FAULT SEGMENTATION AND ANALYSIS OF THE RUPTURE HISTORY OF THE BOULDER FRONT FAULT, NORTHERN BASIN AND RANGE PROVINCE, IDAHO

Active normal faults in central Idaho include the range-bounding Boulder Front fault, which cuts a series of Pinedale-age moraines and fluvial outwash terraces in the upper Wood River watershed. Distinct segments with variable topographic expressions along the south-west dipping fault span at least 18 kilometers, contrary to previous assessments, across the southern slope of the Boulder Mountains from east of Boulder Creek to Galena Summit. Located along strike from the recently active (2020) northeast-dipping Sawtooth fault, seismic linkage may exist between the two faults as polarity reverses across the valley at the Galena Summit accommodation zone. The most prominent segment at Boulder Creek is about 1 kilometer in length. Lateral moraines from the late Pleistocene (~15.76kyr) and the associated outwash terraces are observed to be surficially offset by the fault. Multiple ruptures of the entire segment during the Holocene are evident from diffusional modeling across the length of the scarp (~7.08±2.72kyr) and radiocarbon dating charcoal from the youngest offset fluvial terrace (~2.81kyr ±0.06kyr). Additional radiocarbon dating from fluvial deposits in drainages cut by neighboring fault segments are being evaluated to determine rupture length and correlation to the Boulder Creek segment. Local calibration of a diffusion coefficient in glacial sediments is being conducted using the dated exposures and single-slip diffusion modeling to determine the various rupture ages. Possible extension of the fault system exists to the south toward the travel destination of Ketchum, Idaho posing a seismic hazard to the local and tourist populations.