Hidden Faults Discovered Off California

L O S  A N G E L ES, Oct. 1 -- Two hidden faults capable of unleashing amagnitude-7.6 earthquake lie off the coast of heavily populated LosAngeles, Orange and San Diego counties, researchers reportedtoday.

Though there’s potential for catastrophe, the chances are slim.In the worst-case scenarios detailed in the study, the biggestquakes occur once every 2,100 years on one of the faults — theThirtymile Bank fault — and every 8,800 years on the other — theOceanside fault.

It’s possible the faults release their energy in smaller butmore frequent spurts, the researchers reported in the Octoberedition of the journal Geology.

“Because this is new and we can’t access it easily, we don’thave the knowledge base yet to decide whether it is going torupture in small pieces or in one single event,” said study authorJohn H. Shaw of Harvard University.

“The critical issue for hazard assessment is really justdefining the size of these faults,” he added. “The size obviouslydictates the potential earthquake magnitude.”

A 7.6-magnitude quake would likely cause widespread damage andinjuries. The 6.7-magnitude Northridge quake in 1994 killed 72people and caused an estimated $35 billion in damage in LosAngeles.

The Thirtymile Bank fault runs south from Santa Catalina Island,and the Oceanside fault slices south from Laguna Beach in OrangeCounty. Both extend south to San Diego and possibly beyond theU.S.-Mexico border.

‘Blind Thrust’ and ‘Strike-Slip’ FaultsBoth faults are the same type that unleashed the Northridge and1971 Sylmar quakes. Called blind thrust faults, they are notclearly visible on the surface, whether on land or on the seafloor, and are usually detected when they produce quakes.

“This is the first concrete evidence that we have large thrustfaults in the offshore region here,” said Tom Henyey, director ofthe Southern California Earthquake Center. “It is a significantfinding, if in fact it is the case.”