Study: LA buildings will shake more in earthquakes than previously predicted
There’s a shift happening in earthquake science, and it’s not just in the tectonic plates. Results of a new five-year study reveal that the way experts model potential earthquakes is changing for good.
Experts behind the CyberShake project say they have identified a far more accurate and localized method of simulating earthquakes’ effects at specific locations. Based on the new models, LA is expected to see 10 to 50 percent more shaking across all magnitudes of earthquakes than previous models suggested.
The results have major implications for LA’s skyscrapers—and the future of building safety.
“We can’t predict earthquakes,” says Tom Jordan, a professor of geological sciences at USC and lead author of CyberShake. “But what we’re doing is a much better job of predicting what will happen when they occur.”
In the wake of CyberShake’s findings, developers and builders will have to reassess the safety of “tall buildings”—what experts call buildings of 20 stories or more. They’ll also have to figure out how to build stronger structures in the future, according to John Vidale, director of the Southern California Earthquake Center, which ran the study.
In the past, LA city had refused calls from experts to make a comprehensive list of earthquake-prone buildings. But in the past few years, the city has amped up efforts to supply engineers with the data they’ll need to secure buildings against the ever-impending “big one.”
In 2014, they identified approximately 14,000 buildings, including apartments buildings, that are at risk of collapsing in a major earthquake. Scientists at UCLA have also launched a program to expand earthquake-resistant design for individual skyscrapers and earthquake-resilient design for entire communities.
And city law now requires tall buildings to be outfitted with earthquake data-gathering instruments.
In the decades before CyberShake, scientists relied on retroactive models called Ground Motion Prediction Equations, or GMPEs. These equations are pieced together using historical records of quakes from around the world.
CyberShake, which was launched in response to data showing that the Los Angeles Basin was moving differently than previous calculations predicted, revealed that past methods resulted, not only in very broad estimates, but also in largely inaccurate ones. Instead, it relies on local conditions and existing geology.
“We’re making a shift from thinking about how dangerous the quakes are by looking at past records, to calculating what the next ones will look like based on geology,” says Vidale.
“Instead of going on the track record, we’re looking at the physics of where we are and what’s coming next,” he says.
Historically, GMPEs were used as the basis for building code design. But now, with CyberShake’s technology, its authors say engineers can couple predictive seismograms with structural simulations to better design buildings. That’s especially important civic facilities such as hospitals, dams, and power plants.
It is also possible that, using CyberShake’s technology, engineers will be able to locate a building plan tailored for seismic hazards specific to a proposed site.
The new models also reveal that some geographical regions are more dangerous than previously thought. Those in the large basins, like LA and Mexico City are particularly at risk, because the ground is soft and deep, so much so that experts often equate the shake to the jiggling of a bowl of Jell-O.
“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,” according to the New York Times.
But experts are quick to remind us that estimates are not guarantees.
“It doesn’t mean that the ground motions will go super high at all the locations,” co-author and developer of CyberShake Christine Goulet tells Curbed. “It may increase the risk, effectively, but we don’t know by which amount.”
That might mean there will be more windowless skyscrapers in LA’s future. The strength of a building is determined by factors like how much steel is used, and how many open spaces and windows it has. Buildings with open spaces like atriums and open causeways are much weaker in the event of an earthquake.
Moving forward, engineers and city officials will have to grapple with how to implement these new predictions into safer building codes. But because the building codes change every six years, there are many buildings in LA right now that are technically up to code, but structurally unsound, according to Goulet. Landlords and developers will have to decide whether to reinforce those buildings that were deemed earthquake-safe as far back as the 1930s.
“People have to realize that codes evolve over time,” Goulet cautioned. “As a society, we’ll have to make choices.” Choices, Goulet says, about what era code should be used as the safety standard.
Seattle is one of the first cities to have made that choice.It has announced that it will start using CyberShake’s new maps in determining building codes starting December 1.
For now, in LA, engineers and scientists like Goulet are waiting to hear when the new models will be taken into account.
“We have to incorporate it,” Jim Malley, a structural engineer and co-organizer of the conference told the New York Times. “We haven’t settled on how.”
Author: Marissa Clifford | July 9, 2018
Source: Curbed LA