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

The little studied Boulder Front fault is located in the upper Wood River drainage in the Northern Basin and Range Province of central Idaho. It extends along strike and to the south of the Sawtooth fault in the upper Salmon River drainage and may represent an antithetic segment of the same fault. Ongoing geologic studies of the west-dipping, Boulder Front normal fault (geologic mapping, fault scarp studies, limited surficial dating) were conducted to determine the faulting history of the segment. Late Pleistocene glacial deposits show clear offset by the studied fault segment. Radiocarbon dating of charcoal from the lowest offset fluvial terraced deposit in the footwall of the scarp was <~3kyr old, providing a maximum age constraint for the most recent rupturing event along the Boulder Front fault. Evidence of multiple ground rupturing earthquakes (<~3ka ±1kyr, ~5-7ka ±1kyr, ~10-11ka ±1kyr) since the Last Glacial Maximum are supported by diffusional modeling of scarp profiles. The slip rate on the last several ruptures of the Boulder Front fault is in line with regional values at ~0.1-0.2 mm/yr on a millennial time scale indicating a slow-slipping fault. Catchment-scale models of the faulted Boulder Creek watershed suggest that the fault segment was perhaps active for past several million years. The Boulder Front fault is adjacent to the Sawtooth Wilderness Recreation Area and close to several renown tourist venues in Idaho, creating a potential seismic hazard for the region.