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Friday, March 5, 1999 Published at 00:26 GMT


Hidden fault discovered under LA

The fault system has the potential to do great damage

Scientists have discovered a previously unkown fault system in the rock under Los Angeles which is capable of producing huge earthquakes.

They believe it was probably responsible for the Whittier Narrows earthquake which killed eight people and injured 200 others in 1987.

They warn it could produce even more powerful quakes in the future.

The system, which is not visible on the surface, runs for 40 km (25 miles) from downtown Los Angeles to the Coyote Hills in northern Orange County and toward Brea in the east.

It covers at least 840 square km (324 square miles), John Shaw of Harvard University and Peter Shearer of the Scripps Institute of Oceanography report in the journal Science.

Damaging events

Faults which appear on the surface of the land, such as the San Andreas fault, are much easier to study. But scientists would like to get much more information about so called "blind-thrust" faults similar to the one now identified by Shaw and Shearer.

They have been responsible for a recent string of very damaging events, including the 6.7 magnitude Northridge earthquake in 1994.

Shaw and Shearer used a new seismic model to map the fault system. It was based on data rarely obtained from the oil industry.

They found three distinct segments to the system - one under Los Angeles, another underneath Santa Fe Springs and the third beneath Coyote Hills.

It is closest to the surface in the south - about 3 km (1.8 miles) down - and reaches to a depth of about 17 km (10.5 miles) towards the north.

Unconsidered hazard

"This an important earthquake source for Los Angeles," says John Shaw, "and one that we've been able to establish beyond inference."

It is impossible to say how often the newly mapped fault system has ruptured in the past, but the authors are still concerned that it may pose a significant and "previously unconsidered hazard."

It appears that the Whittier Narrows event (6.0 magnitude) ruptured just 10 percent of the newly mapped fault system. Further ruptures along the full length of any one of the three segments identified "could generate 6.5 to 6.6 moment magnitude earthquakes," Shaw and Shearer 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 quake.

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