The US West Coast already has its next "Big One" to be concerned about when it comes to unstable ground, but researchers have now identified another problem in the region: more than 600 slow-moving landslides. Using satellite imagery, geophysicists with Southern Methodist University found landslides that had almost been entirely undetected in California, Oregon, and Washington -- some within a few kilometers of towns and roads. "These landslides are currently moving slowly. But they're already in a state of force imbalance -- so some other external forces, like earthquakes or rainfall, could shift them into a disaster," said Yuankun Xu, a lead author of the study.

Why This Matters

The study provides a warning map of at-risk areas should any of these slow-moving landslides accelerate. Usually, landslides are only discovered when people report them -- they tend to move fast and noticeably. Slow-moving slides, on the other hand, aren't always detectable by the naked eye. But that doesn't mean they don't have disastrous and damaging potential. In fact, the worsening drought and extreme wildfire seasons coupled with more intense weather could accelerate their movement and even lead to more landslides. However, most landslides worldwide are caused by rainfall -- and scientists have yet to draw a clear line between a warming climate and changes in landslide activity.

Landslides at Sea

In Alaska, scientists are monitoring what would happen if a rockfall avalanche occurred in Glacier Bay National Park and Preserve, where a large cluster of landslides occurred from 2012 to 2016 when the area had record-breaking warm temperatures. Scientists think the thawing permafrost could create an avalanche so big it results in a tsunami -- a "low-probability, high-consequence event" that climate change makes more likely.

"Fundamentally, this landslide probably wouldn't happen, and the tsunami hazard associated with it wouldn't exist, if the glacier [were] positioned where it used to be," geologist Gabriel Wolken of the Geological and Geophysical Surveys Division of the Alaska Department of Natural Resources told Eos. "We have all of these changes in cryospheric variables…such as glacial retreat and permafrost thaw and degradation, that are independently clearly linked to a change in climate."

Permafrost thaw is also suspected of causing increased landslide activity in Denali National Park this year. In August, 50% of the park was closed following a landslide that blocked the park's main access road. The park is home to a slow-moving landslide that has been accelerating in recent years, moving at about 12 inches per day compared to just half an inch in 2018.