Shallow faults mapped with seismic reflections: Lost River Fault, Idaho

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. Six meters of total fault displacement can be interpreted on the bedrock reflector while only 1 to 2 m of displacement can be observed on a shallower refracting interface and the surface fault scarp. This relatively small displacement suggests the western splay has either been active only recently or extremely infrequently since deposition of the bedrock, or that strike-slip motion may be present. 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. 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.