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

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11 shares, 65 points

Eric Niiler

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

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 “hasn’t taken into account whether you are built over a basin or not.”

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.

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