Hidden Quake Risks Cited for O.C.

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.

These concerns could translate to changes in the state’s seismic hazard maps -- which could lead to higher rates charged by the California Earthquake Authority in some areas for earthquake insurance. Orange County rates are now less than in parts of Los Angeles County close to the site of the Northridge and the 1971 Sylmar-San Fernando quakes.

The seismic hazard maps also influence which large building projects require geotechnical seismic assessments and development of quake mitigation plans. Acting state geologist Michael S. Reichle said the maps are a work in progress and are altered every few years as new dangers are determined.

But the kind of quake sequence Grant and Gath are suggesting is so rare that factoring it in may result in little, if any, increase in the seismic hazards, Reichle said.

This is because estimates of quake risk depend on assumptions about the average intervals between major quakes on a fault line. In the case of blind thrust faults, those estimates are difficult to make because the evidence is deep underground.

Depending on the assumptions made about those intervals, “the assessed probability could actually go down, not up,” Reichle said.

Reichle also pointed out that state quake hazard maps already incorporate dangers from the Puente Hills and San Joaquin Hills blind thrust faults. No such danger has been incorporated from a Santa Ana Mountains blind thrust fault because its existence has not yet been proved, he said.

Among the greatest hazards on hazard maps today are along the San Andreas fault, well outside Orange County, and in areas of Los Angeles and Ventura counties close to the transverse ranges, the San Gabriel and Santa Susana mountains.

There have been several quakes larger than magnitude 7.0 in Southern California in the last 60 years or so, including the 7.1 quake in the Imperial Valley in 1940, the 7.7 quake in Tehachapi in 1952, the 7.3 in Landers in 1992 and the 7.1 in Hector Mine in 1999.

Scientists estimate there are 300 faults in Southern California capable of producing quakes of 7.0 or greater, but they are unsure how many potentially very destructive faults are under urban areas. This is because blind thrust faults are difficult to identify. For instance, the Northridge quake took place on a fault that was unknown.