In terms of location it couldn't be worse. It lies right under the main urban area.
Dr. James Dolan
Deep beneath Los Angeles, snaking 25 miles from Puente Hills to Beverly Hills, lies a subterranean fault line that a California seismologist holds responsible for four massive earthquakes in the last 11,000 years. This same fault, he predicts, will be responsible for another mega-quake in the indeterminate future.
Much less famous than the San Andreas fault, the Puente Hills fault may prove even more dangerous and destructive than its more well-known cousin, said Dr. James Dolan, an earth science professor at the University of Southern California. The quake to come will likely register stronger than 7 on the Richter scale, but neither Dolan nor anyone else can say when that will happen.
"In terms of location it couldn't be worse," Dolan said. "It lies right under the main urban area."
Dolan, in collaboration with Harvard seismologist John Shaw, published a paper in the April 4 issue of Science, which details the history of the Puente Hills fault, and the implications it may have for Los Angeles.
"We know we have a threat, and since we chose to live here, we really need to know what we're up against," Dolan said.
In surveying the fault's history, Dolan discovered that the four massive quakes all happened before Europeans came to California. He concluded that the last few centuries have been a time of near-inactivity for the fault line.
"We're in a quiet period, and we have been since the Europeans got here," he said. "This relative seismic quiet is not going to last forever, but that's not a cause of panic."
A lot of stress has been built up along the fault in the last few hundred years, he said.
"We've only released about 10 or 20 percent of the energy we've put in," Dolan said. "That means the rest needs to be released."
The Puente Hills fault gave a small indication of its power in 1987, when it caused the 6.7-magnitude Northridge earthquake that rocked Los Angeles. But that quake was nothing in comparison to what the Puente Hills fault could generate in the future, said Dolan.
Before the 1987 quake, scientists weren't sure the fault even existed. After the quake, "it became sort of a no-brainer," he said.
But it took more than a decade to come up with the proof. Shaw, along with Peter Shearer, of the University of California, San Diego, provided that proof in a 1999 paper that carefully sketched out a three-dimensional map of the fault.
Shaw and Shearer constructed that map by using an unusual tool: a large weight of several tons, dropped from a massive dump truck, which Dolan describes as "the ultimate low rider."
The technique is similar to a bat's echolocation method of "seeing:" sound waves generated by the falling weight traveled deep into the earth, then bounced back to the surface, to be recorded by the seismologists.
Dolan used a different method to collect his data – he drilled 15 holes up to two miles deep in the earth's crust, extracted subterranean sediment and analyzed what he had dug up.
And all of these scientists also relied on information provided by petroleum companies that have been boring holes in the crust of California for decades.
All these techniques have been required, Dolan said, because the Puente Hills fault is much different than more common fault lines, like the San Andreas. The Puente Hills fault is a so-called "blind-thrust system," meaning that it never reaches the surface of the earth – the primary reason why seismologists took so long finding it.
"All the real action is happening 10 miles beneath the surface," Dolan said.
Dolan said the recent Science paper is important because it provides real information about these "blind-thrust systems," which have been largely a mystery to scientists for years.
This data, he said, is helping to put together the "big, thousand-piece jigsaw puzzle," that is the seismic landscape of California. But seismology still has a ways to go.
"We still by no means have the whole puzzle figured out, but we're starting to get a look at the gross picture," he said.
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