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NR 2003-26
September 24, 2003

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

CALIFORNIA GEOLOGICAL SURVEY RELEASES
TWO SEISMIC HAZARD ZONE MAPS FOR L.A. COUNTY

SACRAMENTO – The state today released two maps that identify parts of Los Angeles County that must take precautions to avoid the potentially devastating affects of large earthquakes.

The Department of Conservation’s California Geological Survey produced these Seismic Hazard Zone maps, which improve public safety and impact planners, developers and real estate transactions.

The maps define zones where there is evidence that liquefaction or landslides are more likely to take place during damaging earthquakes, generally those with a magnitude of 5.5 or greater. Liquefaction occurs when water-saturated sandy soil is shaken and, much like quicksand, temporarily cannot support buildings or other heavy structures. Liquefied soil can cause the ground to crack and move, resulting in damage to structures, buried pipelines and utilities.

One of the new maps is of the Acton Quadrangle in central Los Angeles County, centered about 20 miles east of the Santa Clarita Civic Center and 27 miles north of the Los Angeles Civic Center. The other is of the Pacifico Mountain Quadrangle; the center of the area is 10 miles south of Palmdale and 30 miles north of downtown Los Angeles.

If property is located in a Zone of Required Investigation, the local building department must require geologic studies before projects are issued permits. Also, property sellers and real estate agents must inform buyers if property they're selling is in a Seismic Hazard Zone, as is the case when property is in a designated flood zone.

“Knowing where these hazards are improves public safety,” DOC Darryl Young said. “Once you define the problem, you can take steps to minimize the danger.”

In many cases, it is cost effective to retrofit houses and buildings to minimize the effects of severe shaking, which causes most of the damage in big earthquakes. Local public libraries have a number of publications that can be used as guides to making homes more earthquake-ready.

It is generally not as cost effective to retrofit an existing building for the impacts of liquefaction or landslides as it is to build in safety features at the design stage. Thus, design changes to better protect life and property during future earthquakes -- such as deep foundations in liquefaction zones and slope stabilization in landslide zones – are required before new developments are approved and built.

“It’s better for public safety, as well as easier and less expensive for builders, to incorporate design changes and make buildings more resistant to these hazards in the construction phase than to rebuild after liquefaction or landslide damage,” Young said.

The Acton Quadrangle map defines liquefaction zones in the bottoms of Soledad Canyon and its major tributary canyons, where significant development is occurring. Landslide zoning covers nearly a fifth of the evaluated portion of the quadrangle (land inside the Angeles National Forest is not zoned). There are numerous steep slopes where rock falls and slides could be triggered by earthquakes.

Nearly the entire Pacifico Mountain Quadrangle is in rugged San Gabriel Mountain terrain. Development at this time is limited to ranching and rural residential areas, although there is a great deal of recreational use. The liquefaction zones are limited to the bottoms of the area’s numerous canyons. About one-fifth of the evaluated area is in landslide zones.

With these new maps, the California Geological Survey has issued 91 official Seismic Hazard Zone Maps, 75 for Southern California and 18 for the Bay Area. Another 13 maps, including several for Los Angeles and Ventura counties, are under review.

DOC/California Geological Survey geologists use computer models as well as analyses of existing geological mapping and hundreds of engineering borings to produce the maps, which are drawn on a scale where one inch equals 2,000 feet.

Color copies of official maps can be purchased through DOC's California Geological Survey at (213) 239-0878, (916) 445-5716, or (415) 904-7707. The maps also can be viewed and downloaded on the Web here.

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