More than 300 earthquakes have rattled the same region in California over the past month, sparking fears among locals that a big one could soon strike.
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 (USGS) estimates there is a 72 percent chance of this happening by 2043.
The earthquakes began on November 9 with a 3.8 magnitude, and the tremors have not stopped since. The latest, which hit today, measured a 2.7 magnitude.
Sarah Minson, a research geophysicist with the USGS's Earthquake Science Center at California's Moffett Field, told SFGATE: 'This is a lot of shaking for the people in the San Ramon area to deal with.
'It's quite understandable that this can be incredibly scary and emotionally impactful, even if it's not likely to be physically damaging.'
Minson noted that while the swarms seem relentless, they do not indicate a major event is imminent.
'Given the magnitude and locations of the earthquakes that have happened so far, there is no significant risk of something happening on one of the major faults,' she explained.
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.
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.
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,' Baltay told the Patch.
Records from the USGS highlighted similar swarms in 1970, 1976, 2002, 2003, 2015 and 2018.
'This has happened many times before here in the past, and there were no big earthquakes that followed,' Minson told SFGATE.
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.
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 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?'
Over 300 earthquakes rattle California as locals fear the next big one
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