Hidden, Quake-Prone Fault Confirmed Under Los Angeles

Most earthquake hazard assessments in southern California have focused on faults, such as the San Andreas, that reach the surface and are therefore more easily studied. But "blind-thrust" faults similar to the one identified here have been responsible for a recent string of very damaging events, including the 6.7 magnitude Northridge earthquake in 1994. John Shaw of Harvard University and Peter Shearer of the Scripps Institute of Oceanography co-authored the report using a new seismic velocity model based on rarely obtained information from the petroleum industry, including high-resolution, sonogram-like images of the fault itself. Other studies have had to presume the presence of blind-thrust faults with underground images of nearby tell-tale rock folds.

The newly mapped fault is 40 kilometers long and runs from beneath downtown Los Angeles to the Coyote Hills in northern Orange County and towards Brea in the east, covering at least 840 square kilometers. "This an important earthquake source for Los Angeles," says Shaw, "and one that we've been able to establish beyond inference."

The authors' images show that there are three distinct segments to the fault system: one under Los Angeles proper; another underneath Santa Fe Springs; and the third beneath Coyote Hills. The fault comes closest to the surface-about 3 kilometers down-in the southern region, then dips to the north toward Los Angeles, reaching a maximum depth of about 17 kilometers.

Significantly, the segment underlying Santa Fe Springs has the same "strike and dip"-the characteristic angles by which faults deviate from the horizontal-as the fault predicted to exist on the basis of seismic data gathered to explain the Whittier Narrows earthquake. (However, the new fault is distinct from the Elysian Park fault, which had been put forth last spring as a Whittier Narrows candidate.) The authors say that the precision of their images coupled with other evidence strongly suggests they've found the real culprit, and that the fault system is capable of even larger and more destructive earthquakes.

Although it's unknown how often the newly mapped fault system has ruptured in the past, the authors are still concerned that it may pose a significant and "previously unconsidered hazard." That's because the Whittier Narrows earthquake appears to have ruptured only about 10 percent of the newly mapped fault. Ruptures along the full length of any one of the three segments "could generate 6.5 to 6.6 moment magnitude earthquakes," they say. A much larger earthquake of at least 7.0 magnitude could occur if all three ruptured at the same time or if the new fault system turns out to extend below the region involved in the Whittier Narrows temblor.

It's difficult to forecast activity on the fault without knowing the history of past ruptures, but based on an estimated rate of deformation between 0.5 and 2.0 millimeters a year, the authors guess that each fault segment could rupture once every 250 to 1,000 years. A quake that involved all three segments, they say, would occur less frequently-perhaps once every 500 to 2,000 years.