Blind Thrust Fault Runs Under LA

ByPaul Recer

W A S H I N G T O N,  March 4 -- Researchers using a new technique have mapped a hidden fault that may have caused a 1987 Los Angeles earthquake. But government experts say the new work doesn’t affect their current estimate of the region’s earthquake risk.

Geologist John H. Shaw of Harvard and Peter M. Shearer of the Scripps Institute of Oceanography used oil company seismic records and other data to definitively locate a deep “blind thrust” fault that runs under downtown Los Angeles. A report on their study is in this week’s journal Science.

The fault was the source of the Oct. 1, 1987, Whittier Narrows earthquake that caused eight deaths and $358 million in damage, Shaw said. It was a magnitude 5.9–6.0 quake.

Existence Already Known

The existence of a blind thrust fault in the area described by Shaw and Shearer was already known, said Lucy Jones, the chief scientist for southern California of the U.S. Geological Survey. She said the new study “is a nice refinement” that slightly moves the fault’s location, but does not increase the agency’s assessment of the area’s quake risk.

A blind thrust fault is not clearly obvious on the surface. Such faults release energy by suddenly rising, a motion that is particularly destructive to buildings on the surface, Shaw said.

“We have suspected for years that these kinds of faults existed in the Los Angeles basin, but we’ve only been able to infer their presence,” said Shaw. “What allows the faults to escape detection is that their motion is consumed by the folding of rocks.”

The rock folding is “a telltale” sign of a hidden fault, even though the folds are not actually seen on the surface, Shaw said.

Center of Los Angeles Basin

By combining the oil company seismic profiles with other data for the first time, Shaw said he and Shearer were able to plot both the vertical and horizontal path of the blind thrust fault.

The fault goes from Coyote Hills, in northern Orange county, near the town of Brea, and runs for about 25 miles to the west, to beneath downtown Los Angeles. It is about 12 1/2 miles wide. It is about two miles deep in the center of the Los Angeles basin, and then dips to 10 miles as it continues northward.

The entire fault system covers an area of roughly 250 to 300 square miles, he said.

The fault is broken into three segments. Sudden ruptures along any one of those segments could produce quakes of 6.5 to 6.6 magnitude, Shaw said, and if all three ruptured at the same time, there is a potential for a magnitude 7 earthquake.

Important Step Forward

Based on an annual fault deformation rate of about two millimeters, Shaw estimated that segments of the fault will rupture about every 250 to 1,000 years, while all three might rupture together on a frequency of 500 to 2,000 years.

The problem, said Shaw, is that nobody knows when the fault last experienced a quake of 6.5 or high magnitude.

David J. Wald, a seismologist with the USGS in Pasadena, Calif., said the study “is an important step forward” in understanding the complex of faults underlying the Los Angeles basin.

“This gives us a much clearer picture of what’s down there,” said Wald.

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