Finding Fault Beneath The Earth

A hidden fault that caused a 1987 Los Angeles earthquake has been pinpointed by researchers using a new technique, but government experts said the work doesn't affect the current estimate of the region's earthquake risk.

In a study published Friday in the journal Science, geologists John H. Shaw of Harvard University and Peter M. Shearer of the Scripps Institute of Oceanography, used oil company seismic records and other data to locate a deep "blind thrust" fault that runs under downtown Los Angeles.

Shaw says the fault was the source of the Oct. 1, 1987, magnitude 6.0, earthquake that caused eight deaths and $358 million in damage.

Lucy Jones, the chief scientist of the U.S. Geological Survey for Southern California, says the existence of a blind thrust fault in the area described by Shaw and Shearer was already known. The new study, she says, "is a nice refinement" that better pinpoints the fault's location, but doesn't increase the agency's assessment of the area's quake risk.

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

"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," Shaw says. "What allows the faults to escape detection is that their motion is consumed by the folding of rocks."

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

The 12.5 mile wide fault begins in northern Orange County, near the town of Brea, and runs about 25 miles to the west directly under downtown Los Angeles.

The fault covers roughly 250 to 300 square miles and is broken into three segments. Sudden ruptures along any one of those segments could easily produce a 6.5 to 6.6 magnitude quake, Shaw says. If all three ruptured at the same time, there is a potential for a magnitude 7.

Shaw estimates that segments of the fault will rupture every 250 to 1,000 years, while all three might rupture together on a frequency of 500 to 2,000 years.

The problem is that no one knows when the fault last experienced a quake of 6.5 or higher.

David J. Wald, a seismologist with the USGS in Pasadena, Calif., says 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."