New data on a dangerous earthquake fault running directly beneath downtown high-rises here is prompting fresh debate about the type and pattern of future development across the nation's most populous county.
The findings are also generating serious concerns about the safety of hundreds of structures, how to notify occupants of these buildings, and whether these buildings should be reinforced to keep them from collapsing.
A study published April 4 in the journal Science reveals that a fault snaking 25 miles underground from northern Orange County into downtown had produced at least four major earthquakes over the past 11,000 years.
Each of the quakes registered a magnitude of 7.0 or greater - substantially more powerful than the 6.7 magnitude Northridge earthquake in 1994 that resulted in $64 billion in damage. If such an earthquake were centered beneath downtown today, the devastation would be far more severe.
"The type of shaking and ground displacement that would occur with these magnitude earthquakes in downtown would be your worst nightmare for an urban area," says Thomas Heaton, a seismologist and structural engineer at California Polytechnic Institute.
Certain kinds of structures, known as nonductile concrete, as well as some built with steel skeletons, would be in danger of full collapse, he says. "The city and county need to revise building codes for everything that happens from here on out."
Known as the Puente Hills Blind Thrust Fault, the seismic hazard was discovered by a team of paleoseismologists (who study ancient earthquakes) using new methods that could serve as a model for better studies of quakes worldwide. Blind-thrust faults do not extend to the earth's surface and thus have constituted a major gap in understanding regional fault systems. With over 100 active faults, the Los Angeles area's system is one of the world's most complex.
By using three decades' worth of petroleum-industry research, and by boring deep holes into the earth beneath city streets, researchers were able to create three-dimensional pictures that revealed evidence of previous earthquakes - and the likelihood of similar quakes in the future.
"What is new here is stronger evidence that indeed large earthquakes have happened beneath downtown, which is a very different picture from that of the 1970s when much of the downtown was built," says Lucy Jones, the US Geological Survey's coordinator for southern California. "The premise then was that the largest earthquakes possible would be about magnitude 5. We are going to have to dramatically ratchet up our hazard maps for development in the future."
Although the new information could dictate new zoning limitations, emergency-response plans, and other risk-mitigation strategies, Heaton, Jones, and study coauthor James Dolan say residents should not panic or even begin making plans to move.
"The likelihood of one of these earthquakes happening in our lifetime is very, very low, so it's nothing to lose sleep over," says Mr. Dolan, a professor at the University of Southern California. "But for those thinking of how to plan and build this city and others where these kinds of faults lie, this gives sobering information on the most threatening long-term dangers."
If the revelation of this fault line is a primary benefit of study, the first-time application of paleoseismological techniques is also important. Researchers used sophisticated methods of seismic reflection - sound-energy waves that bounce off geological layers of sediment - to produce reflective pictures of flats and folds.
"The actual sites where they drew this information were [in] the middle of city streets, not in the mountains far away where it's far simpler to research," says Tom Jordan, director of the Southern California Earthquake Center. "This could help researchers recognize the need to fund and coordinate such study here and elsewhere to better understand these problems in the future."