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Downtown LA Fault Line Worries Geologists

April 4, 2003

By ANDREW BRIDGES

LOS ANGELES (AP) — A recently mapped, still-active fault line that snakes beneath downtown Los Angeles is capable of generating major earthquakes, but only about once every 2,000 years, according to a new study.

At least four earthquakes of estimated magnitudes 7.2 to 7.5 have struck over the last 11,000 years on the Puente Hills fault, which was first mapped just four years ago. A segment of the fault last ruptured in 1987 with a magnitude-6 earthquake.

The study is the first to show how often and with how much punch large quakes have struck on the fault in the past, said lead author James Dolan, an earthquake geologist at the University of Southern California.

The findings firmly place the fault among the largest of the 100 or more that crisscross the Los Angeles region. Details appear Thursday in the journal Science.

Dolan called the findings a good news-bad news proposition: The major quakes the fault has generated in the past would ravage modern-day Los Angeles, but apparently only strike every 2,000 years or so.

“What we didn’t know was how fast this fault is storing up energy … and whether it releases this energy in the form of numerous, small-sized earthquakes or in less frequent, but much larger events,” Dolan said. “Our results suggest the latter is the case.”

Lucy Jones, who runs the Pasadena office of the U.S. Geological Survey, which partly funded the study, called the findings “disturbing, disturbing information.”

“The conclusion that there have to be pretty big earthquakes underneath Los Angeles is not without consequence,” said Jones, who was not connected with the study.

Many of the buildings in downtown Los Angeles were built to withstand a quake no larger than magnitude 5 directly underneath them. A magnitude-7 earthquake, by comparison, releases about 1,000 times more energy.

“If you had to design the worst place to put a fault in Los Angeles, Puente Hills is it,” Dolan said

But seismologists also caution that the new findings do little to raise the overall risk Los Angeles faces, given the number of faults within the region.

Geologists predict the Puente Hills fault will generate a major quake in the future, yet the likelihood of it happening in the next 50 or so years is slim.

The Puente Hills fault snakes for about 30 miles, formed by the collision of the Pacific and North American plates, which catches the Los Angeles basin in a viselike squeeze. That stress is released in earthquakes.

Dolan and his co-authors used oil industry data and cores extracted from more than a dozen boreholes to gather evidence of the Puente Hills fault’s past.

There is little sign of the fault visible at the surface today and past earthquakes have folded the buried layers of sediments draped over it. When sampled, the layers can be carbon-dated, allowing scientists to build a quake chronology.

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