California has been rattled by multiple earthquake swarms over the past four days, with the latest hitting on Tuesday.
The US Geological Survey (USGS) reported a 3.1-magnitude quake at 5.53am PT (8.53am ET) near San Ramon, the epicenter of the recent seismic activity.
This tremor followed a dozen smaller quakes ranging from 1.1 to 1.6 magnitude.
Over the past four days, San Ramon has been shaken by multiple earthquakes, including magnitudes 2.9, 2.3, and 2.2 on December 13, a 2.8-magnitude quake on December 14 and 2.4 and 2.1 on December 15, along with dozens of smaller tremors.
According to USGS data, San Ramon has recorded at least 21 earthquakes ranging from magnitude 2.9 to 4.0 over the past two months. In the last month alone, the city has been shaken by at least 90 earthquakes, most of them too small to cause damage.
The uptick in seismic activity has raised concerns among some residents, with fears that the clusters of quakes could be a warning sign of a much larger earthquake. However, experts say the small tremors do not indicate that a major event is imminent.
USGS research geophysicist Annemarie Baltay said she is not unusually concerned that the recent earthquakes signal anything larger on the horizon for San Ramon.
'These small events, as all small events are, are not indicative of an impending large earthquake,' Baltay told Patch.
'However, we live in earthquake country, so we should always be prepared for a large event,' she said. 'There is a 72 percent chance of a magnitude 6.7 or larger earthquake occurring anywhere in the Bay Area between now and 2043. So we should all be aware and be prepared.'
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.
San Ramon lies atop the Calaveras Fault, where a network of smaller, interconnected fractures branches off the main fault line.
The Calaveras Fault is capable of producing a magnitude 6.7 earthquake, which would impact millions of people in the San Francisco Bay Area.
Scientists said movement of fluids such as water or gas through these narrow cracks can destabilize the rock, setting off clusters of minor earthquakes that strike in rapid 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,' Baltay told the Patch.
Records from the USGS highlighted similar swarms in 1970, 1976, 2002, 2003, 2015 and 2018.
Sarah Minson, a research geophysicist with the USGS's Earthquake Science Center at California's Moffett Field, told SF Gate: 'This has happened many times before here in the past, and there were no big earthquakes that followed.
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'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.'
Seismic activity in San Ramon started in early November with a 3.8 magnitude, and the tremors have not stopped since.
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.
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.
