Oct 9, 2020 8:00 AM

The Most Sway-Prone Buildings in LA Aren’t Where You Expect

Using a network of sensors, researchers found that the Los Angeles high-rises most likely to rock in a future quake aren’t downtown.

High-rise buildings in some parts of Los Angeles may sway four times as much during an earthquake as those in the downtown area, according to a study that reveals new information about how underlying geology transmits a big quake’s power into buildings above. Most at risk are tall buildings in West LA and parts of the south-central San Fernando Valley, according to a study published this month in the journal Seismological Research Letters. The findings may force changes in how California state officials calculate building codes, says Monica Kohler, lead author and professor of civil and mechanical engineering at the California Institute of Technology.

Much of the greater LA area is built on a giant pit of sand and debris that extends 30,000 feet down at its deepest point before hitting solid bedrock. That deep spot is just south of downtown, and until now, geologists and engineers believed that the deeper the underlying basin, the more a tall building will sway during an earthquake. But the new study found that even buildings erected on relatively shallow areas are vulnerable to the energy generated by a big quake.

“We thought the amplifications would be south of downtown LA, but we didn’t see that,” Kohler says. “We saw them elsewhere, where the basin is not deep, in West LA and the San Fernando Valley.”

West LA includes the Century City commercial and entertainment district and the Wilshire Corridor, home to dozens of high-rise condos and office buildings. The San Fernando Valley sits on a separate geological basin. “That’s of concern,” Kohler says about the new findings, because the California building code may not correctly account for crucial factors other than basins.

On the evening of July 5, 2019, a 7.1-magnitude earthquake struck near the town of Ridgecrest, about 124 miles north of Los Angeles. It was the main shock in a series of several related quakes that began on July 4 and continued for several days. The earthquake was so strong that it created massive cracks in the arid landscape, some as much as 15 feet wide, according to a study on the quake’s effects by the US Geological Survey and other experts. While the big Ridgecrest quake didn’t cause major damage in Los Angeles, people in buildings taller than 15 stories reported frightening swaying that lasted for up to two minutes, the study reported. Some felt nauseous, according to a report in the Los Angeles Times.

What Kohler and her colleagues wanted to know was where the swaying was the longest, and why. They looked at observations recorded by 560 ground‐level devices called accelerometers that measure how much an object moves back and forth. The sensors were part of a citizen science project called the Community Seismic Network and were placed in computer closets at Los Angeles County schools across the region. The accelerometers measured the vibrations from the earthquake 250 times per second, enough data to see how the quake’s force changed over time.

Each building has something called a “resonant period,” which is the amount of time it takes to move back and forth along the horizontal plane during a quake. Buildings taller than 15 floors, suspension bridges, and big petroleum tanks often have a resonant period of more than three seconds. The study found that buildings with a long resonant period experienced more shaking from the Ridgecrest quake than those with shorter periods, and the amount of shaking wasn’t connected to how deep the basin is underneath.

The researchers then used the data from the school sensors to figure out how much sway to expect from LA buildings during future earthquakes. Kohler says that a quake similar in magnitude to Ridgecrest could cause high-rise buildings in West LA and the valley to experience shaking four times greater than a building located in downtown Los Angeles. In a 52-story building, this means that the upper floors might sway back and forth by as much as 3 feet, and could move twice that much during a more powerful magnitude-7.6 earthquake. A sway that big could stress the building’s structural integrity, especially for those built decades ago during an era of less-stringent seismic building codes.

“The building won't necessarily collapse during these types of shaking amplitudes,” Kohler says, “but significant fracturing or buckling could occur in and around multiple beam-column connections or brace-frame connections. These effects may not be visible, and the buildings would be significantly weakened and not prepared to withstand the next earthquake.”

Kohler says that when seismic waves from an earthquake enter the softer sediments that fill the basin underneath Los Angeles, the waves slow down and their energy piles up, creating larger-amplitude waves that lead to stronger shaking. But the study didn’t find a close correlation between the depth of the basin and the amount of swaying in the buildings. Kohler believes that sediment carried from hills and mountains by old rivers that once flowed into the greater LA basin may affect the shaking of buildings today, and so might the removal of oil and gas from geologic deposits during the 20th century.

“I suspect that the locations, depths, and extent of these deep LA river-related sediments and groundwater basins, defined in part by the fault locations, may be playing a role in the seismic amplification patterns,” Kohler says. “We are in the process of testing this idea, but have not published work on it.” Figuring that out will take more sensors in those areas, she adds.

Benson Shing, professor of civil engineering at the University of California, San Diego, who was not involved in the study, agrees that it's a good idea to find out more about the ground beneath LA high-rises. “Perhaps we should revisit the seismic zoning map, which is used in the building code, to see if any changes would be warranted, especially in the LA basin area,” Shing says. “That might be good to investigate. But we cannot draw a quick conclusion that those buildings would be unsafe in earthquakes.”

Predicting how buildings will respond to earthquakes is extremely difficult, because the seismic energy flows around the LA basin almost like a fluid wave, building up in some areas and dispersing in others, depending on the kind of rocks and other geological features it encounters underground, says Robert Graves, a research seismologist at the US Geological Survey in Pasadena, California. “You get channeling of waves through different parts of the basin,” he says.

Graves says the new study is important because it is using a ton of new data from the sensor network, which has spaced the accelerometers much closer than networks in other seismic hot zones on the West Coast, such as the Bay Area or Seattle. “The key is to find some predictive features or measures,” he says. “Hopefully if it works in one basin, in LA, the equations of motion will also apply in San Francisco or Seattle. Earthquakes don’t happen that often, so the opportunity to get ground-truth data comes along infrequently.”

Update 10-12-2020 8:36 PM: This story was updated to correct Monica Kohler's statements on the correlation between basin depth and building sway found in her study, and her comments on the California building code.