Abstract

Geologic hazards have often been overlooked or downplayed in portions of the Thrust Belt. Landslides, earthquakes, and liquefaction are hazards that need to be considered for any type of development in the area.

Many landslide types are found in the area, with most being complexes of slump-debris laden earth flows, debris flows, rock slides, or rock-fragment flows. Broad generalizations can be made as to the type and age of bedrock that movements occur in. In the central and northern portions of the Thrust Belt, landslides are abundant. Cretaceous units prone to movement area are the Bear River, Blind Bull, and Frontier formations in addition to the Aspen Shale and the Gannett Group. Major flows are derived from the Gros Ventre Shale as well as from the talus derived from more resistant units.

Very few detailed seismic risk studies have been done in the central and southern portions of the Thrust Belt. Existing studies indicate that a Maximum Credible Earthquake (MCE) with a magnitude of 7.3 is possible in southern portions of the area, and a MCE of magnitude of 7.5 is feasible for the Star Valley area. Maximum expected horizontal accelerations (% of gravity) estimated for the area partially reinforce the calculated magnitudes for a Maximum Credible Earthquake.

A liquefaction potential exists for three major areas in Uinta, Lincoln, and Teton Counties. The Snake River Valley, Star Valley, and Bear River Valley all have components that may be prone to seismically induced liquefaction.