Hidden Faults Discovered Off California
L O S A N G E L ES, Oct. 1, 2000 -- 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.”
Other offshore faults, such as the Newport-Inglewood and RoseCanyon faults are strike-slip faults, where one side slideshorizontally past the other.
Thrust faults, where one side moves over the other as if on aramp, could pose greater threats because their quakes tend to havehigher vertical acceleration.
“This tends to be very destabilizing for many types ofstructures, including high-rise buildings and other things,” Shawsaid.
And below the ocean, vertical movement might produce tsunamisthat wash over coastal areas, Shaw said. The researchers did notanalyze tsunami potential of the faults.
The researchers from Harvard and the University of Colorado atBoulder used data collected by oil companies exploring forpetroleum. The seismic reflection profiles, created by measuringaspects of sound from small explosions, are sonograms of Earth.
“In this picture, you see the layers of rock, and in some casesyou can see the faults directly,” Shaw said.
Quiet So FarThe faults have little historical record. Just one magnitude-5.3quake off Oceanside on July 13, 1986, likely originated on theThirtymile Bank fault, the researchers said.
“We have such a short history with thrust faults, we just don’tknow over the long term how these things go,” Henyey said. “It’spossible you’ll get small earthquakes now and then on these thingsand then all of sudden the whole thing will just go in one shot.”