California earthquake swarm persists in San Ramon area as Bay Area risk remains long-term

A fourth day of shaking rattled the San Ramon area in California as a 3.1-magnitude earthquake occurred near the East Bay city early Tuesday, the U.S. Geological Survey said. The quake, recorded at 5:53 a.m. PT (8:53 a.m. ET), followed a sequence of smaller events in rapid succession as residents endured a continuing earthquake swarm linked to the Calaveras Fault system. San Ramon, located along the fault line that branches from the San Andreas Fault, has been the epicenter of repeated tremors since the activity began in early November, prompting cautious interest from residents and scientists alike.

Over the four-day period, the region has experienced magnitudes 2.9, 2.3 and 2.2 on December 13; a 2.8 on December 14; and 2.4 and 2.1 on December 15, along with dozens of smaller quakes. USGS data show San Ramon has logged at least 21 earthquakes in the 2.9 to 4.0 range over the past two months, and in the last month the city has recorded at least 90 earthquakes overall, most too small to cause damage. While the ongoing swarm has unsettled residents, scientists say the pattern does not match a forecast for an imminent large quake.

USGS research geophysicist Annemarie Baltay stressed that the swarm, while alarming to onlookers, is not a warning sign of a looming large event. '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.' The agency also notes a long-term Bay Area risk, estimating about a 72 percent chance of a magnitude 6.7 or larger earthquake occurring somewhere in the Bay Area between now and 2043.

The Calaveras Fault sits beneath San Ramon, part of a network of faults that complicate predictions. Scientists say movement of fluids such as water or gas through tight cracks can destabilize rock and trigger clusters of minor earthquakes. While the recent tremors have drawn attention, Baltay and others emphasize there is no consensus that the sequence portends a major rupture.

Sarah Minson, a USGS research geophysicist with the Earthquake Science Center at Moffett Field, told SF Gate that similar swarms have occurred before in this region and were not followed by big earthquakes. 'This has happened many times before here in the past, and there were no big earthquakes that followed,' Minson said. UC Berkeley seismologist Roland Burgmann, who worked on the 2015 San Ramon swarm study, described the current activity as likely the result of several small faults interacting rather than a single fault rupturing all at once. He characterized the sequence as a tense aftershock chain, with each tremor echoing the power of the initial quakes.

Historical records show that San Ramon has hosted multiple notable swarm episodes in the past, including events in 1970, 1976, 2002, 2003, 2015 and 2018. Those swarms highlighted the area’s complex fault geometry and the role of subterranean fluids in triggering micro-movements along interconnected faults. Researchers studying the 2015 swarm found that underground fluids likely contributed to the tremors and that tidal forces offered no clear link to the activity. The overarching lesson, scientists say, is that the fault network under San Ramon is intricate, with several small faults interacting in ways that complicate simple forecasts.

As the region continues to monitor the swarm, authorities urge residents to stay informed, secure loose items, and review emergency plans. While today’s quake did not cause damage, the Bay Area’s geologic setting remains one of the most seismically active in the continental United States, and the long-range probability of a major earthquake in the Bay Area remains a central concern for climate and environment reporting and preparedness planning.