Fault Finding Mission

The newly discovered San Joaquin Hills Fault runs along the South County coast and is capable of producing a magnitude 7.3 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 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 that are trying to determine whether the fault affects their facilities.

As a result of the research, UC Irvine is strengthening the seismic safety at a new 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. The public shouldn’t be alarmed at the discovery, which probably won’t bring higher insurance rates, said Mark Leonard, spokesman for the California Earthquake Authority.

But some said it could ultimately result in changes in the way new buildings are constructed along the fault line. A key question is how quickly stress builds up in the fault and how frequently quakes occur--something that can only be answered with additional research.

In an interview this week, 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 1 million years ago the area was a beach.

Indeed, home builders in the area have found sand and shells on the 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 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 was not known to have existed. Recent earthquakes, including the Hector Mine earthquake and the Northridge temblor, emanated from previously unidentified faults.

The possibility of a fault was first revealed in March, while research was still ongoing. The journal article contains their 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 new fault has the potential to produce 6.8 to 7.3 earthquakes every 1,650 to 3,000 years.

Even though the probabilities of an earthquake seem small, said Ken Hudnut, geophysicist at the United States Geological Survey, 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 a steady program on figuring out where faults are and categorizing them.”

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But Leonard of the earthquake authority said discoveries of faults are so numerous that they do not immediately impact calculated risk in a given area. A discovery 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,” added 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 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 building plans.

Ray Golden, spokesman for Southern California Edison, said officials at the San Onofre Nuclear Generating Station carefully reviewed the UCI data and determined that the fault would not impact the facility. The facility 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.

“Somehow if it is a new fault, the question is whether it could have an impact on our plant. To date, we do not believe it would,” Golden said.

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

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

In the meantime, she hopes her work flags planners and builders.

“The major contribution of this work for now is that there is a potential earthquake source that should be considered in regional planning. There are implications for seismic safety,” she said.

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A New Fault

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

Blind fault thrust upward, without breaking surface.

Source: UCI