California has experienced a series of earthquake swarms recently, culminating in the most recent tremor on Tuesday.

The US Geological Survey (USGS) recorded a 3.1-magnitude earthquake at 5:53 a.m. PT (8:53 a.m. ET) near San Ramon, which has been the focal point of this recent seismic activity.

This quake came after a sequence of smaller tremors, with magnitudes ranging from 1.1 to 1.6.

In the past four days, San Ramon has been subjected to several earthquakes, with magnitudes including 2.9, 2.3, and 2.2 on December 13, followed by a 2.8-magnitude quake on December 14, and further quakes of magnitudes 2.4 and 2.1 on December 15, along with numerous minor shakes.

This pattern is part of a larger trend. According to USGS data, San Ramon has documented at least 21 quakes, with magnitudes from 2.9 to 4.0 in the past two months. Over the last month, the area has experienced at least 90 earthquakes, most of which were too small to cause any significant 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.’

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 US Geological Survey (USGS) reported a 3.1-magnitude quake at 5.53am PT (8.53am ET) near San Roman, the epicenter of the recent seismic activity

San Roman 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.

‘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.

Although small quakes can sometimes whisper warnings of a looming ‘big one,’ California scientists say this swarm does not fit that script. But they noted there is a 72 percent chance of it happening from now until 2043

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.