interludes of low earthquake activity (22). In the United States, geologic studies in Nevada, the eastern California shear zone, and elsewhere have found evidence for periods of high seismic activity across broad regions followed by long intervals with little or no geologic evidence of faulting activity (23).

Questions relating to the repeatability and recurrence intervals of large earthquakes on shorter time scales are of particular importance for the evaluation of earthquake probabilities used in seismic hazard analysis. Current approaches to estimating earthquake probabilities assume either that earthquakes occur randomly in time, but at some fixed rate, or that major earthquakes have sufficient periodicity to permit estimates of probability to be made based on elapsed time from the previous earthquake on a fault segment. Few large faults have ruptured more than once during the instrumental or historical period, and only in rare cases have the ruptures been documented well enough to enable unambiguous comparisons of the sequential ruptures. Hence, discussions of the periodicity (or aperiodicity) of large earthquakes, and the degree to which earthquake source parameters vary through several slip events, are dominated by conjecture. One approach to evaluating repeatability and periodicity of earthquakes employs seismic data from smaller earthquakes. Along the creeping portion of the San Andreas fault in central California, M 4 to M 5 earthquakes have been frequent enough to enable studies of their similarity. Waveforms from these moderate events can be sorted into nearly identical groups, establishing the existence of small, active fault patches, each generating nearly identical characteristic earthquakes with well-defined periodicities (24). These characteristic patches



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