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NR 2006-27
December 4, 2006

Contact: Ed Wilson
Don Drysdale
Mark Oldfield
(916) 323-1886

CALIFORNIA GEOLOGICAL SURVEY RELEASES NEW LANDSLIDE MAPS Residents reminded that rain, earthquakes can trigger destructive slides


SACRAMENTO – As the rainy season approaches, the California Geological Survey (CGS) is releasing five landslide maps intended to protect public safety by guiding future development.

“Wet weather is just around the corner, so people need to be aware of the potential for landslides,” said Supervising Geologist Chuck Real, head of CGS’ Seismic Hazards Zonation Program. “The most likely places for landslides to occur are the places where they’ve happened in the past. By providing that information, these maps will help raise awareness of the need to prepare and to plan.”

These are some of the most detailed landslide maps produced by CGS, showing slides that can be triggered either by earthquakes or by heavy rain. In the coming years, CGS hopes to release a total of about 100 of these “landslide inventory” maps. Three of the first five cover parts of the Bay Area – in and around Morgan Hill and San Jose -- while two cover areas in rural Los Angeles County east of the rapidly developing Santa Clarita area.

Although the maps are non-regulatory, they provide important information to local planners, decision-makers and geotechnical consultants: whether a landslide is considered active or dormant, the direction of movement, the type of movement involved (some landslides are more destructive than others), as well as CGS’ confidence level in their interpretation of the data. CGS, a branch of the California Department of Conservation, produces the maps by incorporating previous mapping work with a detailed review of aerial photography and geologic fieldwork.

“Big earthquakes tend to be more damaging than individual landslides, but landslides are a more frequent phenomena and cause a remarkable amount of damage in California,” said State Geologist Dr. John Parrish, head of CGS. “The potential for landslides can sometimes be mitigated. However, in most cases, it is best to avoid building on or near them. The first steps are determining where they are, how they move, and how often they move. That’s where these maps are helpful.”

A landslide is any mass of earth or rock that slides, flows and/or falls downhill. Landslides can affect land from a few square yards to hundreds of acres in area. They can be a few feet to hundreds of feet thick.

In January of 2005, a landslide claimed 10 lives in the Ventura County community of La Conchita. In June of 2005, a landslide in Laguna Beach’s Bluebird Canyon caused no fatalities but forced the evacuation of more than 1,000 people from about 350 residences. Fifteen homes were destroyed and several others severely damaged.

Large, slow-moving landslides composed of bedrock can cause extensive property damage but usually do not result in loss of life. A debris flow (commonly called a mudslide) is a more dangerous type of slope failure because it is fast moving and can cause both property damage and injuries. Mud, rock and debris caught by these rapid flows can travel faster than 10 mph – in rare cases, up to 100 mph.

Those living on or below slopes are advised to know the signs of impending landslides, to check for them, and to contact local authorities if they are concerned about the potential for landslides.

“If it rains heavily for several consecutive days, it’s prudent to watch and listen for location-specific landslide warnings from federal, state and local agencies,” Parrish advised.

CGS provides technical information and advice about landslides, erosion, sedimentation, and other geologic hazards to the public, local governments, agencies and industries that make land-use decisions in California. More information about landslides, links to landslide maps, and mitigation steps can be found on CGS pages here and United States Geological Survey pages here

“In recent years, there have been several cases of landslides occurring in areas where summer wildfires have burned off the ground cover, making the soil more vulnerable to flowing after heavy rains,” Parrish said. “Many factors can contribute to the formation of landslides aside from rainfall, including improper construction or grading, earthquakes, weak or loose rock and soil, and steep slopes.”

Property owners are advised to consult a licensed geologist before taking any steps intended to mitigate potential risks or harm associated with landslides.

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