for National Geographic News
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.
"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.
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.
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