A thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments. Paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events. Systematic mapping of fault scarps and degradation-equation modeling of relative scarp ages on the Lemhi fault, a range-bounding normal fault in east-central Idaho, provide a method to assess rupture extent and boundaries between rupture segments. The youngest latest Quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault. The boundaries between rupture segments differ in characteristics, although most are at intersections of the currently active range-bounding fault and mid-Tertiary normal faults. Several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or (probably) in the previous cycle. The Lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone.

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