When Landslides Are Misinterpreted as Faults: Case Studies from the Western United States

Abstract

We present several case studies from the western United States where faults are mapped on the basis of geomorphic and structural evidence that is equally likely to indicate landsliding. In some examples, faults have obscured evidence of landslides that utilized fault planes as rupture surfaces. In the Southern California examples, late Pleistocene or Holocene faults are mapped solely based on linear scarps. Such faults are often better explained by landsliding. Similarly, both landslides and faults have been proposed to explain prominent scarps and grabens in the Saddle Mountains of Washington. We note that both faulting and landsliding have been invoked by consultants and reviewers to explain offset Quaternary colluvium in observation pits and linear scarps in a subdivision in central Utah. Several subparallel linear scarps in granitic rock on a ridge top in the Southern California desert have also been mapped as faults. Recent studies, however, show that the features more likely indicate incipient landsliding that grades laterally into fully developed landslides. The Hebgen Lake, Montana, earthquake of 1959 produced landsliding as well as tectonic ground rupture. We suggest that an arcuate scarp that formed north of the primary ground rupture zone, previously interpreted as a fault, was likely produced by reactivation of a 6-mi-wide (9.7 km) landslide. We include a final case study where a combination of normal and thrust faulting mimics landsliding near St. George, Utah. Failure to correctly differentiate between landslides and faults leads to incorrect evaluation of a site's stability as well as incorrect evaluation of seismic hazard and ultimately impacts public health and safety.