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NR 2003-17
July 2, 2003

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

FOUR ALAMEDA COUNTY SEISMIC HAZARD ZONE MAPS NOW OFFICIAL
Liquefaction, Landslide Potential Affects New Construction

SACRAMENTO – Four Seismic Hazard Zone maps – affecting Oakland, Hayward, Union City, Fremont, Castro Valley, San Lorenzo, Newark and other communities -- became official today. The maps, issued by the Department of Conservation’s California Geological Survey, impact planners, developers, property sellers and real estate agents.

If property is located in a Zone of Required Investigation, where liquefaction or earthquake-induced landslides could occur during a large earthquake, 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.

“These maps help improve public safety by ensuring these earthquake hazards are taken into account during new construction,” DOC Director Darryl Young said.

While strong shaking causes most of the damage during an earthquake, liquefaction and landslides are an additional danger. Seismic Hazard Zone maps show areas at risk of these hazards during an earthquake.

The 1989 Loma Prieta earthquake caused extensive liquefaction-related damage in San Francisco’s Marina District. 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. Loma Prieta also caused landslides that blocked two lanes of Highway 17 and damaged residences in the Santa Cruz Mountains.

Retrofitting existing structures to minimize the impacts of liquefaction or landslides generally isn’t as cost-effective as building in safety features at the design stage. Thus, these maps are aimed primarily at new construction. They require that design changes -- such as deep foundations in liquefaction zones and slope stabilization in landslide zones – be made in the planning stage.

“It easier and less expensive – not to mention better for public safety -- to institute design changes as a precaution in the construction phase than to rebuild after liquefaction or landslide damage,” Young said.

With these new maps, the California Geological Survey has issued 89 official Seismic Hazard Zone Maps, 72 for Southern California and 17 for the Bay Area. Another 17 maps are in various stages of public review.

Each map covers about 60 square miles. Here's a closer look at what each new Alameda County map shows:

♦ Hayward quadrangle – There are widespread landslide zones in the hilly regions east of the Hayward Fault, covering about 22 percent of the quadrangle. There are liquefaction zones covering most of the low-lying areas in the southwestern quarter of the quadrangle. A small segment of this quadrangle located in Contra Costa County was not zoned. Some or all of the communities of Oakland, Hayward, Ashland, San Lorenzo and Castro Valley are in this quadrangle.

♦ Mountain View quadrangle – A liquefaction Zone of Required Investigation covers most of the mapped portion of this quadrangle. There are no landslide zones on the map. The portion of this quadrangle located in Santa Clara County was included in an official map released last year. The new map now incorporates the Alameda County portion. The cities of Fremont and Newark fall within the Alameda County portion.

♦ Newark quadrangle – Most of the quadrangle consists of flatlands, tidal marshes and salt evaporation ponds along the eastern margin of San Francisco Bay. The map area includes the communities of Newark, Hayward, Union City and Fremont. The Hayward Hills extend into the northeastern corner. The liquefaction zone is spread across most of the lowland terrain, including most of the land surrounding Alameda Creek. There are small landslide zones in the Hayward and Coyote hills.

♦ Redwood Point quadrangle – Only about five square miles of Alameda County land – including parts of Hayward and Fremont -- are included in this map, which also covers part of San Francisco Bay and San Mateo County. The liquefaction zone covers all of the land in the Alameda County portion of the quadrangle; there is no hilly terrain and thus no landslide zone.

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. Mapping work is ongoing in several Bay Area locales, including Fremont, Morgan Hill, San Jose, Palo Alto, Portola Valley and Menlo Park.

Color copies of official maps can be purchased through DOC's California Geological Survey (415) 904-7707 or (916) 445-5716. The maps also can be viewed and downloaded on the Web at http://gmw.consrv.ca.gov/shmp.

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