The hillside that gave way in a mudslide is seen near Oso, Wash. (Rick Wilking/Reuters)

The massive, destructive landslide in Washington state ravaged a wide stretch of land, killed more than a dozen people and left many more missing. But many people may not have much experience with landslides or know much about them. We talked with Lynn Highland, a geographer with the U.S. Geological Survey’s National Landslide Information Center who has studied landslides for 30 years, to learn a little more.

What makes this landslide so different from others?

“It’s the volume of material, and the fact that it slid. It had something we call a long run-out,” she said. “Usually landslides when they fall, they’re kind of confined. This one had such a volume of material that it ran out on one side of the river, crossed it, dammed it and came out on the other side of the river. The only time I’ve ever seen that happen before was with some huge earthquakes, like the earthquake in 2008 in China. I’ve not seen one that’s happened like that in my career in this country.”

What caused this landslide?

Based on preliminary information, it seems to be caused by heavy rainfall, she said. “This is the type of landslide that is predominant in Washington, is the rain-caused landslides or slope failures,” she said. But, she added, this isn’t something that typically stems from your normal rainfall. “It takes a lot of water to get a slope to fail.”

How can we tell if landslides will occur in a given place?

“It’s not really predictable,” she said. “We have something called susceptibility maps, which are very complicated to make. You have to have geology, the soil type, the rainfall regime of an area….and how much a slope has been disturbed by highway construction and that kind of thing.”

Predicting landslides precisely can’t be done perfectly, so they focus on figuring out what areas may be susceptible to them, she said.

“You can’t look at a place and know it’s going to slide and say because it’s had one before it’ll happen again, because there’s so many complicated factors,” she said. “It sounds like it should be something simple, but it’s not.”

Rain is expected through the weekend. How will that impact the search efforts?

“The more a slope is saturated, the weaker it becomes, and if you keep adding more and more rain before it’s had the ability to drain the water, it starts soaking just like a paper towel,” she said. “And what they say here is it loses its integrity and gravity takes over. This is not good news for the rescue operation. There are bluffs that are nearby that also can experience some kind of failure, and that’s not predictable.”

The way to try and look out for this is to keep an eye on any cracks on top of the bluffs, because that’s how rain can get into the slope, she said.

How does rain cause a landslide in this kind of area? 

“The more rain, the more the infiltration,” she said. “Sometimes it runs off in the river, but other times it infiltrates, it just depends on the soil type.”

So if it rains, the moisture soaks into the ground and it rains again with that water still there, that can lead to over-saturation of the soil. Then, as she said, gravity takes over.

“It can go a week later, a month later and there’s no way of predicting it,” she said. But there are ways to keep an eye on certain areas: “Sometimes we can put monitoring instruments on a slope to see if there’s some kind of movement or cracking.”

Officials said a small earthquake had hit the area this month. Was that related?

No, probably not. “There was a small earthquake 10 days before,” she said. “Usually when earthquakes cause landslides, it’s pretty immediate after the event.” And earthquakes typically have to register between a 6.0 and 7.0 magnitude to cause a landslide.

This particular slide was so big it did register on seismographs, she said.

So are we not adequately aware of where, exactly, landslides could occur?

“I would believe that that is true,” she said. “It depends on the state you live in. California, Oregon and Washington have a lot of public documents that people can go look at before they buy a house about the hazards in the area, mostly because they have so many hazards and mostly because there’s a lot of people moving to those states. But each county is different within each state.”

What can we learn from this landslide?

“There will be people studying the ins and outs of this landslide and what caused it and how it moved,” she said. “And you can get a lot of information even from one that’s already come down.”

That includes what layer slid and how much rain it took, which can be helpful in seeing what the threshold might be for similar areas.

“We’ve had scientists from all over the world calling in to see if they can study that landslide,” she said.