NR 2002-46
November 1, 2002

Contact: Ed Wilson
Mark Oldfield
Don Drysdale
(916) 323-1886

Maps Ensure that Construction Doesn’t Occur on Active Faults

SACRAMENTO – Fourteen new or revised Earthquake Fault Zone maps that direct construction away from areas where earthquake faults have broken through to the ground’s surface were released today by the California Department of Conservation.

The maps affect Riverside, San Bernardino and San Diego counties and the cities of San Diego, Coronado, Corona and Chino Hills.

The first new “Alquist-Priolo” maps in three years from DOC’s California Geological Survey are screening tools. They determine whether extra care should be taken to ensure new construction does not take place on top of active faults, those that have displaced the ground surface in the last 11,000 years. Locating new buildings away from active faults can result in fewer injuries and lower repair costs in the event of an earthquake. Surface rupture generally is the most easily avoidable seismic hazard.

The maps primarily affect developments of four or more homes or commercial building projects. The maps are scheduled to become official on May 1, 2003, following public review. By state law, local planners and developers must refer to the official maps before beginning new construction. The law also requires sellers and real estate agents to inform buyers whether real property being sold is within an Earthquake Fault Zone.

The maps show regulatory earthquake fault zones about 1,000 feet wide. If a proposed development is within that zone, the developer must perform a geologic investigation, typically by hiring a geologist registered in California, to determine whether the construction project area is underlain by active earthquake faults. If an active fault is found, new buildings must be set back from the fault. Setback widths generally are 50 feet from either side of the fault.

The Point Loma quadrangle, including downtown San Diego, was initially zoned in 1991, but recent investigations both onshore and offshore have revealed additional active faults near the southern end of the Rose Canyon Fault Zone. The new zoning could impact new and infill construction in San Diego. New and revised zoning also impacts the cities of Chino Hills, Corona and Coronado. There is a great deal of new development in the Chino Hills area, near the Chino Fault, and Alquist-Priolo zoning there should be timely in preventing structures from being built across active faults.

Most of the new mapping is for sparsely populated areas affected by the 1999 magnitude 7.1 Hector Mine earthquake, which caused observable surface displacement extending for a distance of 30 miles. The earthquake caused some ground to move as much as 18 feet relative to its former position.

The Alquist-Priolo Earthquake Fault Zoning Act was passed in 1972, a direct result of the 1971 San Fernando Earthquake, in which surface fault rupture damaged numerous homes, commercial buildings, and other structures.

There have been 27 earthquakes associated with surface faulting in California since the first Alquist-Priolo maps were issued in 1974. The last six out of seven earthquakes in California that generated ground rupture occurred mainly within established AP zones.

In the past 15 years there have been several earthquakes generated by movement along blind thrust faults, which can’t be seen at the Earth’s surface. These faults generally lie at least 3 miles below the Earth’s surface. The 1994 Northridge Earthquake caused major damage in the Los Angeles and San Fernando Valley areas, but the blind thrust fault that produced the earthquake did not reach the ground surface.

Copies of the Preliminary Review Maps may be examined at the offices of affected cities and counties, and at the California Geological Survey office in Los Angeles, (213) 239-0878. The maps also can be viewed at Copies may be purchased from BPS Reprographic Services, 149 Second Street, San Francisco, California, 94105, (415) 512-6550.