We excavated trenches at two paleoseismic sites bounding a trans-basin bedrock ridge (the Willow Creek Hills) along the northern Lost River fault zone to explore the uniqueness of the 1983 Mw 6.9 Borah Peak earthquake compared to its prehistoric predecessors. At the Sheep Creek site on the southernmost Warm Springs section, two earthquakes occurred at 9.8–14.0 ka (95% confidence) and 6.5–7.1 ka; each had ~1.9 m of vertical displacement. About 4 km to the southeast, across the Willow Creek Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand Springs section occurred at 9.0–14.7 ka and 6.1–7.5 ka with ~1.9 m of vertical displacement each. We synthesize these and previous paleoseismic results into a model of five postgla...