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…