Probable Fault Through County Called Little Threat

Evidence of an earthquake fault cutting a 25-mile swath through south Orange County should not be a cause for alarm, scientists said Thursday.

While such a fault could produce powerful quakes, it’s believed to be much less active than other faults in the region.

“To put it in perspective,” said Egill Hauksson, a seismologist with Caltech, “this is an interesting scientific thing, but I don’t think it has any big implications for people living in the area.”

Scientists confirmed earlier in the week that they had discovered the likely existence of the previously unknown major fault under the San Joaquin Hills. The fault stretches roughly from Newport Bay to Dana Point along the coast, and inland as far as the San Diego Freeway.

“We’ve been studying this possibility and evaluating the evidence, and we think [such a fault] is a good hypothesis that fits the data,” said Lisa Grant, a geologist at UC Irvine. She is the principal author of a paper on the subject that is now undergoing peer review for possible publication in a scientific journal.

Grant emphasized that the findings are preliminary and have not yet been confirmed.

Tom Henyey, director of the Southern California Earthquake Center in Los Angeles, said the probable fault--described as a “blind thrust” type because it is underground--could be as wide as 10 miles and capable of generating an earthquake of 7.1 magnitude--potentially more damaging than the Northridge earthquake of 1994. That quake killed 61 people and caused $40 billion in damage.

“This would be just about the size of Northridge or a fair piece bigger,” said Henyey, a professor of geological sciences at USC who is familiar with Grant’s work.

But the South County fault also appears to be considerably less active than many others, such as the San Andreas, which runs through much of California; the Newport-Inglewood Fault, running roughly from Beverly Hills through coastal Orange County to San Diego; and the Whittier Fault, near the Santa Ana River in northern Orange County.

“We believe that earthquakes might occur on this thing every 2,500 years or so,” Henyey said of the new fault. “We don’t know when the last one was--it’s possible there was one maybe 200 years ago, but we’re still researching that.”

All of that has led earthquake watchers across the state to downplay the significance of the latest find.

“We already live in an area riddled by faults, and the discovery of a new one doesn’t substantially change anything,” said Peter Yanev, president of EQE International. His company, which has offices in Newport Beach, assesses earthquake risks for the state and private companies and advises them on how to respond.

“What apparently we have here,” Yanev said, “is another fault likely to cause a large earthquake but with a fairly low probability of occurrence. Fundamentally, it doesn’t change the big picture.”

Spokesman Mark Leonard of the California Earthquake Authority, which oversees 75% of the state’s residential earthquake insurance policies, agreed.

“There is a great deal happening” in earthquake research, he said, and “the discovery of a new fault is not an uncommon event. Simply, the discovery of a fault doesn’t have a major impact on risk.”

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New Fault

A UC Irvine geologist has found evidence of a fault capable of causing a magnitude 7 earthquake beneath coastal South Orange County. Because the fault lies at least 3 miles below the Earth’s surface, its exact location has not yet been plotted.

Source: Tom Henyey, director of Southern California Earthquake Center.