Scarps on the Warm Spring and northern part of the Thousand Springs segments of the Lost River fault zone, Idaho, were surveyed and analyzed to determ… The Lost River Range has the grand distinction of being Idaho's highest mountain range. • The 1983 earthquake produced 36 km of northwest- trending surface rupture predominantly along the Lost River fault. Abstract A high-resolution seismic-reflection survey, conducted at the intersection of Arentson Gulch road and the western splay of the Lost River fault scarp in central Idaho, defines a bedrock surface about 80 m deep which is segmented by several faults forming graben structures. On the morning of 28 October 1983, the Ms 7.3 Borah Peak earthquake struck central Idaho and formed a Y -shaped zone of surface faults that is divided into a southern, a western, and a northern section.

A westward deflection of the major activity along the Lost River fault was probably responsible for the gap in 1983 surface faulting between the Warm Spring and Thousand Springs segments. DATE: January 17,1993.

Photo: Michael Machette, USGS Hedlund, Christopher A., "Kinematics of fault-related folding in a duplex, Lost River Range, Idaho, U.S.A." (1992).Theses and Dissertations.Paper 141. It boasts seven of the nine coveted “Twelver’s”. Of the 36 km, 14.2 km are found along a westS-trending UT e lund, .hristoph r "TITl : Kinematics of Fault m elated Folding In a Duplex, Lost River Range, Idaho, U.S.A. The total length of the surface faults is 36.4 Lost River fault, identified as a range front fault, is typical of the Basin and Range structural province north of the Snake River Plain [Baldwin, 1951]. The inconsistency in total bed displacement based on reflection, refraction, and fault-scarp evidence suggests tectonic activity on the western splay spans more than just a single episode. Earthquakes 1983 Borah Peak Earthquake At 8:06 a.m. (MST) on Friday October 28, 1983 a magnitude 6.9 earthquake (7.3 on the Richter scale) rocked the towns of Challis and Mackay for 30 to 60 seconds. This rugged range stretches from the Snake River Plain in the south to an area near Challis, for a distance of around 80 miles. Lost River fault scarp formed during the 1983 Borah Peak earthquake, crossing Doublesprings Pass Road.