Geologists are trying to figure out exactly what triggered the deadly landslide in Oso, Washington, last month.

So far there are two primary suspects: rising groundwater that loosened already slippery layers of soil, or the Stillaguamish River, whose current has slowly eroded the base of the hillside. If it's the former, scientists say logging might have played a role.

"We have a full-scale laboratory experiment -- of course, a very tragic one -- and we can learn a lot," Joseph Wartman, a professor of civil and environmental engineering at University of Washington in Seattle, told National Geographic. Wartman is co-leading a six-person team of scientists that plans to examine the disaster for signs of a trigger, whether manmade or natural.

The area surrounding Oso is prone to landslides. Top layers of loose sand and gravel sit atop a stratum of clay, which rejects most descending water. Heavy rains cause the water to pool atop the clay layer, acting as a lubricated slide.

The effects of clear cutting can exacerbate these geological forces. Trees help soak up heavy rains, and their roots hold sediment in place, preventing erosion.

But even if it's clear trees slow the descent of groundwater and mitigate erosion, it's difficult to say what effect logging has on landslides.

Geomorphologist Josh Roering has studied the relationship of logging and landslides in great detail. He said: "I can't even put my finger on a really clear, defensible, definitive study that says 'Yes, logging matters for deep-seated landslides.'"

What role logging played at Oso will require further evidence gathering. And even then, answers might remain elusive.

"The question is, 'What was the trigger? What was the threshold that was exceeded?'" Wartman said. "Whether we will be able to know that, I'm not sure."

[National Geographic]