Background On Washington State Mudslide

This aerial photo of the mudslide near Oso, Washington, was taken Saturday, March 23, 2014. The debris flow was up to 15 feet (4.5 meters) deep in some areas.

PHOTOGRAPH BY MARCUS YAM, THE SEATTLE TIMES/AP

National Geographic:

On Saturday morning, a mudslide moved down the Stillaguamish Rivernear the small former fishing village of Oso, Washington. Authorities have confirmed eight dead, eight injured, and as many as 108 people missing or unaccounted for as of Monday morning. The one-square-mile (2.6-square-kilometer) track of the mudslide also destroyed about 30 homes.
Jim O'Connor, a research hydrologist with the U.S. Geological Survey in Portland, Oregon, told National Geographic that the mudslide, which was up to 15 feet (4.5 meters) deep in some areas, was caused by ground made unstable by heavy rainfall.
"This area has had slides in small increments over the last several years, but this took a huge bite of the hillslope this time," says O'Connor.
Not only has there been a lot of precipitation in the area over the past few months, but the Stillaguamish River also has been eroding away the base of the hillside, or "undercutting it," making it less stable, says O'Connor.
"A whole section of a hillside, about 700 feet [213 meters] high above the river, collapsed all at once," says O'Connor. "It's amazing how much terrain it ended up covering."

There isn't a whole lot that can be done to prevent slides:

Strategies to decrease the risk of mudslides include draining water off hillsides, armoring the bases of hills so they are not undercut by rivers, and "loading the toe," says O'Connor. In the case of "loading the toe," engineers put heavy mass, such as large rocks, at the base of a hill to try to anchor the slope and prevent it from coming loose.
O'Connor says the piles of rock that are often seen at the base of roadcuts are the most visible example of that strategy.
O'Connor adds that when it comes to the Stillaguamish River area, the valley is scalloped with the evidence of many past slides.
"This isn't a situation where [the authorities] should have done something [to prevent it] because there is so much terrain there that this could have happened to," he says.

The failure slope in the picture looks like a diagram straight out of a soil mechanics textbook.  It is hard to imagine that much material moving at one time.

Another photo here:

The size of the landslide became evident from aerial photographs