To many residents, the Seven Trees earthquake served as a terrifying omen, foreshadowing the giant temblor that could hit the Bay Area at any moment: The chances of a "Big One" — an earthquake of at least magnitude 6.7 — are two in three within the next thirty years, according to scientists at the U.S. Geological Survey.
But to 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 motion when the phone is plugged in and stationary, are generating data that has changed scientists' understanding of earthquakes, presenting a much clearer picture of how people experience them in a house, 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 who collect and analyze these recordings are making new observations from dozens of earthquakes that jolted the Pacific Northwest since 2016, the year they launched the MyShake app.
The biggest takeaway so far: Buildings amplify the motion of the quake by as much as a factor of three. When seismologists at Berkeley analyzed reports from smartphones, they found the phones accelerometers' recorded ground movement at three times the amplitude of traditional sensors that are dug in the earth, or placed on rocky outcroppings.
"That's really interesting," said Richard Allen, director of the Seismological Laboratory and leader of a team that developed the app. "Because the phone is on your desk or bedside table. It's recording the same ground shaking that we actually experience."
This might explain why the relatively modest South Bay quake provoked so much fear, and such clamor on Twitter, even though it caused no damage. A person standing inside a house or office might feel three times the clattering and rattling of a person standing outdoors.
Beth Pleasants, a resident of San Jose's Japantown neighborhood, said the lights above her kitchen counter began swaying back and forth when the tremor started. The noise activated her pet camera, which sent video footage to Pleasants' phone.
"It was wild," Pleasants said, recalling the moment that 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 it began rocking, as though someone were pushing on the bumper, she said. Through the windows, she glimpsed telephone poles vibrating outside.
The breadth 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 across the Bay Area.
San Francisco residents, who were right on the edge of the alerting region, got warnings about 18 seconds before the rumbling began. People in San Jose got less notice, and those close to the epicenter may have received alerts just as the earth started convulsing.
Alex Guichet said he got "about a second of warning" from the MyShake app at his office in Palo Alto, where the spasms were relatively benign. Ninety miles away, adjunct professor Ryan Hollister was teaching an earth sciences laboratory at Modesto Junior College, when he heard the shrill cacophony of alarms from his students' phones.
"I told everyone to drop and cover and hold on," Hollister said, recalling how most of his students ducked under tables and desks, even though they never felt tremors. People in classrooms on the second floor and down the hall "felt it slightly," Hollister said, a sign of the earthquake's reach.
Despite the parked cars lurching in downtown San Jose, and the ripplings through a large swath on the Bay Area, the Seven Trees earthquake left no wreckage or injuries. Its impact, or lack thereof, became visible almost instantaneously on the MyShake app, which displayed a regional map awash in blue hexagons, each one representing a person who reported shaking, but who witnessed no destruction.
San Jose Office of Emergency Management Director Ray Riordan credited building codes that have evolved over time to insulate the city, which is now dotted with single-story, woodframe homes that are bolted to the foundation. Allen said he would not expect any devastation from a 5.1 earthquake, which hovers in the "moderate" range on the Richter scale. The 6.9 Loma Prieta earthquake that felled Bay Area freeways and killed 63 people in 1989 was roughly 100 times stronger.
With the distribution of phones throughout the urban environment, we can now get a fairly detailed snapshot of earthquakes, seconds from when they happen, Allen said. Down the line, he added, scientists might detect structural damage to buildings by analyzing the oscillations recorded by smartphones over time.
Yet that hasn't stopped people from using more primitive sources of information. As the shaking subsided, Pleasants did what thousands of other Bay Area residents do when the earth gives way: She logged onto social media.
Correction: An earlier version of this story misstated Ryan Hollister's last name.
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