The Ms 7.3 Borah Peak earthquake that struck central Idaho on October 28, 1983, was one of the strongest historic earthquakes in the Intermountain Seismic Belt. Much of the 34-km-long, northwest-trending zone of fault scarps and surface ruptures that formed during the earthquake follows Holocene and upper Pleistocene scarps of the Lost River fault. Throw along the new fault scarps averages 0.8 m, exceeds 1.0 m along 43% of their length, and attains a maximum of 2.7 m along the broad and complex zone of deformation in the southern section. The net slip was normal sinistral, averaging 17 cm lateral slip for 100 cm of dip slip. The preferred nodal plane from the focal mechanism strikes N22°W, dips 59°SW, and suggests a much larger component of strike slip than do the geologic data.

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