The focus of this study is the age of the earthquake antecedent to the 1983 Borah Peak earthquake in the vicinity of Doublespring Pass Road, where the faulting signature of both events has been studied in detail through trenching operations in 1976 and again in 1984. We sample the morphologic expression of the pre- 1983 event with six profiles along a ≈ 100-m segment of the fault scarps adjacent to and just north of the trench site, where both the 1983 and pre-1983 scarps are remarkably linear and regular. Diffusion-equation modeling of the profiles suggest that the age of the pre-1983 event is 6000 to 8000 yr before present, exclusive of the uncertainties in applying the Lakes Bonneville and Lahontan mass diffusivity κ to this central Idaho setting.

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