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This aerial photo taken on March 23 shows the hillside that collapsed, resulting in a massive mudslide near Oso, Washington, on March 22.


Washington Mudslide's Speed Led to High Death Toll

Slide's large size and unstable conditions also make recovery difficult.

The massive mudslide that wiped out part of a town in rural northwestern Washington Saturday struck so quickly that residents couldn't escape its path, scientists said, leading to the deaths of at least 25 people, with scores still unaccounted for.

And, they said, the slide's large size—which covered nearly a square mile (2.6 square kilometers) with mud and debris—is likely to hamper efforts to clean up the area and rebuild.

Rescue teams, search dogs, and volunteers continued to search for survivors for a sixth day in the aftermath of the devastating mudslide in Washington's Snohomish County. The mudslide, which occurred Saturday morning, crashed down a hillside and destroyed more than 30 homes in the town of Oso. More than 90 people are still unaccounted for.

The mudslide was likely caused by prolonged rainfall, according to Rex Baum, a research geologist with the U.S. Geological Survey. "We know that there have been landslides in that area for a long time, and it's not unusual for deposits from old landslides to be reactivated by prolonged rainfall," he told National Geographic. (See "Mudslides Explained: Behind the Washington State Disaster.")

"This particular landslide has a history of movement, and previously moved in 2006," Baum said. "But this time it moved much farther than any time in the past."

Fast and Deadly

It's not that unusual for landslides to move this much, or to be this large, Baum said. Instead, it may have been how quickly this landslide moved that was key to its deadliness, Baum said.

"Landslides happen frequently, and cause lots of property damage, but it's unusual that they cause this many fatalities," Baum said.

Overall, mudslides and landslides result in an average of 25 to 50 deaths a year in the U.S., according to the Centers for Disease Control and Prevention (CDC).

The massive landslide near the town of Oso, Washington, "moved so far and so fast, and there were so many homes in its path, that it caused so many fatalities," Baum said. "Had it moved more slowly, people would have had time to evacuate," he said.

He added that scientists don't know for certain yet what drove the slide's high speed, although they speculate that it may be some combination of the steepness of the hill, the fact that the mudslide area contained soil that had already been loosened by previous mudslides, and how much the soil had liquefied due to the large amount of rainwater that had soaked into it.

The landslide's deadliness and devastation were caused by a combination of the force of impact and the fact that it covered things in mud and debris. "The landslide debris crushed things that were in its path, and so it's partly a matter of covering things up and partly a matter of breaking, crushing, mashing things," Baum said.

Tough to Rebuild

The mudslide-covered ground is still wet and quicksand-like in many places, making it hard for rescuers to traverse or to bring in heavy equipment, and with more rain in the forecast, conditions could remain treacherous.

The heavy previous rainfall that likely caused the landslide means that "there's a lot of water soaked into the ground," Baum said. "It stays soupy for quite a while, with areas that are soft and easy to sink into, and it takes time for water to drain."

While material from small landslides can usually be moved out of the way, with mudslides this large "it's difficult to remove all of the material that was deposited," Baum said. "There's too much, and it's left where it was for the most part."

Efforts to reconstruct roads and buildings in the affected areas will first require excess water to drain out of the soil, according to Baum. "It's largely a matter of waiting for the soil to drain and become firm, before you can start major reconstruction of anything," he said.