NR 2007-12
May 25, 2007

Contact: Don Drysdale
Mark Oldfield
Carrie Reinsimar
(916) 323-1886


SACRAMENTO – The California Geological Survey (CGS) today released its latest landslide inventory map, to protect public safety and guide future development on the Palos Verdes Peninsula.

“People don’t associate the postcard-perfect weather we’re enjoying now with landslides,” said State Geologist Dr. John Parrish, head of CGS. “But the mechanics of landslides are complicated, and they don’t necessarily care whether it’s 90 degrees or pouring rain when they start to slip. This map is a reminder of the need to be aware of the potential for landslides and to take them into account in the land development process.”

CGS, a branch of the California Department of Conservation (DOC), has created several varieties of landslide maps over the years. The landslide inventory series is the most detailed. CGS hopes to release about 100 of the maps over the next several years.

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, and the type of movement involved (some landslides are more destructive than others). CGS produces the maps by incorporating previous mapping work with a detailed review of aerial photography and geologic fieldwork.

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 and can be a few feet to hundreds of feet thick. 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.

“While the potential for landslides sometimes can be mitigated, many times the best thing to do is to avoid building on or near them,” said CGS Supervising Geologist Chuck Real, who oversees the mapping program. “The City of Rancho Palos Verdes has recognized that reality, as evidenced by its restrictions on development on the Portuguese Bend Landslide. What communities need to know is where the landslides are, how they move, and how recently they’ve moved, and that’s what these maps show.”

The Palos Verdes Peninsula map released today defines 175 landslides of different types, 49 of them active. The best known is the Portuguese Bend landslide. The active movement impacts about 80 acres, and one section has moved more than 600 feet since 1956.

“Local agencies and communities on the Palos Verdes Peninsula are well aware of their landslide issues and established Geologic Hazard Abatement Districts in the 1980s to mitigate hazards in some of the areas,” DOC Director Bridgett Luther said. “Our new map is a ‘one-stop shopping’ overview of the entire peninsula at a user-friendly scale (one inch equals two thousand feet). It also places the well-known and destructive landslides in a regional context along with other lesser-known, but potentially damaging, landslides.”

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 and in rare cases, up to 100 mph.

“Everyone knows that earthquakes present a high potential for damage, but landslides occur more frequently and can be just as devastating as an earthquake in a given locale,” Parrish noted. “In the past few years, the La Conchita and Bluebird Canyon landslides have been reminders of the old saying that ‘civilizations exist by geologic consent.’”

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 here or from  the United States Geological Survey.  

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

In addition to studying and mapping earthquakes and other geologic phenomena, the California Department of Conservation categorizes mineral resources; administers agricultural and open-space land conservation programs, ensures the reclamation of land used for mining; promotes beverage container recycling; and regulates oil, gas and geothermal wells.