Montecito mudslides
Mudslides in Montecito claimed the lives of 23 people in 2018, weeks after the Thomas Fire swept through the area. Photo: OnSceneTV

Fire-prone areas of Southern California can expect to see landslides occurring almost every year, with major events expected roughly every 10 years, a new study found.

The results show residents face a double whammy of increased wildfire and landslide risk caused by climate change-induced shifts in the state’s wet and dry seasons, according to researchers who mapped landslide vulnerability in the southern half of the state.

“This is our attempt to get people thinking about where these hazards are going to be before there’s even a fire,” said Jason Kean, a hydrologist at the U.S. Geological Survey in Denver and lead author of the new study in Earth’s Future, a journal for interdisciplinary research. “By proactively thinking about hazards, you can start to develop more detailed response plans for their inevitability.”

Wildfires make the landscape more susceptible to landslides when rainstorms pass through, as the water hits unstable, dry soil and burned vegetation.

Geologists routinely conduct landslide hazard assessments after wildfires occur, but there is often not enough time between a fire and a rainstorm to implement an effective emergency response plan, Kean said.

In the study, Kean and his colleague combined historical fire, rainfall and landslide data with computer simulations to forecast where post-wildfire landslides are likely to occur in southern California, how big those landslides might be and how often they can be expected to happen.

Their goal was to map which regions of the state are most vulnerable to landslides before they happen, in a manner similar to how geologists map earthquake hazards.

Their results show small landslides can now be expected to occur almost every year in southern California. Major landslides capable of damaging 40 or more structures can be expected every 10 to 13 years – about as frequently as magnitude 6.7 earthquakes occur in California, according to the study.

The results also suggest more intense rainfall, which is likely to happen in the coming decades, could mean Californians face a higher risk of landslides that that damage property and endanger people’s lives.

“We’re going to have a longer season to burn and then when it does rain, it’s going to come down harder,” Kean said. “And that’s a bad recipe for these post-fire debris flows. The reason you can expect one just about every year is because it doesn’t take very much rain to cause one. The rainstorms that can trigger debris flows – they’re kind of garden-variety storms.”