Several examples from the western United States are presented in which faults are mapped on the basis of geomorphic and structural evidence that is equally likely to indicate landsliding.   In some of the examples the presence of faulting has led workers to ignore evidence of landsliding that may have utilized fault planes as rupture surfaces.   In southern California, late Pleistocene or Holocene faults are depicted on published geologic maps solely on the basis of the presence of linear scarps or tilted bedding better explained by landsliding.  Landsliding and faulting have been alternating explanations by various authors for prominent scarps and grabens in the Saddle Mountains of Washington.  Offset Quaternary colluvium in observation pits and linear scarps in a subdivision in central Utah have been the subjects of differing opinions as to their origin by consulting and reviewing geologists even though there is clear evidence of landsliding.  In the desert of southern California several subparallel linear scarps in granitic rock on a ridge top east of a known active fault have also been mapped as faults.  The results of recent studies show that the features are more likely indicative of incipient landsliding that grades laterally into fully developed landslides.  The Hebgen Lake, Montana earthquake of 1959 produced landsliding as well as tectonic ground rupture.  It is suggested that an arcuate scarp that formed north of the primary ground rupture zone previously interpreted as a fault was possibly produced by reactivation of a 6 mi wide landslide.   A final example is presented of a combination of normal and thrust faulting mimicking evidence of landsliding near Saint 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.

Mike Hart, Engineering Geologist