A method is described for determining recurrence times as a function of distance to the causative fault and magnitude, for earthquakes distributed along a linear source zone. The method takes into account rupture length, which is scaled to magnitude, and permits direct calculation of approximate return periods for peak ground-motion parameters for large earthquakes, when the appropriate attenuation functions are known. Several examples are presented using instrumentally determined seismicity along the San Andreas fault zone. Results illustrate the necessity of incorporating rupture length in calculations related to seismic risk; for large earthquakes, it is also necessary to use a source region large enough to contain the rupture zones of all such events. For a site in the San Andreas fault zone we find that the recurrence time to be within 10 km of the causative fault of an earthquake with M ≧ 8 is 200 to 300 years, depending on the choice of maximum magnitude (8.6 or 8.4). For a site on the fault in the Hollister region, we find that recurrence times to be within 10 km of the rupture due to events of M ≧ 5, 6 and 7 are, respectively, 12, 45 and 105 years.

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