One of the deadliest landslides in U.S. history was unleashed when part of a mountain collapsed onto a rain-sodden slope, sending a wall of mud shooting through a Washington state neighborhood, according to a federal landslide expert.
The new account from the U.S. Geological Survey (USGS) differs from the explanation offered by the Geotechnical Extreme Events Reconnaissance (GEER) Association, which on Tuesday unveiled the first published analysis of the 22 March slide. The accident killed 43 in the little town of Oso at the edge of Washington’s Cascade Mountains.
The difference revolves around a critical question: What caused a hillside with a history of relatively minor landslides to suddenly turn into a tsunami of mud and debris that sped about a kilometer across a valley? The findings could influence what signs scientists look for when trying to detect other potentially explosive slides.
The GEER team, funded by the U.S. National Science Foundation, blamed rain and ground water soaking a hillside weakened by decades of smaller slides. In their analysis, first the slope began to slide. The movement, combined with ample water, turned the sliding ground into a fast-flowing semiliquid. The process, known as liquefaction, occurs when water pressure in the soil builds high enough that it pushes particles of earth apart and the slope can’t hold together. Four minutes later, a second portion of the mountain fell after losing support from below. The report pointed to two spikes recorded by nearby seismic devices as evidence of a two-stage scenario.
But Richard Iverson, a landslide expert at USGS’s Cascades Volcano Observatory in Vancouver, Washington, says that a closer look at the seismic data tells a different story. “The USGS disagree[s] significantly with several aspects of the GEER report,” Iverson says. According to the unpublished analysis by scientists with the USGS and the University of Washington, all the important action appears to have been compressed into a rapid chain reaction in the first few minutes. The lower slope began to slide slowly, but just 50 seconds after that began, an upper part of the mountain broke loose and collapsed onto it. The violence of that impact could cause the soil to rapidly liquefy and shoot across the valley, much like a foot slamming down into a mud puddle, he said. Iverson and scientists working with him concluded the second incident recorded in the seismic data was an “extremely small event.”
Iverson’s explanation relies on a new method of analyzing low-speed shaking in the earth. Newer seismometers capable of detecting slower, less intense motion are enabling scientists to gain a more detailed, second-by-second picture of how a landslide unfolds. Iverson surmised that the GEER scientists relied on a less sophisticated analysis of the seismic data.
Responding to Iverson’s critique, GEER co-leader Jeffrey Keaton, an engineer at AMEC Americas, a private engineering firm, says that observations in Oso helped convince them that much of the mountain collapsed a few minutes after the initial slide. Large chunks of relatively intact earth still covered with trees would probably have broken up if they had been part of the first slide, he said. And swaths of sand had flowed up onto the back end of the first slide, suggesting it happened after the first slide slowed to a halt.