Surficial geologic map of the Nephi segment of the Wasatch fault zone, eastern Juab County, Utah (M-170)

By: K. M. Harty, W. E. Mulvey, and M. N. Machette The Nephi segment of the Wasatch fault extends 33 kilometers from north of Payson to south of Nephi. Quaternary deposits along the segment are middle Pleistocene to late Holocene in age, and consist primarily of coalesced alluvial fans and Lake Bonneville lake-bottom sediments. The Bonneville lake cycle spanned approximately 16,000 years, but the lake occupied Juab Valley for only about 1,000 years during the lake's highstand. Other Quaternary deposits in the mapped area include stream alluvium, landslides, glacial till, eolian deposits, and colluvium. Three surface-faulting earthquakes have occurred on the Nephi segment during the middle to late Holocene. Morphology of the scarp formed by the most recent event and its lack of vegetation indicate that it may be the youngest surface-faulting event on the Wasatch fault. Radiocarbon dates from trenches indicate that it is younger than about 1,200 years, but the surficial expression