Abstract

The 1983 Borah Peak earthquake was accompanied by extensive surface faulting along a part of the Lost River fault that has abundant evidence of latest Quaternary (last 15,000 yr) offset. This fault and two similar range-front normal faults along the Lemhi Range and Beaverhead Mountains lie in an area of basin-and-range structure in central Idaho that is part of a roughly V-shaped belt of latest Quaternary surface faulting that extends from the Wasatch fault, through the Yellowstone area, to the Lost River fault. The position of this belt may be related to the outward migration of a thermal front associated with the northeastward progression of late Cenozoic silicic volcanism along the Yellowstone-Snake River Plain axis. The central segments of the Lost River, Lemhi, and Beaverhead faults have been active more recently, and probably more active throughout Quaternary time, than the southern and northern segments. The main 1983 surface faulting occurred in an area of high structural relief along a central segment of the Lost River fault that has ruptured in latest Quaternary time, which suggests that comparable areas along other range fronts in the area should be regarded as likely sites of future surface faulting. Other perspective of fault behavior suggest additional possible sites, and all segments of the range-front faults are regarded as capable of surface faulting.

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