Table of Contents Why should I care? What should I do? What should I know?

Putting down roots in earthquake country (a service of SCEC)



Other earthquake-related hazards in Southern California

The previous pages have described where earthquakes have happened, the many faults in Southern California capable of large earthquakes, and the expected shaking from future earthquakes. In addition to these regional aspects of the earthquake hazard, there are location-specific hazards that can cause additional damage: surface rupture, liquefaction, and landslides. The California Geological Survey produces maps that identify Earthquake Fault Zones and Seismic Hazard Zones where these hazards may occur. State laws require that every person starting to “put down roots” by buying a home or real property in California be told if the property is in one of these zones.

Larger imageRed zones are Earthquake Fault Zones with potential for surface faulting. Blue and green zones are Seismic Hazard Zones with a potential for landslides and liquefaction, respectively. Shaded areas indicate future Seismic Hazard evaluation and zoning.

Earthquake Fault Zones (EFZs) recognize the hazard of surface rupture that might occur during an earthquake where an active fault meets the earth's surface. Few structures can withstand fault rupture directly under their foundations. The law requires that within an EFZ most structures must be set back a safe distance from identified active faults. The necessary setback is established through geologic studies of the site. EFZs are narrow strips along the known active surface faults wherein these studies are required prior to development. Being located in an EFZ does not necessarily mean a building is on a fault. Most of the important known faults in California have been evaluated and zoned, and modifications and additions to these zones continue as we learn more.

Larger imageEarthquake Fault Zone (EFZ):
see "sample area" above

 

Seismic Hazard Zones (SHZs) identify areas that may be prone to liquefaction or landsliding triggered by earthquake shaking. Liquefaction is a temporary loss of strength in the ground that can occur when certain water saturated soils are shaken during a strong earthquake. When this occurs buildings can settle, tilt, or shift. Landsliding can occur during an earthquake where shaking reduces the strength of the slope. These hazards can usually be reduced or eliminated through established engineering methods. The law requires that property being developed within these zones be evaluated to determine if a hazard exists at the site. If so, necessary design changes must be made before a permit is granted for residential construction. Being in an SHZ does not mean that all structures in the zone are in danger. The hazard may not exist on each property or may have been mitigated. Mapping new SHZs in urban and urbanizing areas is ongoing statewide.

Current zones, as established by the California Geological Survey, are indexed at www.consrv.ca.gov/cgs.

Myth #3: Don't be fooled!

"AND THE EARTH OPENED..."

A popular literary device is a fault that opens during an earthquake to swallow up an annoying character. But unfortunately for principled writers, gaping faults exist only in novels. The ground moves across a fault during an earthquake, not away from it. If the fault could open, there would be no friction. Without friction, there would be no earthquake.

Larger imageSeismic Hazard Zone (SHZ):
see "sample area" above

LA County Juvenile Hall, Sylmar, California, damaged by liquefaction during the magnitude 6.6 San Fernando earthquake of February 9,1971. The broken floor is due to settling of the earth and is not the fault itself. Photo by Jack Meehan, structural engineer.


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