Seismic activity has returned to a California city after weeks of unusual calm, rattling nerves in a region already on edge from months of earthquake swarms.
The US Geological Survey (USGS) reported two tremors on Friday that struck within seconds of each other near San Ramon, marking the first notable shake since activity appeared to subside earlier this year.
The USGS detected more than 300 earthquakes from November through December in the region, stoking fears that the swarm could be a precursor to a larger event.
From November through December, the USGS detected more than 300 earthquakes in the area, prompting concerns that the swarm could signal mounting stress along nearby faults and the potential for a larger event.
While the renewed tremors may revive fears, USGS research geophysicist Annemarie Baltay cautioned that the recent activity does not indicate an imminent major earthquake in San Ramon.
‘These small events, as all small events are, are not indicative of an impending large earthquake,’ Baltay told Patch.
Still, she emphasized that the region’s long-term risk remains high.
‘We live in earthquake country, so we should always be prepared for a large event,’ Baltay said, noting there is a 72 percent chance of a magnitude 6.7 or larger earthquake occurring anywhere in the Bay Area between now and 2043.
The largest of the two earthquakes measured a 3.4 magnitude
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 first earthquake, measuring 3.4 magnitude, was detected at 10:33am PT (1:33pm ET), followed by a 2.9 magnitude tremor just 19 seconds later.
According to Michigan Tech University, quakes under 2.5 are rarely felt, while those between 2.5 and 5.4 can be felt and sometimes cause minor damage.
More than 600 people reported feeling shaking to the USGS.
The previous seismic activity to strike San Ramon was reported on January 11, when at least three earthquakes were detected.
And another earthquake swarm had hit just two days prior.
San Ramon lies atop the Calaveras Fault, where a network of smaller, interconnected fractures branches off the main fault line.
A magnitude 6.7 earthquake on the Calaveras Fault would be classified as a major seismic event capable of causing significant damage in densely populated East Bay communities.
By comparison, the 1989 Loma Prieta earthquake, a magnitude M6.9, widely labeled ‘the Big One’ at the time, caused widespread destruction, and the USGS uses the 6.7 threshold when discussing the long-term probability of a ‘Big One’ in the Bay Area.
While experts said the relentless seismic activity is likely not a warning sign of something major to come, Baltay told Patch: ‘We live in earthquake country, so we should always be prepared for a large event.’
Scientists say that when fluids like water or gas flow through small cracks in rock, they can weaken the surrounding rock, triggering clusters of minor earthquakes that occur in quick succession.
‘It is also possible that these smaller earthquakes pop off as the result of fluid moving up through the earth’s crust, which is a normal process, but the many faults in the area may facilitate these micro-movements of fluid and smaller faults,’ said Baltay.
Records from the USGS highlighted similar swarms in 1970, 1976, 2002, 2003, 2015 and 2018.
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.
