A method is described for determining expected acceleration return periods, based on calculations involving magnitude, fault length and distance to the causative fault. The method permits earthquake magnitude and duration of strong motion to be associated with these return periods. In addition, because attenuation equations are in terms of distance to the causative fault, instead of focal distance, sites can be considered which are in the immediate vicinity of potential faults. Results of calculations indicate that for an average site in the western Nevada region maximum-amplitude, maximum-duration ground motion has a recurrence time of the order of thousands of years. This result, based on a relatively brief sample of instrumental data, is entirely consistent with geological field data representing time periods two to three orders of magnitude longer. Smaller ground motions have correspondingly smaller return periods, down to about a decade for accelerations greater than 0.1 g, when caused by all earthquakes with magnitude 5 or greater. Our results indicate that evaluation of seismic risk in terms of a single peak ground-motion parameter may lead to risk estimates which are several times too high.

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