LOS ANGELES, April 3 (UPI) -- A large active fault line running directly under downtown Los Angeles could loose future earthquakes with 15 times more energy than the quake dubbed the most expensive natural disaster in U.S. history, scientists reported Thursday.
"The results are rather sobering," researcher James Dolan, an earthquake geologist at the University of Southern California, told United Press International. "This is one of the biggest faults in L.A. It's a terrible location for a fault."
Investigators discovered the fault has bred at least four very large quakes in the past 11,000 years directly beneath where the city now sits.
"By very large, I mean magnitude 7.2 to possibly 7.5," Dolan said. "To give you an idea of the size, the 1994 Northridge earthquake was magnitude 6.7. That earthquake caused upwards of $50 billion in damage, and it struck in the far northwest corner of Los Angeles, rather than directly in the heart. So that's the bad news."
Nevertheless, Dolan thinks the findings also are good news.
"The whole point of doing all this work is not to scare people. It's to understand what exactly the threat is. I've always been a believer in the axiom, 'Forewarned is forearmed,'" Dolan said. "This is exactly the kind of information emergency planners at the city, state and national level need to respond to this hazard."
The fault is named after the Puente Hills in northern Orange County, where its east end starts. It snakes 30 miles almost directly westward under downtown Los Angeles "and my office" toward Beverly Hills, Dolan said.
He called it a "blind thrust" fault, which refers not to some exotic move in the sport of fencing. "'Blind' is an old oil company term that means they don't see the surface," he explained. The fault starts nearly two miles deep and extends down to 10 miles below the surface.
Co-researcher John Shaw of Harvard University in Cambridge, Mass., and colleagues had mapped the fault earlier in great detail by sending acoustic energy into the ground and analyzing the echoes. They also reviewed petroleum industry records from well drilling over the past century.
With a clear idea of the fault's location and geometry, Dolan and his team then could begin to figure out its ancient quake history. They focused on the floodplain of the San Gabriel River by Los Angeles. "Sand and gravel was being deposited there every winter storm, so we had continuous sediment deposition, and the area is almost dead flat," Dolan said.
In findings appearing April 4 in the journal Science, analyses of 15 borehole samples drilled from this sediment, each about three inches wide and 50 to 100 feet deep, revealed the normally flat layers of sand and gravel and clay had four large folds in them. Carbon dating confirmed when this folding took place, and the extent of these interruptions helped the investigators determine each quake's magnitude.
"It's pretty amazing that they're seeing the history of a fault you can't look at," said Lucy Jones, the seisomologist who supervises the southern California area for the U.S. Geological Survey. "This is a new innovation that opens up the possibility of looking at other buried faults. Scientifically, that's quite cool, and this is a big issue in L.A., where there at least 100 faults we know about in the metropolitan area," most of which have had all their surface features obliterated by construction before the geologists ever got there."
Areas in China and northern Africa also might possess buried faults under populated areas, though Los Angeles is the largest U.S. metropolis known to scientists to have buried fault issues. The 1987 Whitter Narrows earthquake, a magnitude 6.0, happened when a small piece of this fault ruptured, "so we know it's still active," Dolan said. Larger earthquakes also generate seismic waves with lower frequencies and longer wavelengths, "which will most affect large structures like high-rises or skyscrapers, dams, bridges," he explained.
The energy of a magnitude 6.0 quake is measured by a standard called the Richter scale, in which each whole number increase -- from 6.0 to 7.0, for example -- represents 31 times more energy than the preceding number.
"Think about the difference between a violin and a standup bass. They're essentially the same instrument. It's just that one is bigger than the other. A violin produces high notes, high frequency sounds, whereas a bass produces low notes, lower frequency waves. Earthquakes are just the same," Dolan said. The longer the wavelength, the more the waves match up with objects of equal size, and the more such items will vibrate.
Still, the geological record shows it took 11,000 years for four such quakes happen, so "the take home message is to keep it in perspective, this is happening only every 3,000 to 5,000 years. We're probably not going to see it," Jones said.
"The flip side is that most of the buildings in L.A. were built when they thought the most likely earthquake, the biggest under L.A., was a 5. We now have better proof these big earthquakes really did happen, and when, and how often," Jones added. Retrofitting buildings for added strength is possible, but given the billions of dollars it would likely cost, it becomes a tricky political issue, she said.
"We can make sure the education is out there, tell people this would be a building we expect to fall down in the largest earthquake in the region. Then people might not want to work in it, and then you have incentive to update the building, don't you?" she said.
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(Reported by Charles Choi, UPI Science News, in New York)
