Seismic Hazards Mapped

Homes throughout south Orange County could be heavily damaged by liquefaction or landslides in a major earthquake, according to maps released Thursday by the state Department of Conservation.

The maps, which cover 120 square miles, identify dangers in the event of a magnitude 6.0 or greater earthquake in Aliso Viejo, Dana Point, Irvine, Laguna Beach, Laguna Hills, Laguna Niguel, Mission Viejo, Rancho Santa Margarita, San Clemente and San Juan Capistrano.

They show large coastal sections where a quake could turn soil saturated with water into quicksand, and vast stretches of potential landslide zones extending from the ocean to the mountains.

Liquefaction, which makes the ground jiggle like Jell-O when loosely packed, water-saturated soil gets hit by earthquake shock waves, caused an estimated $2.5 million in damage in Redondo Beach during the 1994 Northridge earthquake. The threat of liquefaction is greatest when ground water is less than 40 feet from the surface, which is common throughout Orange County. The Northridge quake also triggered more than 11,000 landslides.

State officials said most of the homes in the “hot spots” identified on the new maps were built with such risks in mind. But because building standards have been inconsistent in some cities, homeowners are encouraged to make sure their homes meet current standards.

“The older the subdivision, the more suspect it might be in terms of the standards that were used,” said Jack McMillan, senior engineering geologist with the state Department of Conservation’s Division of Mines and Geology. He said the maps will “level the playing field,” holding all builders in areas with seismic hazards to the same high engineering standards.

In areas where the state identifies a hazard, builders will be required to test the stability of the ground before getting local approval for any projects. Those tests will determine whether special anchoring must be added to the homes’ foundations. Sales of any homes in the zones must be accompanied by full disclosure to the buyer of the seismic hazards.

“Knowing where liquefaction and landslides are most likely to occur means that local officials can require special engineering steps on new construction to make people and buildings safer,” Darryl Young, director of the Department of Conservation, said in a statement.

Any homes already built in high-risk areas that don’t meet the standards can be retrofitted--but at a cost of up to $70,000. When foundations are anchored during construction, it typically costs less than $5,000.

State officials caution that the maps come with no guarantees.

“These maps don’t mean everything in the zone will fail, and they don’t mean everything outside the zone is safe,” McMillan said.

The state has produced seismic hazard maps for more than 3,000 square miles in California, including most of Los Angeles, Orange and Ventura counties.

State geologists create the maps with computer modules, guided by analysis of existing maps and data from engineering borings.