Geological Hazards

Geological hazards affect how a house performs in an earthquake. Many people think that the proximity of a house to an earthquake fault is the principal geological hazard, but strong ground shaking can occur miles from an earthquake epicenter or a fault line.

Listed below are high-hazard areas:

• Fault zone--Quarter-mile-wide Special Study Zones have been designated around major earthquake faults. In a large quake, the ground in these zones may rupture. Structures in these zones may be seriously damaged, especially if they are built on a fault line. Maps of these Special Study Zone areas can be obtained from the California Division of Mines and Geology or from your local building department.
• Geological hazard zones--In these areas, a quake may cause intense ground shaking, settlement, landsliding, or liquefaction (earthquake-induced flow of water-saturated sandy soils). Unconsolidated, sandy soils, which underlie a large portion of the Bay Area's heavily developed areas, can shake much more strongly in an earthquake than firmer soils nearby. Many counties have geologists who may have specific data on stability of land parcels. A history of landslide, settlement, or major soil problems on a land parcel may be known to the county geologist.

You may want to consult with selected professionals about the potential for earthquake-induced landslides on or near your property. For instance, if your land parcel has been cut or filled, it is a good idea to hire a geotechnical engineer to evaluate the property. If your parcel is on a natural steep slope, confer with an engineering geologist about potential slope stability problems.

Maps showing hazardous areas are included in the Planning Scenarios for the San Andreas and Hayward Fault (Special Publications 61 and 78, respectively), which can be obtained from the California Division of Mines and Geology. The United States Geological Survey has similar maps.

• Flood and inundation zones--Areas that are subject to inundation from large storms or earthquake-ruptured dams and reservoirs have also been mapped. City or county planning departments, as well as local Offices of Emergency Services, have these maps.

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