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Newfound L.A. Fault Threatens Major Quake

 

Stefan Lovgren
for National Geographic News
April 3, 2003
 
Between the sun and the stars, Los Angeles sometimes seems like paradise. But life in the City of Angels comes at a price: earthquakes.

Now the threat of "the big one" may be greater than previously feared. Researchers have identified a buried fault that may have caused at least four large-magnitude earthquakes in the past 11,000 years and is still active.

Known as the Puente Hills Blind Thrust System, the fault is three to 17 kilometers (2 to 11 miles) deep and extends for almost 50 kilometers (31 miles) from northern Orange County, through Los Angeles, up to Beverly Hills.

 

 Los Angeles

Earthquakes pose a constant threat to California. A recently identified fault found beneath the Los Angeles basin suggests the threat to the city is greater than previously thought.

Photograph copyright Jodi Cobb/National Geographic Society

"In terms of location, it couldn't be much worse," said James Dolan, a professor at University of Southern California's department of Earth sciences, who led the study. "Downtown L.A. is sitting on top of this thing."

Paleoseismologists have previously pinpointed the locations, magnitudes, and dates of ancient earthquakes, but never in so-called blind thrust faults. These are faults that don't extend to the surface of the Earth. Scientists have in fact debated if such faults exist beneath Los Angeles. The new study shows they both exist and could pose a credible earthquake hazard.

Earthquakes New and Old

The researchers received help for their study from an unexpected source: the oil industry. Companies like Texaco, which have spent millions of dollars on geologic drilling research in California, provided scientists with invaluable research data.

Using that information and high-resolution seismic reflection data, Dolan and colleagues drilled 15 bore holes, up to 40 meters (130 feet) deep, to study sediment layers overlying the hidden fault. What they found was subtle folding of the sediments revealing a history of ancient earthquakes.

The study shows the occurrence of at least four earthquakes with a magnitude of 7.2 to 7.5 on the Richter scale during the past 11,000 years. Perhaps most importantly, the 6.0-magnitude Whittier Narrows earthquake occurred in 1987 along a segment of Puente Hills, demonstrating that the fault system remains active and dangerous.

Squeezing L.A.

Geodetic studies show that Los Angeles is contracting. The northern point of the L.A. basin is moving closer to the southern point. "L.A. is being squeezed from north to south at about 4 to 5 millimeters [0.15-0.2 inch] per year," said Dolan.

This shortening, part of which is happening on top of recognized fault systems, literally bends the rock in the ground. The process stores energy, and when this energy exceeds the strength of the system, the fault breaks, triggering an earthquake.

Scientists believe that up to half of the energy stored in this process could be released on the Puente Hills fault.

"The good news is that major earthquakes along this fault are very infrequent, it may not happen again for thousands of years," said Dolan. "The bad news is that it could be very strong when it does happen."

An earthquake with a magnitude of about 7.5 on the Richter scale probably occurred on the Puente Hills fault 7,000 to 8,000 years ago. That means it was 15 times stronger than the 1994 earthquake that hit Northridge, north of Los Angeles, and killed 51 people.

The Northridge earthquake, which measured 6.7 on the Richter scale, caused U.S $44 billion in economic damage and is the largest natural disaster in U.S. history.

The tremblor proved that an earthquake smaller in magnitude can cause greater damage than a more powerful earthquake. The shaking in Northridge was some of the worst ever felt.

Bowl of Jello

In bigger earthquakes, shaking lasts longer and is felt over a larger area. There is a difference in frequency content between small and large earthquakes. Small earthquakes have higher frequency energy and can be particularly harmful to residential homes. Big earthquakes have low frequency energy and may cause more damage to large structures like skyscrapers.

What makes Los Angeles particularly vulnerable to any earthquake is that part of the basin the city is built on is filled with weak sediments. "The fault will pump energy directly into the basin and cause it to shake like a bowl of Jello," said Dolan.

Mexico City has a similar problem. That city was severely damaged in a 1985 earthquake, even though the epicenter lay in far-away Acapulco.

Establishing what kind of earthquakes could happen is critical for seismic hazard zoning, emergency response, and risk mitigation strategies.

"You want to find out as much as you can about the threat," said Dolan. "That gives you a batter chance to prepare for it."

The main challenge is to build earthquake-proof structures. As seismologists are fond of saying, "Earthquakes don't kill people, buildings do."

A summary of the research appears in the current issue of Science, the journal of the American Association for the Advancement of Science.

 
 


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