Landsliding has historically been one of the most damaging geologic hazards in western Wasatch County, Utah. Accordingly, we mapped and analyzed landslides (slumps and debris slides) in the area to provide an empirical basis for regional landslide-hazard evaluation. The 336 landslides in the 250-sq-mi (650-km2) area involve 20 geologic units, including Mississippian- to Quaternary-aged rock and unconsolidated deposits. Landsliding in western Wasatch County is characterized by a strong correlation between geologic material and landslide-slope inclination. From a simple statistical analysis of overall slope inclinations of late Holocene landslides, we determined “critical” slope inclinations above which late Holocene landsliding has typically occurred and used these as the primary basis for defining relative landslide hazard. The critical slopes vary for individual geologic units and range from 15 to 50 percent (9°–27°). The critical slope values and landslide locations were used in conjunction with geologic and slope maps to construct qualitative landslide-susceptibility maps for use by county planners. The maps delineate areas of low, moderate, and high relative hazard and indicate where studies should be completed prior to development to evaluate site-specific slope-stability conditions. Critical slopes as determined in this study provide a consistent empirical reference that is useful for evaluating relative landslide hazard and guiding land-use-planning decisions in large, geologically complex areas.
