From the hills in Yorba Linda to the tract homes along Serrano Creek in Lake Forest, vast chunks of Orange County could be damaged by liquefaction or landslides in a 5.5 magnitude earthquake or larger, state officials said Tuesday.
Maps of three areas of Orange County pinpoint danger spots in eight communities, areas where the soil under homes could be turned to quicksand and where hillsides--even those as modest as the slopes near UC Irvine--could give way in landslides. The extent of the impact on the communities--Anaheim, Orange, Yorba Linda, Irvine, Tustin, Lake Forest, Mission Viejo and Laguna Hills--is revealed in a set of seismic hazard maps released by the state Department of Conservation on Tuesday--one day short of the seventh anniversary of the 1994 Northridge earthquake.
The maps represent a second phase in an effort to draw Orange County's seismic portrait. The western section of the county was detailed in maps made public two years ago and a final stretch, from Coto de Caza to San Clemente, will be studied next.
The mapping in Orange County is unrelated to a seismic report also released Tuesday, a study that revealed the "hot spots" in Southern California that will shake the most violently during an earthquake.
Orange County homeowners who live in the zones where liquefaction and landslides pose a threat will be compelled to disclose the fact to potential home buyers.
Although the maps will be used primarily by civil engineers and government officials to determine the stability of proposed construction, the documents offer a sobering glimpse into the potential dangers of the ground beneath neighborhoods that have been built near creek beds and hillsides.
"The purpose is not to alarm the public but to provide more details," said Darryl W. Young, director of the Department of Conservation. "If I lived in one of those areas, I would protect my house and do things like anchoring [the foundation] and also I would think about protecting my valuables by putting them in a safe place."
Protection is not cheap, however. Although homes can be fortified for $3,000 to $5,000 during construction, the cost of digging out soil and shoring up an existing residence can easily reach $70,000, state officials said.
The new data will help city engineers and geologists locate problem areas that may lead to modified construction regulation, state officials said.
Orange County does sub-surface testing of proposed developments by engineers, geologists and civil engineers, said Tom Matthews, director of planning for the county.
"The topic of liquefaction is not new to Orange County," Matthews said. "The mapping detail that the state has provided is much better than we had before and helps us pinpoint the geotechnical evaluations, including new development areas."
Matthews said the county's larger developments, including Ladera, Newport Coast and Newport Ridge, are "all the highest standards, including the state's standards for liquefaction and slope stability."
Liquefaction occurs when loosely packed, fine-grained soil saturated with water is hit with shock waves, causing the soil to behave like jelly. It is blamed for causing $2.5 million in damage at the King Harbor Marina in Redondo Beach during the Northridge quake. Likewise, the Loma Prieta earthquake near San Francisco was blamed for liquefaction that damaged a runway at the Oakland Airport and a Marine station as far away as San Jose.
The threat of liquefaction is greatest when ground water is within 40 feet of the surface, said Theodore C. Smith, a supervising state geologist.
"In some areas of Orange County, like Fountain Valley, you have water 3 feet down," Smith said. "And there's plenty of sand. But we're not saying that if you live there, you're going to have liquefaction, but that you could have liquefaction."
The maps are a tool to help communities minimize earthquake damage by being prepared, Young said.
They scrutinized three clumps of county geography, but the zone that includes Tustin and part of Irvine was studied two years ago when much of the western portion of the county was examined. There was only one area of concern added to the Tustin map: the channel of the San Diego Creek east of Jeffrey Road.
In a zone that includes Lake Forest, Mission Viejo, Laguna Hills and part of Irvine, the maps show liquefaction areas running along the Aliso, Serrano, Oso, Santiago and San Diego creeks and along the rim of the former El Toro Marine base. In a zone that takes in canyon areas in Yorba Linda, Anaheim and Orange, hot spots showed up in Featherly Regional Park, the Green River Golf Course, Irvine Lake, Irvine Park and Fremont Canyon.
"Local officials can place special engineering requirements on new construction to make buildings and people safer now that we know where these hazards are most likely to occur," Young said.