Landslides are more common than you think

Every second Friday in the Morning Edition, the Outside/In team answers a listener’s question about the natural world.

This week Phil from Maine asked this question: “My wife and I were hiking and noticed a landslide that has probably happened in the last few years. How often do they occur, why do they occur, and what are the notable events? Will climate change make landslides more common?”

Wow, that’s a lot of questions! To organize this, I’m going to break what I’ve learned into three basic facts about landslides.

The term landslide is sort of a catch-all term, and there are many different types. For example, a “rockfall” is a type of landslide where rocks fall down a slope or cliff. (Here’s a chilling example.) Einsteinslide,“In contrast, a section of rock detaches and slides down a slope, much like a layer of snow might give way in an avalanche.

One of the types you see most often in the news is a mudslide (technically called a “garbage flow”), where water-laden earth and debris suddenly liquefy to form mudflows.

But also the slow rolling of stones on an eroded path
can be considered a landslide! In other words, landslides can move at a rate of inches per year, or as fast as a city bus — and they happen all the time.

“They happen often, they happen in all 50 states,” says Corina Cerovski-Darriau, a researcher with the US Geological Survey’s Landslide Disaster Assistance Team. “Every year they cause billions of dollars in damage and unfortunately kill on the order of 25 to 50 people each year in the US alone.”

United States Geological Survey

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So why do landslides happen? First of all, because of gravity. Take an incline between 25 and 40 degrees, put some roughly round rocks on it and you’ll probably see some falling off over time.

But many landslides are what you might call a “secondary disaster.”

“Landslides are usually triggered by water – be it rain or changes in river or lake levels,” says Cerovski-Darriau. And there are other triggers: “Earthquakes, volcanoes, and then people.”

People can trigger landslides during road construction and other construction projects, or during mining. But again, many of the most widespread and destructive landslides in recent history involve rainfall.

In 2014, a month and a half of unusually heavy rains saturated an already landslide-prone slope in Oso, Washington triggered the deadliest debris flow in recent American history. The mudslide spread over half a mile in less than a minute, killing 43 people. According to Cerovski-Darriau, there was so much debris that it would have covered “700 football fields 10 feet deep.”

Another example of the secondary catastrophe concept came in 2017 when Hurricane Maria struck 70,000 landslides in Puerto Rico. The torrential rains, the island’s mountainous topography, and road infrastructure (Puerto Rico has one of the highest “road densities” in the world) all contributed.

Precipitation is such a big factor that you can see it in the data – a global database of fatal landslide events from 2004 to 2016 shows a very predictable and steady increase in landslides during annual cycles associated with rain – particularly in East and South Asia.

But fire is also part of the equation. Runaway wildfires can strip the land of the vegetation that anchors the ground and pave the way for more mudslides during the rainy season.

Jonathan Godt, U.S. Geological Survey

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extreme weather events etc Global precipitation is increasing. So it goes without saying that our warming climate is also changing the nature of global landslide risk.

California has seen this winter Hundreds of landslides caused by heavy rains from the ‘atmospheric flow’ (and exacerbated by wildfire damage). Given current trends, it would be fair to expect an increase in landslides there in the coming years.

But there are other areas that can see reduced also the risk of landslides due to climate change.

“On the other hand,” says Cerovski-Darriau, “these prolonged droughts could reduce the risk of deep-seated landslides.” She says some of the largest landslides caused by rising lake or groundwater levels may occur less frequently.

Because of these regional differences, it is difficult to say whether climate change will lead to an increase in total frequency of global landslides; at least loud global landslide database I mentioned earlier.

But one thing it does show is a significant increase in fatalities triggered by people Landslides from mining and construction.

One way or another, the ground is literally moving beneath our feet.

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