Can Los Angeles rise out a stronger quake?

A giant lurking beyond the Los Angeles horizon, the San Andreas fault has long loomed in the fears of southern Californians, who wonder when the expected "Big One" will strike. But seismologists in recent years have also urged the city to consider the threat from lesser quakes on hidden faults directly beneath Los Angeles, a point punctuated by the Northridge quake that struck on Jan. 17.

Although it racked up $15 billion in damages and killed 61 people, the magnitude 6.7 quake spared tall buildings. But the city cannot count on such luck next time, say engineers, seismologists, and emergency-response officials, who met last week in Pasadena, Calif., to discuss the effects of a magnitude 7 quake beneath downtown Los Angeles.

This type of shock would apparently shift the ground enough to topple some 20-story buildings built to current safety standards, says John Hall, an engineer at the California Institute of Technology in Pasadena.

The hypothetical-earthquake exercise, planned long before January's disaster, addressed a shock occurring on the Elysian Park fault, a fracture that produced a magnitude 5.9 quake in 1989. A magnitude 7 jolt would release three times the energy of the Northridge shock but only 3 percent of the energy of a magnitude 8 San Andreas quake.

Although not huge, the modeled quake destroyed some tall buildings in Hall's study because they could not withstand the large, fast ground shifts that occur close to a fault. Such pulses of motion travel wavelike up tall buildings, causing them to lean. If the period of the pulses matches the building's height, the tilting gets amplified and the structure can fall over.

Unlike shaking, which spreads far from a quake's epicenter, severe ground shifts happen only near a fault in a large shock. Seismologists have warned of fast displacements in the past, but they have lacked any direct evidence of them. As a result, construction codes have not taken this motion into account. "These near-fault pulses are what's missing in the code. We really haven't had much experience with them," says Hall.

Last week, Caltech researchers reported measurements from the 1992 Landers earthquake proving that land nearby does shift rapidly At a site only 2 kilometers from the fault, the ground moved 2.5 meters in 4 seconds.

Besides the new findings about ground movements, engineers must also grapple with unanticipated problems in steel-frame buildings that surfaced during the Northridge quake. So far, inspectors have found cracked joints between beams and columns in at least 50 buildings.

No steel-frame buildings collapsed during the January jolt. But the fractured joints have weakened some structures, reducing their safety in future shocks. In light of the joint problem, Hall wonders whether even undamaged steel buildings would survive a stronger quake. He notes, however, that most deaths probably would occur in smaller buildings already known to be seismically unsafe.

Highway bridges should ride out a magnitude 7 shaker without major damage, says James E. Roberts of the California Department of Transportation in Sacramento. Although several freeway bridges collapsed in January, strengthened bridges survived and should withstand stronger jolts, he says.

Officials have less optimism about the city's ability to fight quake-sparked fires, which traditionally cause much of the damage during such disasters. Robert Canfield, emergency preparedness coordinator for Los Angeles, noted that the city devoted half its fire-fighting resources to save one burning high-rise in 1987. Should several such blazes break out, the city could rescue people, but it could not save those buildings. "We are going to have to walk away from them and devote our first efforts to saving lives," Canfield says.

He and others stressed that Northridge claimed so few lives in part because it struck at 4:30 a.m. on a holiday, a time when the garages and freeways that collapsed were nearly empty.

Although the scenario involved a hypothetical earthquake, seismologists say Los Angeles sits on several faults that could produce quakes of magnitude 7 or greater. What's more, the city has not had enough Northridge-size quakes in its history to relieve the strain building underground, says James E Dolan from Caltech. Magnitude 7 shocks, if they occurred every few hundred years, could release this pressure. But the city has not seen such a killer in at least 2 centuries, a fact that sets seismologists on edge.