For about seven jaw-dropping seconds on a Tuesday morning in October, a magnitude 5.1 earthquake ruptured the Calaveras Fault near San Jose, rocking parked cars, rocking skyscrapers and alerting 100,000 phones laptops.
For many residents, the Seven Trees earthquake was a terrifying omen, foreshadowing the giant quake that could hit the Bay Area at any time: the chances of a “Big One” – an earthquake of at least 6.7 in magnitude – are two out of three in the next thirty years, according to scientists from the US Geological Survey.
But for experts at UC Berkeley, the October quake became a sprawling research project in which thousands of individual smartphones served as seismic instruments. These phones, equipped with a MyShake app that records ground motions when the phone is plugged in and turned off, generate data that has changed scientists’ understanding of earthquakes, presenting a much clearer picture of how whose people experience them in a home or a classroom. , or on the eighteenth floor of San Jose City Hall.
Six hundred phones across the Bay Area recorded waveforms from the Seven Trees earthquake and uploaded the data to the Berkeley Seismological Laboratory. Researchers collecting and analyzing these records are making new observations from dozens of earthquakes that have rocked the Pacific Northwest since 2016, the year they launched the MyShake app.
The biggest takeaway so far: Buildings amplify earthquake motion by up to a factor of three. When Berkeley seismologists analyzed reports from smartphones, they found the ground motion recorded by the phones’ accelerometers to be three times the amplitude of traditional sensors that are dug into the ground or placed on rock outcrops.
A person checks the ‘MyShake’ app on their smartphone in Hollywood on October 17, 2019. – California launched the nation’s first earthquake warning system on Thursday in hopes residents will be alerted within seconds of impending doom and will be able to “get down, cover up and hold on.” (Photo by Chris Delmas/AFP) (Photo by CHRIS DELMAS/AFP via Getty Images)
CHRIS DELMAS, Contributor / AFP via Getty Images“It’s really interesting,” said Richard Allen, director of the seismology lab and leader of a team that developed the app. “Because the phone is on your desk or bedside table. It records the same earthquake that we are actually experiencing.
This might explain why the relatively small South Bay earthquake caused so much fear and outcry on Twitter, even though it caused no damage. A person standing inside a home or office can experience three times more rattles and oscillations than a person standing outside.
Beth Pleasants, a resident of San Jose’s Japantown neighborhood, said the lights above her kitchen counter began to swing back and forth when the shaking started. The noise activated his pet’s camera, which sent video footage to Pleasants’ phone.
“It was wild,” Pleasants said, recalling the moment she and thousands of other Bay Area residents felt the earth rumble, at 11:42 a.m. on Oct. 25. Pleasants was sitting in her parked car when she started rocking, like someone was pushing on the bumper, she said. Through the windows, she saw telephone poles vibrating outside.
The magnitude of the Seven Trees earthquake made it a compelling test of the MyShake alert system, Allen said, in that it distributed alerts to smartphone users throughout the Bay Area.
Residents of San Francisco, who were just on the edge of the alert zone, received warnings about 18 seconds before the rumble began. Residents of San Jose were less warned, and those near the epicenter may have received alerts just as the earth began to convulse.
Alex Guichet said he received “about a second warning” from the MyShake app in his office in Palo Alto, where the spasms were relatively mild. Ninety miles away, Assistant Professor Ryan Patterson was teaching an earth science lab at Modesto Junior College, when he heard the shrill cacophony of alarms coming from his students’ phones.
“I told everyone to get down, cover up and hang on,” Patterson said, recalling how most of his students hid under tables and desks, even though they weren’t. have never felt tremors. People in classrooms on the second floor and down the hall “felt it slightly,” Patterson said, a sign of the earthquake’s reach.
Despite wobbling parked cars in downtown San Jose and ripples across a wide swath in the Bay Area, the Alum Rock earthquake caused no damage. Its impact, or lack thereof, became visible almost instantly on the MyShake app, which displayed a regional map awash with blue hexagons, each representing a reported person shaking, but no destruction.
San Jose Department of Emergency Management Director Ray Riordan credited building codes that have evolved over time with insulating the city, which is now dotted with single-story wood-frame homes that are bolted down at the foundation. Allen said he wouldn’t expect any devastation from a 5.1 earthquake, which is in the “moderate” range on the Richter scale. The 6.9 Loma Prieta earthquake that destroyed Bay Area highways and killed 63 people in 1989 was about 100 times stronger.
With the distribution of phones throughout the urban environment, we can now get a fairly detailed snapshot of earthquakes, seconds after they occur, Allen said. Ultimately, he added, scientists could detect structural damage to buildings by analyzing oscillations recorded by smartphones over time.
Yet that hasn’t stopped people from using more primitive sources of information. As the tremors subsided, Pleasants did what thousands of other Bay Area residents do when the ground shatters and shakes: she took to social media.
Rachel Swan is a staff writer for the San Francisco Chronicle. Email: rswan@sfchronicle.com Twitter: @rachelswan
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