News Release #2008-07
March 18, 2008
Contact: Don Drysdale
Ed Wilson
916-323-1886
SACRAMENTO " The California Geological Survey (CGS) has released preliminary Seismic Hazard Zone maps showing areas in western Alameda County with potential for earthquake-induced landslides and liquefaction. The maps designate areas where special precautions may have to be implemented in new development and construction projects to protect life and property in the event of a large quake.
"Large, damaging earthquakes are inevitable in California," State Geologist Dr. John Parrish, head of CGS, said. "However, new construction projects within the zones outlined on these maps will be better prepared to withstand the next ‘big one' than they would have otherwise been."
Communities covered fully or in part by the maps include Livermore, Pleasanton, Dublin, Castro Valley and Hayward. Each map covers an area about 60 square miles in size known as a "quadrangle." The maps will become official after a 90-day review opportunity for governments and geotechnical experts, followed by a 90-day revision opportunity for CGS.
Earthquakes of magnitude 5.5 or greater can trigger landslides or liquefaction, a phenomenon in which soil temporarily loses its ability to support structures. Liquefaction caused underground gas pipes to rupture in San Francisco's Marina District during the 1989 Loma Prieta earthquake and leaking gas fueled a large, hard-to-extinguish fire. The temblor also produced landslides that blocked Highway 17 for days. The Seismic Hazards Mapping Act mandating the maps was passed the year after Loma Prieta.
Seismic Hazard Zone maps are aimed at new development, as beginning construction with safety features in mind is usually easier and more cost effective than retrofitting an existing building for the impacts of liquefaction or landslides.
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. Examples of these modifications include deep foundations in liquefaction zones or slope stabilization in landslide zones.
"It's relatively cost-effective and simple to take pre-construction measures to minimize the risk to life and property once we define the areas with the highest potential for these hazards," said Charles Real, CGS supervising geologist in charge of the mapping program.
Property sellers and real estate agents must inform potential buyers if property they're selling is in a seismic hazard zone, as is the case when property is in a designated flood zone.
For those in existing homes or structures, the Governor's Office of Emergency Services, American Red Cross and FEMA have all released publications about becoming more earthquake-ready. Those publications are available online or at local public libraries.
The Seismic Hazard 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 covering all or portions of nine counties. Upon the official release of the newest two maps, CGS will have completed 26 seismic hazard zone maps in the greater Bay Area.
In creating Seismic Hazard Zone Maps, CGS uses Geographic Information System (GIS) technology, which allows the manipulation of three-dimensional data. Information analyzed in these studies includes topography, surface and subsurface geology, borehole log data, recorded ground-water levels, existing landslide features, slope gradient, rock-strength measurements, geologic structure, and probabilistic earthquake shaking estimates.
What the new maps show:
Dublin Quadrangle: About 10 square miles of land in the Dublin Quadrangle, 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.
About the California Department of Conservation: 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.
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