Swarm of earthquakes strike San Ramon Friday night

A swarm of earthquakes, once again, rattled the East Bay near San Ramon on Friday night.

OAKLAND, Calif. - There were at least six separate earthquakes that measured above a magnitude of 2.5 in an earthquake swarm in the San Ramon area on Friday night. No damage was reported, but locals were somewhat rattled by the frequency.

The first quake in this series, measured at magnitude 2.5, struck at 7:41 p.m. It was followed by two simultaneous quakes that struck at 7:49 p.m. They measured at 2.9 and 3.6 respectively.

A 4.0 earthquake, which would be the strongest of the evening, struck at 7:46 p.m. Two subsequent quakes, within the next four minutes, measured at magnitudes of 3.1 and 2.9 respectively.

The epicenters were east of the 680 Freeway, south of San Ramon.

That region has seen steady seismic activity over the last week. As reported by the US Geological Survey, Friday night’s quakes were just the latest in a cluster of dozens of quakes that struck on Monday and Tuesday.

Whole lotta shaking…

What they're saying:

People out in San Ramon on Friday night said the quakes caught them off guard.

"Really, really very loud. We were at the top floor, so everything was shaking," said Krishna, who only gave her first name.

"My car started shaking back and forth," said Anabelle, who did not give a last name. She said she was a little bit scared.

"It kind of felt like I…I dunno…had too much to drink. But I didn't. It was an earthquake," said Sarah who also did not give a last name.

Swaying chandelier

KTVU received video of a chandelier shaking during the quake swarm. We received calls into the newsroom from viewers who felt shaking across the East Bay. People called from Lafayette, Walnut Creek, Castro Valley and Newark.

It’s been roughly a decade since residents of San Ramon experienced such high levels of seismic activity. The city in November experienced nearly two dozen earthquakes above a magnitude 2. As of Friday evening, the number of earthquakes in December was over twice that.

The only other recorded instance of such high activity in the last 50 years was a two-month period in 2015, which actually saw even more earthquakes.

A large number of quakes in a short period is referred to as an earthquake swarm, which is a series of small earthquakes repeatedly occurring in the same region.

Experts say earthquake swarms should not be seen as any indication that a larger quake is imminent.

"Based on past research, we understand that swarms probably indicate that an extra ‘ingredient’ is involved where the earthquakes are happening… Sometimes this ingredient might be that a fault is slipping slowly, and small sticky patches are popping off and generating numerous small earthquakes," a 2019 USGS report states. "Sometimes, magma might be pushing up into the crust, opening up a pathway for itself by breaking the rock in front of it. But most often, swarms are probably caused by fluids (dominantly water) interacting with faults."

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