December 12, 2008

Contact: Don Drysdale
              Ed Wilson


SACRAMENTO – With rain in the forecast, the California Geological Survey (CGS) today reminded Southern California residents – especially those in areas impacted by recent wildfires -- to be aware of the potential for landslides and mudslides.

“If you live on, below or atop a slope, especially in areas where recent fires have burned away plant life, be alert when it’s raining,” advised geologist Charles Real, who heads CGS’ landslide mapping activities under California’s Seismic Hazards Mapping Act. “Check for new cracks in soil, bare hills or material moving downhill. Any unusual rumbling sounds may indicate that soil is giving way.”

A landslide is any mass of earth and rock that moves downhill by sliding, flowing or falling. Large, slow-moving landslides composed of bedrock can cause extensive property damage, while debris flows, commonly called mudslides, move very quickly.

“When it’s raining hard, the single most important action you can take is to avoid sleeping in lower-floor bedrooms on the sides of houses that face hazardous slopes,” Real said.

More than 100 Californians have been killed by debris flows during the past 25 years. Most of those deaths occurred when debris flows buried people sleeping in lower-floor bedrooms adjacent to slopes that gave way.

Landslides can happen at any time but are more likely once the rainy season begins. Fire damage causes soil to become less absorbent because ash clogs pores in the soil and oil is given off by burning plants, laying down a water-resistant veneer. As a result, rainwater runs off faster instead of sinking into the ground and erosion increases.

Areas that have experienced landslides in the past are more susceptible to repeat occurrences and regions that received significant amounts of rain last year have an increased potential for a landslide this winter. Exceptionally heavy rain – three or more inches a day or a quarter-inch an hour – also increases the likelihood of a landslide because soil becomes waterlogged. Homeowners should take precautions to make certain house gutters are clear of debris and properly functioning, and runoff water is properly channeled to storm drains -- particularly in terraced hillside developments.

“Californians are well aware of the dangers of wildfires and earthquakes,” Real said. “Although landslides don’t impact widespread areas like those other phenomena, they can be locally devastating and they’re much more common than damaging earthquakes. Fortunately, whereas earthquakes don’t give us any advance warning, landslides do. Knowing the warning signs can save lives and property.”

When California rainfall exceeds normal levels by 150 percent or more, landslides cause, on average, property damage in excess of $100 million and as many as five deaths each year. 

CGS, part of the California Department of Conservation, provides technical information and advice about landslides and other geologic hazards to the public, local governments, agencies and industries that make land-use decisions in California.
More information about landslides – including tips for residents and mitigation steps – can be found here​.