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January 12, 2015
VANCOUVER, Wash. — The large landslide that occurred on March 22, 2014 near Oso, Washington was unusually mobile and destructive.

VANCOUVER, Wash. — The large landslide that occurred on March 22, 2014 near Oso, Washington was unusually mobile and destructive. The first published study from U.S. Geological Survey investigations of the Oso landslide (named the “SR530 Landslide” by Washington State) reveals that the potential for landslide liquefaction and high mobility are influenced by several factors, and the landslide process at Oso could have unfolded very differently (with much less destruction) if initial conditions had been only subtly different. 

A major focus of the research reported this week is to understand the causes and effects of the landslide’s high mobility. High “mobility” implies high speeds and large areas of impact, which can be far from the landslide source area. Because high-mobility landslides overrun areas that are larger than  normal, they present a significant challenge for landslide hazard evaluation. Understanding of the Oso event adds to the knowledge base that can be used to improve future hazard evaluations.

Computer reconstructions of the landslide source-area geometry make use of high-resolution digital topographic (lidar) data, and they indicate that the Oso landslide involved about 8 million cubic meters (about 18 million tons, or almost 3 times the mass of the Great Pyramid of Giza) of material.  The material consisted of sediments deposited by ancient glaciers and in streams and lakes near the margins of those glaciers. The landslide occurred after a long period of unusually wet weather. Prolonged wet weather increases groundwater pressures, which act to destabilize slopes by reducing frictional resistance between sediment particles.

The slope that failed at Oso on March 22, 2014 had a long history of prior historical landslides at the site, but these had not exhibited exceptional mobility.

The area overrun by the March 22 landslide was about 1.2 square kilometers (one-half square mile), mostly on the nearly flat floodplain of the North Fork Stillaguamish River. Additional areas were affected by upstream flooding along the river, which was partially dammed by the landslide. Eyewitness accounts and seismic energy radiated by the landslide indicate that slope failure occurred in two stages over the course of about 1 minute. During the second stage of slope failure, the landslide greatly accelerated, crossed the North Fork Stillaguamish River, and mobilized to form a high-speed debris avalanche. The leading edge of the wet debris avalanche probably acquired additional water as it crossed the North Fork Stillaguamish River. It transformed into a water-saturated debris flow (a fully liquefied slurry of quicksand-like material) that entrained and transported virtually all objects in its path.

Field evidence and mathematical modeling indicate that the high mobility of the debris avalanche was caused by liquefaction at the base of the slide caused by pressures generated by the landslide itself. The physics of landslide liquefaction has been studied experimentally and is well understood, but the complex nature of natural geological materials complicates efforts to predict which landslides will liquefy and become highly mobile.

Results from a suite of computer simulations indicate that the landslide’s liquefaction and high mobility were very sensitive to its initial porosity and water content. Landslide mobility may have been far less if the landslide material had been slightly denser and/or drier. Computer simulations that best fit field observations and seismological interpretations indicate that the fast-moving landslide crossed the entire 1-km-wide river floodplain in about one minute, implying an average speed of about 40 miles per hour.  Maximum speeds were even higher.

Only one individual landslide in U.S. history (an event in Mameyes, Puerto Rico in 1985 that killed at least 129) caused more fatalities than the 43 that occurred in the 2014 landslide near Oso.

The full paper, “Landslide mobility and hazards: implications of the 2014 Oso disaster” by R.M. Iverson et al. is published in the journal, “Earth and Planetary Science Letters” and is freely available online. 

Video Transcript
Video: Oso landslide simulation.
Image: Oso, Washington Landslide
Oblique aerial photograph of the Oso, Washington landslide, showing the entire extent of the landslide source area and path.Location: State Route 530, Oso, WashingtonPublic domain

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