Regional landslide evaluation: Two Utah examples

Landslide-susceptibility evaluations based on bedrock units, slope inclination, and aspect (slope orientation) incorporate the lithologic, stratigraphic, structural, and topographic passive conditions contributing to landsliding as outlined by Sharpe (1938). Generalized results based on passive condition analyses may define potential for failure over entire states or large parts of states, establishing an approximate landslide susceptibility for specific areas that are still in need of detailed study. This provides a preliminary susceptibility for use in land management and identifies high-priority areas for future detailed investigations. Landslides in the Bear River Range, north-central Utah, and the northern Wasatch Plateau, central Utah, illustrate application of these factors to regional landslide-susceptability evaluation. The greatest landslide potential exists on slopes of between 10 and 40%, with a westerly aspect, underlain by Wasatch or Salt Lake formations in the Bear River Range and on slopes of from 20 to 60% that are underlain by either North Horn or Price River formations in the northern Wasatch Plateau. Results from the two study areas are compared to a statewide landslide evaluation (Schroder 1971) in terms of passive conditions. Lithologic and stratigraphic conditions of the state and of the two study areas are nearly identical. Aspect data detect the presence or absence of structural conditions favoring failure in the two study areas rather than climatic factors as concluded in the statewide analysis. Topographic condition is more precisely stated in quantitative as opposed to qualitative terms to define failure-prone slopes in the two study areas.