Pinning L.A. quakes down to a fault.

Seismologists hunting for faults in Los Angeles have always seemed like fishers surveying a well-stocked lake: Both groups know that their quarry hides beneath the surface in large numbers, but neither can say exactly where the prey lurks.

Until now. A pair of researchers has just reeled in a major catch by pinpointing the location of one of these elusive faults.

The concealed cracks beneath Los Angeles are called blind-thrust faults because they remain out of sight, covered over by thick layers of surface sediments. In an earthquake, these thrust faults serve as giant ramps, with one wedge of crust riding up another. Seismologists tuned into the dangers of blind thrusts during the 1987 Whittier Narrows earthquake, a magnitude 6.0 tremor that struck directly beneath metropolitan Los Angeles but never broke the surface. Although the blind thrust faults are far smaller than the San Andreas Fault, they pose a major hazard because of their proximity to the city.

By studying the topography throughout Los Angeles, geologists have been able to infer the general locations of some of these blind thrusts. In the new study, John H. Shaw of Harvard University and Peter M. Shearer of the Scripps Institute of Oceanography in La Jolla, Calif., probed deeper by gaining access to tightly held data collected by oil companies. To ferret out petroleum, such firms have scanned the hidden geology of Los Angeles by vibrating the ground with explosions and other means.

Shaw and Shearer discovered a large fault, which they named the Puente Hills thrust, running 40 kilometers in an east-west direction underneath the city. The plane of the fault dips down to the north at an angle of 27 [degrees] to the horizontal, and the structure appears to be broken into three distinct segments, they report in the March 5 Science. When Shaw and Shearer reanalyzed data from the Whittier Narrows earthquake, they found that the location of the quake precisely matched the position of the Puente Hills thrust.

Each of the newly identified fault segments could produce a magnitude 6.5 jolt, say the researchers. If the segments broke together, they could trigger an earthquake of magnitude 7.0, which would carry 30 times the energy of the Whittier Narrows shock.

"This study is a vast leap forward in understanding blind-thrust faults," says geologist James F. Dolan of the University of Southern California in Los Angeles. The new discovery, however, should not heighten the concern of jittery Angelinos because geologists knew that a blind thrust must have been down there, says Dolan. "This doesn't change our overall perception of the seismic hazard facing L.A."