A LOOK AHEAD * Seismic experts scramble to reassess possible earthquake danger to buildings as . . . New Fault Is Found in Southern Orange County

The newly discovered San Joaquin Hills fault runs along the south Orange County coast and is capable of producing a 7.3-magnitude temblor, UC Irvine researchers reveal today in a scientific journal.

The fault line, which stretches from Huntington Beach to Dana Point, could have been the source of California’s first recorded earthquake in 1769 and might be one of several hidden faults in the area, according to the researchers.

“It doesn’t tell us when we would experience an earthquake, but we know there would be significant ground motions,” said researcher Lisa Grant, one of the authors of an article in Geology. “It raises questions about what we don’t know . . . it’s the tip of the iceberg.”

The work has already drawn the interest of dozens of consultants who work for various state agencies and private companies, who are trying to determine whether the fault affects their facilities.

As a result of the research, UC Irvine is strengthening seismic safety features at a faculty housing development now under construction, Grant said.

Outside seismic experts compared the findings to the discovery a few years ago of faults under central Los Angeles. Experts said the public shouldn’t be alarmed at the Orange County discovery, which probably won’t bring higher insurance rates, said Mark Leonard, spokesman for the California Earthquake Authority.

But some said the news could ultimately result in changes in the way buildings are constructed along the fault. Two key questions are how quickly stress builds up in the fault and how frequently quakes occur--which can be answered only with additional study.

In an interview, Grant explained how her nearly 10 years of research began at some geological structures in the San Joaquin Hills that look like bath rings. They proved that a million years ago, the area was a beach.

Indeed, home builders in the area have found sand and shells on hilltops. From there, Grant and her team set out to prove that the land was moving and to find out the rate of motion. They concluded that the hills are rising a quarter of a millimeter a year.

The work has been recognized as important because it helps understand more about a fault that until recently was not known. Some recent earthquakes, including the Oct. 16 Hector Mine quake and the 1994 Northridge temblor, emanated from previously unidentified faults.

The possibility of a fault was revealed in March, while research was still underway. The journal article contains the researchers’ completed findings.

Other faults in Orange County include the Newport-Inglewood fault, which generated the 1933 Long Beach earthquake and runs parallel to the coast.

“Prior to this work, most people considered the Newport-Inglewood to be the principal source of seismic activity in Orange County,” said Tom Rockwell, geology group leader at the Southern California Earthquake Center. “One possible outcome of this research is that there are other blind thrust faults that we do not know about in the area.”

Grant said the newly discovered fault has the potential to produce a 6.8 to 7.3 earthquake every 1,650 to 3,000 years.

Though the probability of a quake seems small, Ken Hudnut, a geophysicist at the U.S. Geological Survey, said the work is important.

“The recent earthquake in the desert showed us that slow-moving faults with long recurrence intervals still pose finite hazards,” he said. “This research helps us make steady progress on figuring out where faults are and categorizing them.”

But Leonard of the earthquake authority says discoveries of faults are so numerous that they do not immediately affect calculated risk in a given area. A fault undergoes intense scrutiny before being considered a factor in determining insurance rates, he said.

“If you look at a map of faults, Southern California is a giant spider web. It doesn’t make a difference that we have one more line,” said Steve Shekerlian, vice president of Newport Beach-based EQE International.

Still, Shekerlian said that if Grant’s preliminary findings are widely accepted, building codes could require that new buildings along the San Joaquin Hills fault would need to be designed to tolerate high levels of force.

Geologist John Waggoner of URS Greiner Woodward Clyde Consultants of Santa Ana, which does seismic design for several public and private agencies, incorporated the new research into his firm’s building plans.

Ray Golden, spokesman for Southern California Edison, said officials at the San Onofre Nuclear Generating Station carefully reviewed the UC Irvine data but determined that the fault would not affect the facility. The power plant is already structured to withstand a 7.0 earthquake and it is 10 to 12 miles from the new fault, far enough away for officials to dismiss possible impact.

Like some, Golden questions whether Grant has found a unique fault, or nothing more than an extension of the existing Newport-Inglewood fault. A Caltrans spokeswoman said her agency reviewed the findings and agreed with Golden.

Grant believes the two faults are separate entities and plans to study their relationship in coming years. She also will research whether the newly discovered fault has moved in the past few hundred years.

She hopes to prove the fault was the source of a quake recorded by Spanish explorer Gaspar de Portola, who experienced it on July 28, 1769, while camped along the Santa Ana River, about 10 miles from the San Joaquin Hills.

In the meantime, Grant hopes her work is a heads-up for planners and builders.

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A New Fault

UC Irvine scientists have discovered a blind thrust fault in south Orange County capable of generating a 7.3 magnitude earthquake. A look at a blind-thrust fault:

Blind fault thrusts upward, without breaking surface.

Source: UCI