A seismic change in predicting how earthquakes will shake tall buildings

Los Angeles, Salt Lake City, San Jose and Seattle are among the cities that have buildings that could sustain more damage than anticipated or, in the worst case, have a greater potential for collapse, engineers said.

“There are going to be large changes coming,” Norman Abrahamson, a seismologist at the University of California, Berkeley, told hundreds of engineers gathered for the conference. “We now know how far-off our ground motion models have been.”

In some areas of Los Angeles County like Century City, Culver City, Long Beach or Santa Monica, the new projections nearly double the previous estimates for the type of ground shaking that is most threatening to a tall building.

Ibbi Almufti, a researcher with the engineering firm Arup, said the significance of the new projections was “huge.”

“It’s going to amplify the shaking in terms of intensity but also the duration,” he said. “Those two things combined can have quite a damaging effect that right now we are probably not capturing.”

Greater shaking could also “bring out the vulnerabilities” in older buildings already known to have defects, Almufti said.

The revised estimates for Los Angeles are the result of a five-year project by the Southern California Earthquake Center, a research organization of seismologists and engineers, that used some of the country’s most powerful supercomputers to study how earthquake shaking moves through local ground conditions.

For decades experts have arrived at their calculations of shaking by observing conditions in California, Japan, Taiwan and other seismically active places and taking an average. But they discovered that grouping far-flung regions created imprecise estimates.

The crucial changes in the new models are that they rely on local conditions, not global averages, and they model the ground more deeply. The Los Angeles model relied on measurements of thousands of local earthquakes, most of them imperceptible but which offered more precise information on how seismic shock waves travel through the earth.

Understanding how earthquakes affect cities like Los Angeles, Mexico City and Seattle have long bedeviled earthquake experts because they sit in large basins where seismic waves are trapped and amplified. The effect is often compared to the way a bowl of Jell-O reacts when jolted.

The new projections of shaking in Los Angeles and other cities apply only to buildings of about 20 stories or more. But Abrahamson said calculations that would be rolled out over the next few years would offer revisions of shaking for all structures and areas across the West. In some cases, the revisions will predict lower shaking estimates than previously thought.

California has not had a major earthquake since 1994 when a 6.7 magnitude earthquake struck the northern Los Angeles neighborhood of Northridge, killing more than 60 people and causing widespread damage. But the seismic faults in California can produce earthquakes that release well more than 50 times more energy than Northridge. There are no reliable ways of knowing where and when the next big earthquake will strike.

The new projections have been met with resistance by some engineers, some of whom fear that it could drive away developers.

In Seattle, where earthquakes have the potential to be even stronger than in California, engineers will be required to take into account new projections of shaking that are 33 percent higher than the old ones, said C.B. Crouse, an expert in ground motions who helped write the new guidelines.

“That’s a significant increase from the standpoint of building design,” Crouse said. But because of pushback by engineers in Seattle, the use of the new projections in building codes has been delayed until December.

“The structural engineers said this is really going to cause a problem with developers up here,” he said. “They said, ‘We can’t institute this immediately.'”

Jim Malley, a structural engineer who helped organize the conference, said implementation was a concern.

“We have to incorporate it,” he said of the new data. “We haven’t settled on how.”

Even more difficult is the question of how to handle existing buildings in areas where the ground shaking is projected to be significantly higher.

“These cities and buildings are already in place,” said Thomas Heaton, the director of the Earthquake Engineering Research Laboratory at the California Institute of Technology. “Now what do we do?”

At a time of a severe shortage of housing in California, where the median price of a home is above $600,000, requiring retrofits would be a heavy financial burden.

John Vidale, the director of the Southern California Earthquake Center, said the revised projections would ultimately help engineers.

“What we are doing is mapping out things more precisely,” he said. “We are making more accurate maps. And we are shrinking the uncertainties.”

This article originally appeared in The New York Times.

A seismic change in predicting how earthquakes will shake tall buildings

The research, presented Wednesday at a gathering of earthquake experts in Los Angeles, has significant consequences in the ways tall buildings are designed.

JOIN OUR PULSE COMMUNITY!