Scientists trying to find out the cause of the landslide in western Washington have identified two possible causes: it can either be the water movement of the Stillaguamish River or the groundwater weakening the soils.
The Stillaguamish River's water direction has been consistently eroding the bottom part of the hillside, and this could lead to a landslide. The rising groundwater, on the other hand, may have been made more dangerous by the excessive logging done in the area.
"We have a full-scale laboratory experiment-of course, a very tragic one-and we can learn a lot," civil engineer at the University of Washington in Seattle, Joseph Wartman told National Geographic. Wartman is the co-leader of the Geotechnical Extreme Events Reconnaissance Association, a team funded by the National Science Foundation.
The geology of the Stillaguamish River Valley is very prone to landslides. There is a layer of gravel and loose sand on top of the clay layer which cannot absorb the water. During rainy months, the clay layer is subjected to an increased water pressure, and this can destroy the structural integrity of the steep slope, making it vulnerable to landslides.
However, the role of logging in catalyzing these landslides has not yet been proven. Geomorphologist at the University of Oregon, Eugene, Josh Roering told the National Geographic, "I can't even put my finger on a really clear, defensible, definitive study that says 'Yes, logging matters for deep-seated landslides."
He further explained that there is a strong link between debris flow and logging. Since logging removes the trees that absorb excessive water from the ground, this water continues its way downstream and picks up pieces of debris as it goes along.
As of Tuesday, the death toll of the Oso, Wash.- landslide has reached 28 with 20 people still missing. Police officers, detectives, and other staffers have already joined the search.