Threat From San Andreas Quake Upgraded : Disasters: Study using computer model finds it would harm high-rises and bridges more than temblors on closer faults, which were believed more dangerous.

A magnitude 7.75 earthquake along the San Andreas fault in Southern California--one that researchers say has a one in four chance of occurring in the next 30 years--would shake the densely populated Los Angeles Basin far more severely than previously believed, scientists announced Thursday.

The slow, rolling tremors generated by such an earthquake--strong enough to set buildings 40 miles away swaying like ships in high seas--would pose a greater threat to the high-rise buildings and bridges of the Los Angeles region than did the 1994 Northridge quake, even though the epicenter would be much farther away.

“This may require a shift in thinking because many people have recently tended to believe that large earthquakes in the L.A. Basin represent a greater threat” than those on the San Andreas, said Ralph J. Archuleta, a seismologist at UC Santa Barbara who helped develop the computer model.

However, scientists said that because of the physics of the shock waves, the ground motion from the San Andreas quake would pose less danger to most homes and low-rise buildings in the Los Angeles Basin than it would to skyscrapers.

The study by researchers at UC Santa Barbara and the Massachusetts Institute of Technology, published today in the journal Science, is the first serious reevaluation of the effects of a San Andreas earthquake since the 6.7 Northridge quake, which occurred on a much smaller fault system. The study, funded by the National Science Foundation, is also the first to take into account the complete geological complexity of Southern California.

The findings are the product of what other experts called the most sophisticated computer simulation of a major earthquake ever achieved. An MIT computer, composed of 512 processors linked in parallel, churned for 23 hours to perform all the necessary calculations.

“This is a very significant advance on a long-standing problem in engineering and seismology--the issue of how the ground in Los Angeles will move in a large San Andreas earthquake,” said Caltech earthquake expert Thomas H. Heaton. “A San Andreas earthquake is pretty inevitable for us.”

Archuleta and his colleagues were inspired, in part, by the devastating effects of the 1985 Michoacan earthquake on Mexico City and the 1989 Loma Prieta earthquake on San Francisco. In each instance, the temblors caused unexpectedly severe damage in distant urban areas.

The researchers used their computer to simulate the effects of a major earthquake striking under Quail Lake near the Tejon Pass, on the segment of the 700-mile-long San Andreas fault closest to Los Angeles. The simulation was based on a major earthquake there in 1812; a similar quake is believed to occur every 150 years.

The computer-generated quake lasted 68 seconds at its center, sending out shock waves that reverberated for another minute. By comparison, it took barely 10 seconds for the Northridge fault to rupture, and it generated shock waves that lasted about 30 seconds.

Until recently, there was simply not enough computer power to simulate all the variations in the region’s subterranean basins, sedimentary folds and rock formations, which would all act to reflect, refract or deflect the shock waves radiating from the epicenter.

“For the first time, we could run a simulated earthquake of a magnitude comparable to what ruptured in 1812, and we could watch the waves as they propagated into Los Angeles,” Archuleta said.

Buildings respond to the shock waves of an earthquake the way a tuning fork responds to musical pitch, and only the buildings taller than 10 stories or very long horizontal structures such as bridges are likely to be affected by the kind of seismic ripples that would reach the metropolitan region, the scientists said.

The most damaging seismic waves, he said, are like the motion of a slow playground swing. “If you were pushing a swing, [the long-period motion] would take 2 1/2 seconds to come out and come back,” Archuleta said.

Houses and other one- or two-story structures are most affected by high frequency “short-period” ground shaking, which is quickly dampened by the earth and normally does not travel very far.

“One does not expect really violent shaking for most of our inhabitants and most of our buildings,” Heaton said.

But the scientists estimate that the long-period ground motion generated in the 8,500-square-mile Los Angeles region by the imaginary temblor would be four to 10 times greater than that of the 7.5 Landers earthquake, which struck in 1992 and was California’s most powerful temblor in 40 years.

The long-period ground motion also would be more severe than that caused by an earthquake like Northridge along faults under the metropolitan area, the scientists reported.

“We have simulated the ground motions from a 6.75 earthquake on the Palos Verdes fault, Northridge and the Elysian Park fault that, compared to San Andreas, are significantly less, even though they are closer,” Archuleta said.

They found that in the San Andreas quake, the area between the Santa Ana and San Diego freeways from Anaheim to Santa Ana would experience strong swaying motions for up to a minute.

Cities closer to the fault, such as Ontario, Pomona, San Bernardino and Redlands, might have more intense ground motions, the scientists said.