The 25 May 2014 West Salt Creek landslide had a volume of 30 million cubic meters of rock and a runout of 4.5 kilometers (about 7 times its fall height).
Long-runout landslides, like the 25 May 2014 West Salt Creek landslide with a volume of 30 million cubic meters of rock and a runout of 4.5 kilometers (about 7 times its fall height), have long puzzled scientists, but researchers are beginning to understand how wave processes within slides can reduce friction, allowing materials to travel unexpectedly long distances. Credit: Jon White/Colorado Geological Survey
Source: Journal of Geophysical Research: Earth Surface

When earth, rocks, and debris on a slope give way, the resulting landslide can obliterate everything in its runout path. Landslides, like many geological disasters, are difficult to predict, but long-runout landslides—where a mass of earthen material travels unexpectedly lengthy horizontal distances after a comparatively short vertical fall—are especially puzzling for scientists.

Existing models of long-runout landslides can reproduce observed slide events relatively well. But they can’t explain why long-runout slides—with more than 1,000,000 cubic meters of material—experience a counterintuitive reduction of friction. Scientists have put forth many potential explanations: the debris might glide overtop a layer of trapped air, water could lubricate the slide’s path, or friction-induced melting of ice or rock could ease the way for a large landslide. However, the occurrence of long-runout landslides elsewhere in the solar system makes any entirely air- or liquid-based explanation unlikely.

Here Johnson et al. use a soft particle code first presented by Campbell et al. (J. Geophys. Res., 100(B5), 8267–8283, 1995, doi:10.1029/94JB00937) to elucidate the mechanism responsible for the reduced friction. The reprised, two-dimensional model approximates landslides down 45° slopes. The authors looked at variations in pressure on the ground within simulated landslides and found that sliding was more likely to occur when the pressure dropped below estimates of the overburden—the mass of material above the land at the base of the slide.

According to the researchers, sliding when the pressure on the ground is below measures of the overburden is characteristic of a process known as acoustic fluidization, where vibrations caused by sound waves traveling through the landslide affect friction. The authors conclude that acoustic fluidization could create the low friction necessary for long-runout landslides. (Journal of Geophysical Research: Earth Surface, doi:10.1002/2015JF003751, 2016)

—Kate Wheeling, Freelance Writer

Citation: Wheeling, K. (2016), What makes long-runout landslides so mobile?, Eos, 97, doi:10.1029/2016EO050987. Published on 22 April 2016.

Text © 2016. The authors. CC BY-NC-ND 3.0
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