At least 90 small earthquakes have shaken California’s Bay Area this month, prompting scientists to dig into what’s driving the unusual burst of activity.
San Ramon in the East Bay has been the epicenter of this seismic activity, which sits on top of the Calaveras Fault, an active branch of the San Andreas Fault system.
The Calaveras Fault is capable of producing a magnitude 6.7 earthquake, which would impact millions of people in the San Francisco Bay Area.
The US Geological Survey estimates there is an 18 percent chance of this happening by 2030.
This month’s seismic activity began on November 9 with a 3.8 magnitude, and the tremors have not stopped since.
Although small quakes can sometimes whisper warnings of a looming ‘big one,’ California scientists say this swarm does not fit that script.
Sarah Minson, a research geophysicist with the U.S. Geological Survey’s Earthquake Science Center at California’s Moffett Field, told SFGATE: ‘This has happened many times before here in the past, and there were no big earthquakes that followed.
‘We think that this place keeps having earthquake swarms due to a lot of fluid-filled cracks, thanks to very complex fault geometry, unlike, say, the San Andreas Fault, which is this nice clean edge.’
At least 90 small earthquakes have shaken California ‘s Bay Area this month, prompting scientists to dig into what’s driving the unusual burst of activity
The last major earthquake on the Calaveras Fault was a magnitude 5.1 event in October 2022 near M. Hamilton.
While not the largest in California history, it was the biggest on the Calaveras fault since 2007 and the largest in the Bay Area since 2014. The largest historical quake on the fault was a magnitude 6.6 in 1911.
And scientists consider the Calaveras Fault to be overdue for a major earthquake.
This month’s activity marks at least the sixth swarm to rattle the area since 1970, the most recent one shaking loose in 2015.
Scientists studying the 2015 San Ramon earthquake swarm found that the area contains several small, closely spaced faults rather than a single big one.
The quakes moved along these faults in a complex pattern, suggesting the faults interact with each other.
The study also found evidence that underground fluids may have helped trigger the tremors.
Researchers looked into other possible causes, like tidal forces, but found no clear connection.
San Ramon in the East Bay has been the epicenter of this seismic activity, which sits on top of the Calaveras Fault, an active branch of the San Andreas Fault system
Overall, the findings showed that the fault system under San Ramon is more complicated than previously thought, which could help explain why these earthquake swarms occur.
Roland Burgmann, a UC Berkeley seismologist who worked on that study, told SFGATE that because the first quake in November was the strongest, he believes the entire series is more than just a swarm; it’s a tense aftershock sequence, each tremor echoing the power of the one that started it all.
Minson echoed the conclusion, saying the smaller earthquakes were likely aftershocks from the 3.8 magnitude earlier this month.
Clusters of earthquakes often appear in regions with volcanic or geothermal activity, but San Ramon does not fit that profile.
Scientists suggested the tremors could be driven by underground fluids forcing their way through the crust and triggering a series of small faults.
Minson noted that the area’s fault system is intricate, with the Calaveras Fault ending nearby and the movement potentially leaping to the Concord-Green Valley Fault to the east.
‘We think that what’s going on, which makes this like geothermal areas or like volcanic areas, is that there are a lot of fluids migrating through the rocks and opening up little cracks to make a bunch of little earthquakes,’ Minson told SFGATE.
Emily Brodsky, a seismologist at UC Santa Cruz, warned that the recent tremors in San Ramon are puzzling, making it hard for scientists to draw any firm conclusions about what’s really happening beneath the surface.
Although small quakes can sometimes whisper warnings of a looming ‘big one,’ California scientists say this swarm does not fit that script
‘Although it’s the kind of thing you might expect to happen before a big earthquake, we can’t distinguish that from the many, many times that have happened without a big earthquake,’ she told SFGATE. ‘So what do you do with that?’
The analysis of the 2015 seismic activity also found that the dangerous Hayward Fault is essentially a branch of the Calaveras Fault that runs east of San Jose, which means that both could rupture together, resulting in a significantly more destructive earthquake than previously thought.
The Hayward Fault, stretching roughly 43 miles through densely populated parts of the East Bay, is considered one of the nation’s most dangerous faults.
It runs from Richmond on the northern edge of San Pablo Bay down to just south of Fremont.
In a seismic hazard update released last month, the USGS estimated a 14.3 percent chance of a magnitude 6.7 or higher earthquake on the Hayward Fault within the next 30 years, while the Calaveras Fault carries a 7.4 percent risk.
These calculations assume the two faults act independently, with the largest Hayward quake expected to reach around magnitude 6.9 to 7.0.
However, because the Hayward and Calaveras faults are connected underground, a simultaneous rupture could unleash far more energy, potentially triggering a magnitude 7.3 earthquake, 2.5 times stronger than a solo Hayward event.
