Abstract
The Borah Peak Earthquake of October 28, 1983 occurred near MacKay, Idaho with about 42 km (26 mi) of complex normal-oblique-slip faulting on the Lost River fault. Associated with the surface faulting was strong ground shaking and dramatic cold-water geysers and hydrologic changes related to regional ground subsidence. Faulting occurred on pre-existing Quaternary-Holocene scarps as both single and en echelon traces with net vertical displacement of 2 to 3 m (6.6 to 9.8 ft) and up to 1 m (3 ft) of left lateral displacement. Although no earthquakes prior to 1983 are known to have been associated with this fault, there is abundant geomorphic evidence of late Quaternary activity. Other normal-slip faults in the basin and range of Idaho, Montana, Utah, and Nevada show similar geomorphic evidence of activity.
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