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NR 2003-04
February 20, 2003

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

SIX EAST BAY SEISMIC HAZARD ZONE MAPS NOW OFFICIAL
Liquefaction, Landslide Potential During Quakes is Highlighted

Note: To view the liquefaction map for parts of Oakland and Berkeley, click here.

SACRAMENTO – Six Seismic Hazard Zone maps – affecting the communities of Oakland, Berkeley, Hayward, Alameda, San Leandro, Emeryville, Piedmont and Albany -- became official today. The maps, issued by the Department of Conservation’s California Geological Survey, impact planners, developers, property sellers and real estate agents.

One preliminary map for part of San Jose and Santa Clara County also was released. The Santa Teresa Hills map will become official after a six-month review.

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. 

“The release of these maps will increase awareness of these earthquake-related hazards and prompt better construction practices, thus improving public safety,” DOC Director Darryl Young said.

Strong shaking causes most of the damage during an earthquake. Seismic Hazard Zone maps, produced by DOC's California Geological Survey, show areas at risk from landslides and liquefaction during an earthquake of magnitude 6.0 or greater.

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 sand 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.

“These proactive changes can lessen the impact of seismic hazards and better protect life and property during future earthquakes,” Young said.

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

♦ San Leandro quadrangle: The San Francisco Bay occupies about half of the area on this map. However, parts of the cities of Oakland, Alameda, San Leandro and Hayward are shown, as well as some unincorporated portions of Alameda County. Liquefaction zones cover most of the mapped land area, from the Oakland Coliseum south to the Salt Evaporators, and from the east bay coastline east to the MacArthur Freeway (580). The northeastern corner of the quadrangle includes some landslide zones on the East Bay Hills.

♦ Richmond quadrangle: The Richmond map zones were only done for Alameda County, covering the southern 20 percent of the map. A liquefaction zone covers a small area in Albany from the Golden Gate Fields area east to San Pablo Avenue, from Cedar Street north to about Solano Avenue. Also, a narrow zone follows a creek bed easterly from San Pablo Avenue to Hopkins Street. Broad landslide zones occur in the Grizzly Peak areas of Cragmont, and some areas in the Thousand Oaks areas. Small landslide areas occur on the slopes of Albany Hill along Interstate 80.

♦ Oakland East and Oakland West: Liquefaction zones cover all of Alameda and western Oakland from the bay coastline to a few blocks east of Highway 24. Liquefaction zones have been mapped along the Nimitz Freeway across from Alameda, spreading out southeast to the Oakland Coliseum. The zones extend along the beds of Samsal Creek and Peralta Creek. Landslide zones have been mapped in the Oakland and Berkeley hills. The zones extend from the UC Berkeley campus northeast of Ashby Avenue to a two-mile wide region between Skyline Blvd and the MacArthur Freeway southeasterly through Knowland Park to San Leandro.

♦ Briones Valley: No Zones of Required Investigation for liquefaction were mapped in the Alameda County portion of this quadrangle. However, historically the combination of dissected terrain and weak rocks has produced abundant landslides. A landslide zone covers about 55 percent of the Alameda County land area in the quadrangle.

♦ Hunters Point: No Zones of Required Investigation for landslides were mapped in the Alameda County portion of this quadrangle. While the Hunters Point Quadrangle covers approximately 60 square miles in Alameda, San Francisco, and San Mateo counties, San Francisco Bay occupies most of the area. An official Seismic Hazard Zone map covers the portions of the City and County of San Francisco in this quadrangle. The Alameda County land shown on the new map consists of approximately half a square mile at the north end of Bay Farm Island, which includes a southern section of the City of Alameda. The liquefaction zone covers the entire land area of Bay Farm Island.

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 Palo Alto, Morgan Hill, north Hayward and Gilroy.

Black and white copies of preliminary maps can be purchased from BPS Reprographic Services in San Francisco, (415) 495-8700. 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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