San Andreas looms larger in L.A.'s future.

When the San Andreas fault eventually unleashes the Big One, it will batter Los Angeles with waves of seismic energy far greater than seismologists had ever imagined, according to a ground-breaking computer simulation-the largest yet attempted for a San Andreas quake.

"We're looking at numbers that are two to three times what had been predicted previously," says seismologist Ralph J. Archuleta of the University of California, Santa Barbara. Archuleta worked with colleague Kim B. Olsen and with Joseph R. Matarese of the Massachusetts Institute of Technology to simulate the effects of a magnitude 7.75 tremor originating north and east of Los Angeles.

Seismologists put the chances of such a quake at one in four by the year 2024.

"When I look at the calculated ground motions, they are just breathtakingly large," says seismologist Paul G. Somerville of Woodward-Clyde, a consulting firm in Pasadena, Calif.

Angelenos shouldn't rush to pack their bags, however. Somerville and other researchers warn against placing too much faith in the computer simulation. Despite its massive size, they say, it suffers from limitations and uncertainties.

Even Archuleta hesitates to interpret the results. "Is this bad news? I don't know. I don't know if this will be significant or not," admits the seismologist, who discussed the work this week in San Francisco at a meeting of the American Geophysical Union. Archuleta and his coworkers also published their results in the Dec. 8 Science.

In the simulation, the quake ruptured a 170-kilometer-long stretch of the San Andreas fault from a region near the town of Gorman to the San Bernardino area east of Los Angeles. The virtual vibrations rippled through a three-dimensional representation that included realistic details of the local geology.

Other research groups have conducted simulations with the same algorithm, but Archuleta's team is the first to attempt computations for so big a quake. The simulation took 23 hours on a supercomputer with 512 parallel processors.

"It's a calculation that many have dreamed about for years," says Thomas H. Heaton of the California Institute of Technology in Pasadena.

Despite all this computing power, Archuleta's group had to limit the simulation to keep it manageable. The team looked only at vibrations with periods greater than 2.5 seconds. The rolling motion from these long-period waves threatens tall buildings and long bridges but does not usually harm buildings of a few stories or less, which constitute the majority of structures in Los Angeles.

Not surprisingly, the simulated vibrations were strongest in regions close to the San Andreas fault, including heavily populated San Bernardino. Unexpectedly, however, the computer exercise also showed the ground moving powerfully in parts of Los Angeles some 60 kilometers from the fault.

The waves are amplified as they pass beneath Los Angeles, explains Archuleta, because the city sits on a vast, sediment-filled basin. The soft sediments slow the waves as they enter, and the basin structure traps them-both factors that boost the size of the waves. The ground in some areas of Los Angeles moved at 1.4 meters per second, several times the rate expected.

If they struck an area with tall buildings, these exceptional waves could cause tremendous damage. But the simulation lacks sufficient resolution to show engineers whether the waves would hit the few sections of Los Angeles with high-rises. "It's going to take a fair amount of additional work to tell what this means for existing structures," says Heaton.

The study does not address short-period vibrations, the rapid jerking that tears small buildings apart. Some seismologists fear that the short-period waves from a San Andreas temblor would also exceed expectations.

Heaton disagrees, arguing that a giant quake outside Los Angeles is unlikely to shake the city with short-period waves much more destructive than those of last year's magnitude 6.7 Northridge shock. If big San Andreas jolts did create monster short-period waves, then the last superquake in 1857 should have devastated the city. Most of its 5,000 residents survived the tremor, however.