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.

Gravity-induced rock mass damage related to large en masse rockslides: Evidence from Vajont

“…These principal features of damage can affect creeping rock slopes at shallowintermediate depths (20-50 m), as well as at considerable depth (50-300 m), presenting highly variable intensity. A major difficulty during the field survey of a large rockslide or of a creeping unstable rock mass is the recognition of rock mass damage caused by gravity, especially when differentiating these effects from tectonic-induced phenomena (Chigira, 1992;Hart et al, 2012;Jaboyedoff et al, 2013). In these circumstances, disintegrated and/or pulverized rocks related to basal or internal shear zones can be misinterpreted as a tectonic fault gouge.…”

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

“…strata, in the east Traverse Mountains. This contact is clearly not displaced by landsliding as inferred by Hart et al (2012). (3) Their interpretation also requires the basal slip surface of Qls?1 to be in Oquirrh Group orthoquartzite, a unit nowhere noted for its landslide potential.…”

“…As correctly noted in their introduction, ''…every pattern of tectonic faulting can be produced by landsliding.' ' Hart et al (2012) illustrate this fact with seven examples from the western United States, one of which, the Traverse Mountains of Utah, warrants clarification. Figure 7 of Hart et al (2012) shows their interpretation of three landslides on the south flank of the Traverse Mountains, based in part on my 1:24,000 scale geologic map of the Lehi quadrangle (Biek, 2005a).…”

“…' Hart et al (2012) illustrate this fact with seven examples from the western United States, one of which, the Traverse Mountains of Utah, warrants clarification. Figure 7 of Hart et al (2012) shows their interpretation of three landslides on the south flank of the Traverse Mountains, based in part on my 1:24,000 scale geologic map of the Lehi quadrangle (Biek, 2005a). The focus of their summary relates to ''landslide A,'' and what I interpreted to be a downto-the-northeast normal fault bounding its southwest margin.…”

“…Not only are the volcanic rocks altered, but they also consist of a heterogeneous sequence of inherently resistant andesitic lava flows and ash-flow tuffs (volcanic mud-flow deposits) and inter-bedded non-resistant ash-fall units, all generally tilted gently to the southeast. The presence of both non-resistant intensely altered and resistant unaltered volcanic rock and their overall gentle southeast dip create an unusual knobby and locally stair-stepped topography (the ''anomalously low terrain gradient'' and ''subdued topographic benches'' of Hart et al, 2012) that locally mimics characteristic landslide geomorphology. Biek (2005a).…”