Seismic hazard maps issued by the state may underestimate the quake risk in Orange County and should be reevaluated in light of evidence that significant faults are hidden underground, according to two UC Irvine researchers.
So-called blind thrust faults, which do not reach the surface, are buried under the Puente Hills, the San Joaquin Hills and probably the Santa Ana Mountains.
They could trigger quakes on nearby surface faults that would be several times stronger than the 1994 Northridge quake in Los Angeles County, say UCI geologist Lisa Grant and doctoral candidate Eldon Gath.
The two presented their findings at a meeting of the Seismological Society of America in Palm Springs last month.
It's not the first time Grant has explored hidden faults in Orange County. In a 2002 research paper, she wrote that a blind thrust fault under the San Joaquin Hills could have been the source of a 1769 quake that was the first in California for which there is a written record. Its magnitude is believed to have been about 7.0.
Gath said in an interview that after additional research it seemed more likely that at very long intervals a quake on a blind thrust fault could trigger one on a nearby surface fault.
Gath said the San Joaquin Hills blind thrust fault could trigger a quake on the Newport-Inglewood fault, which runs along the county's north coastline. In 1933, a magnitude-6.3 quake along this fault killed 120 people.
The same triggering sequence could occur between the Puente Hills blind thrust fault, on the county's northern tip, and the Whittier fault, which follows a similar course, he said. The same could also occur if a suspected blind thrust fault under the Santa Ana Mountains triggered a quake on the Elsinore fault in southwestern Riverside County.
Scientists believe no major quake has occurred on the blind thrust faults under Orange County in 150 years or more. But the risk of a blind thrust fault triggering a quake on a so-called strike-slip fault at the surface was shown by a massive Alaska quake in 2002, Gath said.
The Denali quake, which caused $44 million in damage but only one injury, registered a magnitude of 7.9.
"This is the new lesson for Orange County," Gath said in his presentation at the Seismological Society.
