Earthquake fault maps show where not to build

SACRAMENTO – New maps released in September of areas prone to earthquakes will ensure that new construction in San Diego does not take place over seismic faults that could shatter the surface.

Developed by the California Geological Survey (CGS), these regulatory maps of the Alquist-Priolo earthquake fault zone detail where local governments should require site-specific geological and engineering surveys for proposed developments in order to ensure that this hazard is identified and avoided. Generally, new construction for human occupation must be set back 50 feet from the active surface trace of a fault. The map can be viewed at conservation.ca.gov/cgs/geohazards/eq-zapp.

“Surface fault rupture is the easiest seismic hazard to avoid because you can see the evidence of where it occurred,” said Steve Bohlen, acting geologist for the state of California and head of CGS. “The rupture of a surface fault means that one side of a fault moves vertically or horizontally with respect to the other side. The deformation caused by movement is potentially devastating for buildings and infrastructure. “

The Alquist-Priolo Law was enacted in the wake of the San Fernando 6.6 magnitude earthquake in 1971, which caused an extensive surface rupture that damaged many buildings. Not all large earthquakes cause a surface fault to rupture. For example, the Loma Prieta earthquake of 1989 was devastating without breaking the surface. However, the 1992 Landers earthquake in San Bernardino County caused a 50-mile-long surface rupture with displacements ranging from one inch to 20 feet.

“Since the first earthquake fault zone maps were published in 1974, there have been approximately 30 earthquakes associated with surface faults in California,” Bohlen said. “For the most part, the surface displacement is relatively minor. But there were seven earthquakes that produced offsets of more than a foot – more than enough to shatter a building’s foundation, which could cause a collapse. “

Two revised seismic fault zone maps have been prepared for the Rose Canyon fault where it lands on the coast of Coronado Island and crosses the San Diego area in the northwest and back offshore near La Jolla. A revised area map has been prepared for the San Cayetano Fault, which affects the east side of the town of Fillmore and the unincorporated Ventura County. Each of these maps covers approximately a “quadrangle” of territory of 60 square miles.

When construction is proposed in a fault zone, a licensed geologist in California must assess the plot to be developed and submit a report to the local agency that issues building permits. If a fault is located and found active, the risk is mitigated by ensuring that no building is placed in the fault traces.

Zones do not affect existing developments unless significant additions or renovations are proposed. Disclosure that the property is in an area must be made in real estate transactions.

Ultimately, the local lead agency, not the CGS, has the final say over whether building permits are issued in zoned areas and how much, if any, of a setback required.

The new maps were finalized after review by local government entities, the public and the National Council of Mines and Geology. Members of the public who want to know if their property is in a CGS Alquist-Priolo seismic fault zone or a seismic risk zone can enter an address into CGS’s EQZapp tool. There are now 558 Alquist-Priolo fault zone maps statewide

“Risk management is a fundamental pillar of the work done by CGS and the Department of Conservation, of which we are a part,” said Bohlen. “While our new maps will help protect lives and property, we still encourage Californians to make sure they and their families are prepared for earthquakes and other natural disasters.”


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