FOR IMMEDIATE RELEASE
September 4, 2008
Contact: Ed Wilson
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SACRAMENTO – Following a 90-day technical review period, two maps that establish earthquake hazard zones in the East Bay Area are now official. The maps designate areas where special precautions may have to be implemented in new development and construction projects in order to protect life and property in the event of a large quake.
“These maps are similar to those that define potential flood or wildfire areas,” said State Geologist Dr. John Parrish, head of the California Geological Survey, which issued the maps. “Although the maps prompt a disclosure element in real estate transactions for existing properties, the earthquake hazard zones are created with an eye toward future construction and are part of our efforts to be prepared.”
Communities covered fully or in part by the maps include Livermore, Pleasanton, Dublin, Castro Valley and Hayward. Each map covers an area of about 60 square miles known as a “7.5-minute quadrangle.”
Earthquakes of magnitude 5.5 or greater can trigger landslides or liquefaction, a phenomenon in which soil temporarily loses its ability to support structures. The Seismic Hazards Mapping Act mandating the maps was passed after the 1989 Loma Prieta earthquake, which triggered significant landslide and liquefaction damage throughout the Bay Area.
The maps establish Zones of Required Investigation. For development within a zone, the local building department must require – before permits are issued – that a registered geologist investigate the site for evidence of liquefaction or landslide potential. If such evidence is found, design modifications must be made in the planning stage.
“It is much easier and more cost effective to address landslide and liquefaction hazards during construction than to retrofit,” said Charles Real, supervising geologist in charge of the mapping program. “Construction modifications include deep foundations in liquefaction zones and slope stabilization in
For individuals in existing homes or structures, the Governor’s Office of Emergency Services, American Red Cross and FEMA all have released publications about becoming more earthquake-ready. Those publications are available online or at local public libraries.
The Seismic Hazards Zonation Program has identified 345 California communities as high-risk areas for liquefaction and/or landslides; 171 have been zoned. There are now 115 official Seismic Hazard Zone maps – 26 in the Bay Area – covering all or portions of nine counties.
CGS also announced the release of a preliminary Seismic Hazard Zone map for the Altamont Quadrangle, which includes part of the City of Livermore east of the City Civic Center complex. Like the maps released today, this map will become official after a 90-day review opportunity for governments and geotechnical experts, followed by a 90-day revision opportunity for CGS.
What the new maps show:
Dublin Quadrangle: About 10 square miles of land, mainly east of Highway 680, are in Zones of Required Investigation for liquefaction. The zones run from the border with Contra Costa County in the north to the county fairgrounds in the southeast, with a finger continuing south along Arroyo de la Laguna. There are additional liquefaction zones on the floors of several canyons flowing to the west, such as Dublin and Hollis canyons, and along Palomares Creek. Borehole logs of test holes drilled in Livermore Valley indicate the widespread presence of near-surface soil layers composed of saturated, loose sandy sediments.
Roughly 30 square miles of hilly land in the southwestern half of the Dublin Quadrangle are designated as Zones of Required Investigation for earthquake-induced landslides. Portions of Castro Valley and the Castlewood Country Club also are included in landslide zones.
Livermore Quadrangle: About 19 square miles of land are designated as Zones of Required Investigation for liquefaction. The liquefaction zones encompass about two-thirds of the Livermore Valley floor and most of the stream valleys and canyons leading into the surrounding hills. Liquefaction zones include much of downtown Livermore, from Highway 580 to just north of Bess Avenue; Livermore Airport; the Santa Rita Rehabilitation Center; and city hall as well as Amador Valley High School in Pleasanton.
The combined total area within the Livermore Quadrangle designated as Zones of Required Investigation for earthquake-induced landslides is roughly 15 square miles. Most of the zones are concentrated in two separate hilly areas, one north of Highway 580 in the Mount Diablo foothills to the Contra Costa County border, the other southeast of downtown Pleasanton.
Altamont Quadrangle (preliminary): About nine of the 20 square miles of valley floor within the
Altamont quadrangle are designated Zones of Required Investigation for liquefaction, within which there is evidence of the widespread presence of near-surface soil layers composed of saturated, loose sandy and silty sediments.
About 14 of the 40 square miles of upland area within the quadrangle are designated Zones of Required Investigation for earthquake-induced landslide hazard. Most are concentrated in the northeast, southeast, and southwest parts of the quadrangle, where slopes are generally steeper and/or rock strengths are generally weaker. Numerous historical landslides, including two of the largest mapped in the quadrangle, occur along the trace of the active Greenville Fault.
About the California Department of Conservation (DOC): In addition to studying and mapping geologic phenomena such as earthquakes, DOC 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.