Your Local Risks · Hazard Guide
Landslides and debris flows kill 25 to 50 people in the U.S. every year and cause billions in property damage. The triggers are heavy rain, earthquakes, and wildfire burn scars. The warning signs are visible if you know what to watch for.
Understanding the hazard
Landslides happen when the forces pulling a slope downhill exceed the forces holding it in place. Three events commonly tip that balance: prolonged heavy rainfall that saturates soil and adds weight, earthquakes that shake loose material already at its stability limit, and wildfire that destroys the root systems holding hillsides together.
Post-wildfire debris flows are particularly dangerous because they can be triggered by relatively modest rainfall on burned slopes. The soil, stripped of vegetation and hardened by heat into a water-repellent layer, channels water and loose material into valley bottoms at high speed. The 2018 Montecito debris flow killed 23 people when rain hit the Thomas Fire burn scar.
The 2014 Oso landslide in Washington buried an entire community under a wall of mud traveling 600 mph. Forty-three people died. Geological surveys had flagged the slope as unstable years earlier. Risk assessment, early warning recognition, and evacuation planning are the only defenses.
Know your exposure
Your risk depends on slope, soil, vegetation, and proximity to known slide areas. These indicators help you assess your exposure.
Living on, below, or adjacent to a steep slope, especially with exposed soil or rock. Slopes steeper than 25 degrees are highest risk. Cut slopes from road construction can be especially unstable.
Living below or in a valley downstream of a recent wildfire. Post-fire debris flow risk is highest in the first two rainy seasons after a fire. USGS publishes post-fire debris flow hazard assessments for major wildfires.
Extended wet seasons, multiple storms in sequence, or rapid snowmelt saturate soil and increase pore pressure. Springs or seeps appearing on slopes that were previously dry are a warning sign.
Slopes that have failed before will fail again. Your county geological survey, USGS landslide inventory, and neighborhood history all provide data. Ask long-time neighbors about past slope movement.
The key skill
Landslides often telegraph their arrival. Watch for these indicators during and after heavy rain, especially if you live near slopes.
Cracks in your foundation, driveway, retaining walls, or the ground surface near a slope. Cracks that widen over hours or days indicate active movement.
Trees, utility poles, or fences that were vertical and are now leaning. This indicates the ground beneath them is moving.
Streams suddenly turning muddy, water seeping from hillsides where it never has before, or water levels in creeks rising or falling abnormally. All suggest subsurface water movement that may be destabilizing a slope.
Rumbling, cracking, or snapping sounds from uphill, especially at night or during rain. If you hear these, do not investigate. Move away from the slope and to higher ground on a stable surface. Alert neighbors.
Official resources
Next step
Post-fire debris flows are the fastest-growing landslide risk category. If you live downhill from fire-prone terrain, wildfire preparedness and landslide preparedness overlap significantly.
Wildfire preparedness"The best time to repair the roof is when the sun is shining."
— John F. Kennedy
Go deeper
Affiliate disclosure: New World Survival earns a small commission on purchases made through links on this page, at no cost to you. We only recommend gear we'd put in our own kit.