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Study: fault line could threaten older L.A. neighborhood

Los Angeles
Los Angeles   
April 8, 1998
Web posted at: 5:21 a.m. EDT (0921 GMT)

LONG BEACH, California (CNN) -- A fault line running under Los Angeles could deliver a powerful jolt to an an area full of older buildings, geologists said in a study Tuesday.

The fault, which runs through the hills north and east of Los Angeles, could deliver a quake with a magnitude of 6.5 to 6.8 -- roughly the strength of the deadly 1994 Northridge quake. There is no way of knowing if such a quake is imminent, according to Kerry Sieh, a geology professor at the California Institute of Technology in Pasadena and co-author of the report.

Such a quake would threaten hundreds of thousands of buildings, mainly in older areas of the city, says Tim McCormick. McCormick is director of Anchor L.A., an organization that encourages reinforcing older woodframe buildings against earthquakes.

"It's confirming our worst fears, but not surprising," McCormick said.

Because of the area's population density and the prevalence of older buildings, experts say the losses of life and property would likely be worse than the Northridge quake of 1994, which killed 72 and caused more than %25 billion in property damage.

According to Michael Oskin, a CalTech grad student, the new study concerns an 11-mile-long, previously known geological fold that runs through the hills north and east of downtown Los Angeles. This fold provides indirect evidence for an underlying fault.

"Our evidence from the surface is that the fold is still growing," says Oskin. "This indicates that the fault that lies beneath it must also be active."

The fold, first associated with earthquakes at the time of the Whittier Narrows Earthquake, in 1987, is formally known as the Elysian Park Anticlinorium and runs northwest-southeast from Hollywood to Whittier Narrows. Three smaller "wrinkles" formed upon the the southwest-facing flank of this fold have been investigated in detail by the CalTech scientists.

Their studies of sediment deposited by the Los Angeles River and its tributaries indicate that these small folds have been active during the past 60,000 years. During that time, the area has been contracting north-south at a rate of at least a half-millimeter per year.

"Our evidence that this structure is active does not increase the overall hazard in the metropolitan region," according to coauther Kerry Sieh, a professor of geology at CalTech. "Rather, it allows us to be more specific about how, where, and how fast deformation is occuring in the area.

The length of the surface features suggests that the underlying fault is about 11 miles in length and may extend 10 or so miles into the Earth. The rate of deformation suggests that an event on the scale of the Northridge quake might occur, on average, about once every one to three thousand years, Sieh said.

 
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