NR 2005-17
August 16, 2005

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


Note: The map can be viewed here.

SACRAMENTO – The Department of Conservation’s California Geological Survey today released a new regulatory Seismic Hazard Zone map that shows the risk of liquefaction and landslides in portions of Los Angeles and Orange counties, as well as a county not previously zoned, San Bernardino.

“Seismic Hazard Zone maps are good examples of the state’s commitment to earthquake preparedness,” said Mike Chrisman, Secretary for Resources. “They are important tools in the effort to protect public safety.”

Liquefaction and landslides are potential side effects of earthquakes in the magnitude 5.5 or greater range that can cause destruction over and above the damage done by shaking.

“This map identifies the areas where liquefaction and landslides are more likely to occur,” said State Geologist John Parrish, head of the California Geological Survey. “With that knowledge, local planners and the building community can take steps to minimize the danger by ensuring that new construction takes into account not only that we’re in earthquake country, but also that there are concerns other than shaking.”

The Yorba Linda Quadrangle map covers approximately 60 square miles in eastern Los Angeles, northern Orange and western San Bernardino counties. The Orange County portion includes parts of the cities of Anaheim, Brea, Fullerton, Placentia and Yorba Linda. The community of Chino Hills in San Bernardino County is covered. Within Los Angeles County, most of Diamond Bar and a small part of the City of Industry are covered in the northern part of the map.

Earthquake-induced landslide zones encompass more than a third of the map due to a combination of hilly terrain and relatively low rock strength. Liquefaction zones are restricted to stream channels and their adjacent floodplains, some canyon bottoms, and a small area in the map’s northwest corner.

Liquefaction – which occurs when water-saturated, sandy soil is shaken violently and temporarily loses its ability to support structures – was a major cause of damage in the Kings Harbor area of Redondo Beach during the Northridge earthquake of 1994. The Northridge earthquake also caused more than 11,000 landslides, some of which damaged structures or blocked roads.

If the site-specific geotechnical studies conducted prior to development reveal that liquefaction or landslide hazards are present, mitigation measures are incorporated into development plans.

The program has identified about 345 California communities as high-risk areas for liquefaction and/or landslides; 160 have been zoned. There are now 109 Seismic Hazard Zone maps covering all or portions of eight counties.

To establish its priority list, the program looks at the level of seismic hazard in each locale as well as the amount of new development going on. Work is ongoing in Riverside County.

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