New Quake Maps Focus on O.C. Danger Zones

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 are part of an ongoing project to identify seismic hazards throughout California. Similar maps pinpointing seismic risks have already been released for most of the rest of Orange County. Large portions of Los Angeles and Ventura counties, as well as other counties throughout the state, have also been mapped.

The new maps identify dangers in the event of a magnitude 6.0 or greater earthquake in 10 South County cities, from Irvine to San Clemente. 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 into 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 earthquake also triggered more than 11,000 landslides.

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

Maps Will Help Builders Fortify Their Projects

“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 Division of Mines and Geology in the state Department of Conservation.

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 to resist earthquake damage.

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,” Department of Conservation Director Darryl Young said in a written statement.

Any homes already built in high-risk areas that don’t meet the standards can be retrofitted--but at a cost of as much as $70,000. When foundations are anchored at 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.

Coverage Tops 3,000 Square Miles in State

Besides Irvine and San Clemente, other cities affected by the maps released Thursday are Aliso Viejo, Rancho Santa Margarita, San Juan Capistrano, Dana Point, Laguna Hills, Laguna Beach, Laguna Niguel and Mission Viejo.

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

More than 135 cities are using the maps in their permit process.

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

The maps may be viewed online at www.conservation.ca.gov/dmg/shezp. Printed copies may be purchased from the state by calling (415) 904-7707 or (916) 445-5716.