In Northern California, the ground has slipped back into one of its most unnerving habits: a burst of small quakes that seem to come out of nowhere and refuse to stop. The latest swarm centered near San Ramon is part of a broader pattern of clustered shaking that has rattled nerves from the East Bay to Sonoma and The Geysers. I want to unpack what scientists say is driving this activity, how it fits into California’s long seismic story, and what it really means for the next big rupture.
San Ramon’s swarm and a region on edge
The current swarm is focused around San Ramon, a city tucked along the Interstate 680 corridor where the Calaveras and related faults quietly store strain. Residents have reported repeated jolts and a sense that the shaking is “back” rather than brand new, which matches the record of at least six swarms in the community over the past few decades. In earlier activity, The San Ramon sequence included a largest event of magnitude 3.6, a size that is widely felt but rarely damaging, according to the U.S. Geological Survey reports that tracked the cluster. That history helps explain why locals immediately recognized the familiar rolling and rattling as another swarm rather than a one-off quake.
Scientists who study The San Ramon area describe the fault geometry here as more complicated than the simple, straight trace people picture when they think of the San Andreas. Instead of one clean break, the crust is sliced into smaller blocks and strands, which can encourage bursts of small quakes as stress redistributes. Reporting on The San Ramon swarm notes that this is one of at least six such sequences in the community, underscoring that what feels like a sudden onslaught is actually part of a recurring pattern. When I look at that record, I see a region where swarms are not an anomaly but a recurring mode of how the crust relieves stress.
What an earthquake swarm actually is
To understand what is happening, it helps to be precise about the term “swarm.” Seismologists use it for a burst of earthquakes in a confined area over a relatively short time, with no single mainshock that clearly dominates the sequence. One detailed explanation of What an earthquake swarm is emphasizes that these clusters can involve dozens or even hundreds of small events, all tapping into the same patch of stressed crust. The key is that energy is spread out among many quakes rather than being dominated by one large rupture followed by a classic aftershock decay.
Researchers have also stressed that swarms are not simply foreshocks in disguise. A widely cited overview of earthquake swarms notes that they are not aftershocks or foreshocks of a major earthquake, but instead occur in response to more subtle changes in local crustal stress. Those changes can be driven by tectonic loading, by the movement of fluids in the crust, or by a combination of both. When I compare that framework with the pattern in San Ramon, the swarm looks less like a countdown clock to a single big event and more like a snapshot of how the crust is constantly adjusting.
From The Geysers to Ontario, swarms are part of California’s normal
The Bay Area is not the only part of Northern California and the wider state dealing with clustered shaking. At Northern California’s geothermal field known as The Geysers, swarms are so common that they are almost a defining feature of the landscape. A detailed look at The Geysers explains that injection and production activities there cause local stress changes that, combined with regional tectonic stresses, induce quakes on preexisting faults. The same report notes that the field is the world’s largest geothermal development, which means human operations and natural tectonics are tightly intertwined in driving the swarm behavior.
Farther south, swarms have also rattled communities that sit away from the iconic San Andreas trace. In Ontario, for example, a spate of shaking a little more than a year ago topped out with a magnitude 4 quake on Oct. 6, 2024, part of a broader cluster that reminded inland residents that they, too, live on active faults. Another report on why CALIFORNIA is seeing an earthquake cluster right now highlighted that in the USA, M4. 6 and M4. 7 EARTHQUAKES HIT NEAR Eureka, while a series of 24 tremors struck The Geysers on March 24, 2018. When I line up these examples, the pattern is clear: swarms are not rare outliers but a recurring feature of California’s seismic life.
Is this about the “Big One,” or something else?
Whenever a swarm hits the Bay Area, the question that hangs over every conversation is whether it signals the approach of a much larger rupture. Recent coverage of an earthquake swarm that has broken out in Northern California and the Bay Area noted that experts consider it almost certain the region will suffer a major quake within the next 20 years. That long term probability is driven by the steady accumulation of tectonic strain on big faults like the San Andreas and Hayward, not by any single swarm. In other words, the risk of a large event is real and well documented, but the swarm itself is not a reliable countdown.
Seismologists repeatedly emphasize that most swarms do not culminate in a damaging mainshock. The broader explanation of swarms as responses to local stress changes, rather than as foreshock sequences, is central here. That does not mean swarms are meaningless. They reveal where the crust is actively adjusting and can highlight segments of faults that are currently loaded. When I look at the San Ramon activity through that lens, I see a reminder that the Calaveras system is very much alive, but not a specific prediction about when or where the next large rupture will occur.
How residents are feeling the swarm in real time
On the ground, the science intersects with something more visceral: the lived experience of repeated shaking. In the Bay Area, one Reddit user posting under the name One described a cluster of 30 earthquakes that hit Northern California near Sonoma, capturing the sense that “it wasn’t just one shake.” Another commenter, Bonzer, chimed in with the reminder that, IIRC, there is no real way to know whether small quakes are relieving tension or simply rearranging it, and that Small quakes do not necessarily release enough energy to change the long term risk. Those conversations mirror what I hear from residents in San Ramon: a mix of curiosity, anxiety and a desire for clear answers that science cannot always provide on the timescale of a news cycle.
Another thread, posted in Jan, captured the same tension in the East Bay. “There are earthquakes all the time, just depends on if they are big enough for us to feel,” one commenter wrote, adding that they feel them often but that most pass unnoticed by the broader public. That perspective lines up with the instrumental record, which shows constant microseismicity humming away beneath Northern California. When a swarm nudges more of those quakes into the “felt” category, it does not create the hazard so much as reveal it.
Why the crust keeps popping and how to live with it
Geologically, the San Ramon swarm sits at the intersection of several forces. The region lies within the broader Bay Area fault network, which is part of the plate boundary where the Pacific and North American plates grind past each other. The area around Northern California’s coastal ranges is riddled with smaller structures that transfer stress between the main faults. Some of those structures run near Sonoma and The Geysers, others cut beneath San Ramon and the I‑680 corridor. In geothermal areas, reports on activities at The Geysers show how fluid injection and production can tweak local stress enough to trigger swarms on existing faults, a reminder that human operations can modulate, but not create from scratch, the underlying tectonic setting.
For residents, the practical question is how to respond. The recurring swarms are a cue to revisit basic safety steps rather than a reason to panic. Preparedness campaigns urge people to practice “Drop, Cover, and Hold On,” guidance laid out in detail at ShakeOut drills that run across the state. In San Ramon and other Bay Area communities, that means knowing how to protect yourself at home, at work and on the road when the next jolt hits. When I connect the dots between the science and the lived experience, the message is not that swarms are harmless, or that they guarantee a larger quake, but that they are a recurring reminder to treat California’s seismic reality as a constant, not a surprise.
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