Series of Earthquakes Jolt the Bay Area. It’s a Good Reminder to Be Prepared

A series of earthquakes rattled the East Bay on Thursday, adding a jolt of quake anxiety to what was already a rain-soaked day.

The largest of the bunch, a 3.7 magnitude earthquake, struck shortly before 2 p.m. just east of Hayward, sending a short but strong shockwave through much of the Bay Area. It followed two other small quakes — likely foreshocks — in the hours before.

The shaking could be felt as far north as Vallejo and south throughout San José, according to U.S. Geological Survey data.

The quakes’ epicenters surrounded the Hayward fault, which spans from San Pablo Bay in the north to Fremont in the south. It runs under the cities of Berkeley, Oakland and Hayward before connecting to the longer Calaveras fault.

Hayward’s slip-strike fault hasn’t been hit with a strong earthquake since the 1860s, according to USGS, but do these smaller shakes mean the Big One is happening soon?

According to Peggy Hellweg with UC Berkeley’s Seismology Lab, who spoke to KQED in 2019, it’s not so easy to know.

She said at the time that small quakes happen along the Hayward fault all the time, as they did in January of that year.

In the past 20 to 30 years, those worrisome, but fairly harmless, shakes had never led to a “big one.”

The longer the fault goes without a significant quake, though, the more pressure it builds up, Hellweg said. And the widespread myth that small earthquakes release some of this tension isn’t true.

The average interval between major seismic events on the Hayward fault is about 140 years, plus or minus 50. USGS researchers say there’s about a 33% chance a magnitude 7 quake could take place on the fault in the next three decades, and there’s a slightly lower but still significant chance that two other faults that run through the Bay Area, the Paicines and San Andreas, could see some action.

One concern on the East Bay fault specifically is that it is made of a combination of locked and creeping lines. While the creeping ones move, normally causing some slight shaking, locked faults don’t. Instead, shaking on those could be foreshocks of a larger quake to come, UC Berkeley seismologist Roland Burgmann previously told KQED.

Whenever a quake — big or small — occurs, it’s a good time to check on earthquake kits and make sure to have the MyShake app downloaded.

The app, created by Berkeley’s Seismology Lab, gave Californians a few seconds’ notice before significant earthquakes last year and could make a real difference when a big one hits.

The farther from the epicenter, the longer the warning time you can usually get from the app, which sends out a notification instructing people to do what they’ve been taught since kindergarten but could forget in a panic: duck, cover and hold on.

“We’ve seen that for many earthquakes, particularly on the West Coast, in California, the people that are injured are injured by things falling on them because they’re trying to get to safety,” said Angie Lux, a project scientist for the Berkeley Seismology Lab’s earthquake early warning program. “It’s really hard to move during an earthquake.”