Downtown LA Fault Line Worries Geologists (Puente Hills fault)

LOS ANGELES - A recently mapped, still-active fault line that snakes beneath downtown Los Angeles is capable of generating major earthquakes (news - web sites), 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 (news - web sites).

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 (news - web sites), 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.

FROM 1999 article in the Guardian.

New quake menace under LA

By Martin Kettle in Washington
Guardian

Saturday March 6, 1999

American scientists have discovered an unknown geological fault beneath the centre of Los Angeles capable, they fear, of producing an earthquake bigger than any yet to have hit southern California.

The fracture, christened the Puente Hills fault, is believed to be at least nine miles underground and runs in three broad segments for almost 25 miles under downtown Los Angeles.

Scientists believe that the active fault zone covers an area of around 250 square miles of densely settled urban land. The Puente Hills fault "is clearly a source of major earthquakes and likely could produce much damage to the Los Angeles area", John Shaw, the Harvard University geophysicist whose research on the fracture is published in the latest issue of the journal Science, told the Los Angeles Times.

"This fault system has not been considered in previous hazard assessments." The discovery of the potentially disastrous fault was made possible after geological survey information was given to the researchers by oil companies. It adds to the theory, which has gained weight over the last decade, that Los Angeles is ultimately more seriously threatened by so-called "blind thrust faults" like the Puente Hills fault, than by the more famous, larger and more visible San Andreas fault, which is much closer to the surface.

Recent findings of this kind have led many disaster experts to warn that engineering safety standards and building codes in areas of southern California threatened by earhquakes are inadequate and need strengthening.

The new fault is thought to have caused a 5.9 magnitude quake in Whittier Narrows, in the Los Angeles area, in 1987 which killed eight people and injured 200. The Puente Hills fault is similar in structure to the fault that caused the serious Northridge quake in 1994, which measured 6.7, killed 57 people and left 20,000 people homeless. About $40 billion (£25 billion) worth of damage was caused.

Unlike in that quake, however, a major earth movement from the Puente Hills fault would take place right under heavily populated areas.

At its highest point, the newly discovered fault lies 1.8 miles underneath the stadium of the Los Angeles Dodgers baseball team, which is owned by Rupert Murdoch.

By themselves, each of the fault's three segments could produce a magnitude 6.0-6.5 earthquake, Mr Shaw says. But if all three were to shift simultaneously, the effect would be a quake measuring 7 - catastrophic in a built-up area. Like all of southern California, the Los Angeles basin area is caught in a massive geological vice, in which the enormous Pacific and North American tectonic plates are pressing against one another.

The resulting slippage of rock along a fault line can move at anything from a fraction of an inch in a year to 5,000 miles an hour during major earthquakes. In 1998, southern California recorded more than 11,700 measurable earth movements.

"Finding a new fault doesn't mean that the overall hazard has changed, but that we know more about where an earthquake might occur and how large it is likely to be," a seismologist with the US Geological Survey, David Ward, was reported as saying. "This gives us a much clearer picture of what's down there."

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