6. Determine if You Live or Work in Particularly Hazardous Areas

Even reasonably detailed maps give only an overview of potential for shaking, liquefaction, landsliding, faulting, and damage. To be sure about a particular building site, you should consult an engineering geologist, geotechnical engineer, or foundation engineer.

Particularly severe damage is likely where structures are built directly on top of active faults. The Alquist-Priolo Special Study Zones Act of 1972 required the California Department of ConservationÕs Division of Mines and Geology to map all known active faults in California and to designate areas within 500 feet of these faults as Special Study Zones. Buildings for human occupancy must be at least 50 feet away from an active fault trace. Significant development in these zones can proceed only after geologic studies are done to ensure that structures are not placed directly on top of ground likely to rupture during major earthquakes. Most realtors have maps showing these fault zones, and they are required to inform you if you are considering buying land within a Special Study Zone. You can learn more about these zones and how to obtain detailed maps by ordering Special Publication 42 from the Division of Mines and Geology. You may also be able to examine these maps at your local government planning office or building department.

This portion of U.S. Geological Survey Map I-1257-G shows the potential for liquefaction during an earthquake. The broad, dark-reddish area shows where there is a potential for liquefaction. Where holes have been drilled into the soils, the sites are shown as bright red if a liquefiable layer was found and yellow if no liquefiable layer was found. This map demonstrates how it is possible to determine risk on a site-by-site basis.

Earthquake risk is high throughout the Bay Area, but the risk is particularly high in regions with steep slopes prone to landsliding, on soft soils, and near faults. The map on pages 12 to 13 provides a regional overview of the general areas where the greatest hazards exist. The four types of geologic conditions shown are simplified, and the boundaries are imprecise, but the map can alert you to potential problems in your area. If you are concerned, you may be able to get more detailed information from maps listed in additional materials and from your local government planning office or building department. None of the available maps can substitute for an examination of your particular site by a geologist or geotechnical engineer.

Building new structures and reinforcing older structures throughout the Bay Area should involve careful attention to seismic-resistant design and construction. If surveys show that your site is at higher risk than typical, you will probably be required by your building department to provide for the specific hazards of your site in your design and construction practices even though such provisions may increase the costs. Given the high probability of earthquake damage in the next few decades, it is prudent for all residents to learn more about whether they may face particularly high risks because of location.