Predicting The Effects of a 7.75 Earthquake in The L.A. Basin

When The San Andreas Fault Moves: Predicting The Effects of a 7.75 Earthquake in The L.A. Basin

Cheryl Dybas

National Science Foundation scientists report that a magnitude 7.75 earthquake along the San Andreas fault in southern California – one they give a 27 percent chance of occurring in the next 30 years – would shake the densely populated Los Angeles basin far more severely than they had expected. The evidence comes from a detailed computer simulation, which they describe as the most ambitious ever attempted for a major earthquake.

“I never expected that the ground motion would be so great over such a large area from an earthquake of this size,” says Ralph Archuleta of the University of California at Santa Barbara, one of the pioneering study’s authors.

The simulation showed that long-period ground motion the motion created by successive seismic waves arriving at intervals greater than 2.5 seconds in the Los Angeles area would be four to 10 times greater than that of the 1992 magnitude 7.3 Landers earthquake, California’s most powerful in 40 years.

The study, published in the December 8th issue of Science, was undertaken by seismologists Kim Olsen of UCSB and Joseph Matarese of MIT, along with Archuleta, to explore the risks of living in large, sediment-filled bowls like the Los Angeles basin, even at some distance from a major fault. It was funded by the National Science Foundation Southern California Earthquake Center (SCEC).

As was shown by the devastating effects of the 1985 Michoacan earthquake on Mexico City and the 1989 Loma Prieta earthquake on San Francisco’s Marina District, the compliant sediments filling such basins can greatly amplify ground motion, prolonging shaking and increasing its intensity.

Still, explains Archuleta, no one had ever undertaken a detailed study of the likely shaking in the 8,500-square mile greater Los Angeles area with its population of more than 10 million people, in part because the three dimensional nature of the problem presented an enormous computing challenge. The scientists overcame this obstacle by developing special computer techniques that enabled them to simulate an earthquake originating on the section of the San Andreas fault closest to the L.A. basin.

They chose the San Andreas fault – rather than other faults in the L.A. region because it has been the source of the area’s largest quakes in the past, and is likely to be in the future. The San Andreas is California’s great 700 mile seismic divide, marking the boundary where two of the earth’s major crustal slabs (“tectonic plates”) slide past each other.

Their postulated event began with a rupture six miles deep under Quail Lake, near Tejon Pass. It continued southeastward “like a zipper opening,” Archuleta says racing through the Mojave Desert and San Bernardino Mountains until it reached Mill Creek, east of the city of San Bernardino, 104 miles away. Total ground displacement on opposite sides of the fault was just under 16 feet. The rupture itself lasted 68 seconds, though the ground vibrated for nearly another minute.

The simulation indicated that the area between Interstates 5 and 405 from downtown Los Angeles to Anaheim and Santa Ana would experience strong swaying motion for about 60 seconds. Even so, the authors point out, such longperiod motion is generally not associated with damage to structures less than 10 stories high. Nor would it probably pose a serious threat to most family dwellings in the L.A. basin, they add. However, the simulation suggested that cities closest to the fault, such as Ontario, Pomona, San Bernardino, and Redlands, might feel even more intense ground motion.

In spite of the “what-if” nature of their exercise, the scientists emphasized that it was firmly grounded in reality: on December 12, 1812, the same stretch of the San Andreas fault was hit by an earthquake of similar size. Moreover, because large earthquakes along the fault’s Mojave and San Bernardino segments recur at similar intervals about once every 150 years – it was “reasonable,” they felt, to consider a scenario that had both segments rupturing together.

Archuleta and Olsen, who have done similar computer studies of Northridge-type earthquakes on faults in Los Angeles, said that the simulation showed that a major earthquake on the San Andreas fault would trigger still larger long-period ground motions. “This may require a shift in thinking, because many people have recently tended to believe that large earthquakes (between magnitude 6 and 7) within the L.A. basin present a greater threat than major earthquakes on the San Andreas,” says Archuleta.

“And there are still other scenarios that have to be examined for us to get a better sense of the range of ground motion that might be expected from a real earthquake on the San Andreas. This work is part of a larger program by SCEC scientists to produce realistic ground motion estimates for future earthquakes on the San Andreas and other faults within southern California,” he adds.