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Last year in March, a huge landslide overwhelmed the neighborhood in Oso, Washington. The movement shocked even geologists. Over the last two millennia, the Stillaguamish River's North Fork is shown on average to undergo a huge slide in every 140 years, according to a study from the University of Washington, published in Geology.

"The takeaway is that this is a very dynamic landscape," said Sean LaHusen, a UW doctoral student and lead author on the paper, in an article in the Seattle Times. "It's very unlikely that landslides will stop happening in this valley."

The disaster had killed 43, for which the researchers used a laser-scanning technique called lidar to survey the maps in the region. By clearing the vegetation, the technique is able to show past landslides that had occurred in the valley, says a release.

Researchers gathered deposits from earlier landslides by wading into the water, and then climbing on nearby slopes to find if trees or pieces of wood are left under the remnants of slides, which in turn could be dated through radiocarbon.

Just a few miles from the Oso spot, another huge slide called the Rowan slide was two times bigger than the Oso slide. Due to the slide material had gone skidding across the valley like the Oso slide. Through dating, the scientists understood that the Rowan slide had happened 500 years ago.

That was just some time ago, according to the team's understanding of previous slides. "So Oso can't be dismissed as an outlier," LaHusen said.

Hence, as the glaciers retreated about 15,000 years ago, the river valley became vital for landslides. Once the glaciers had left, there was a "rubble of till, clay, sand and lake sediments" that were not stable and were open to slides.

"...Knowing this place is so unstable, we ought to take a careful look at our land use practices," UW geologist and co-author Alison Duvall explained.