The city of San Ramon in the East Bay has been experiencing an unusual and prolonged sequence of earthquakes since late 2025, and as of April 2026 the swarm shows no sign of stopping. With more than 300 recorded earthquakes — including several felt by residents — since December 1, 2025, federal scientists have now deployed a new network of sensitive underground sensors to try to understand what is driving the activity beneath this suburban community.
What Is an Earthquake Swarm?
An earthquake swarm is a sequence of many small to moderate earthquakes that occur in a relatively small area over an extended period, without a single dominant mainshock followed by the usual pattern of decreasing aftershocks. Swarms are relatively common in certain geological settings — areas with geothermal activity, volcanic regions, and places with complex fault networks — and they are not necessarily a precursor to a larger event.
What makes the San Ramon swarm notable is its duration and volume. More than 162 earthquakes of magnitude 2.0 or higher have been recorded in the area since November 2025, and when smaller events are included, the total exceeds 300. Several of the earthquakes have been large enough to be widely felt across the East Bay, including events measuring magnitude 3.8 and 4.2.
The New Sensor Network
In early April 2026, the USGS deployed dozens of new battery-operated seismometers in and around San Ramon in an effort to gather more detailed data about the swarm. The sensors, buried at multiple locations across the area, record ground motion at approximately 200 times per second — a level of resolution that should allow researchers to detect very small earthquakes and to pinpoint the locations and depths of events much more precisely than the existing regional network allows.
The goal is to build a clearer picture of the fault structures generating the swarm and, potentially, to understand why the sequence has persisted for so long. USGS seismologists involved in the project have said they expect the new sensors to capture a large number of small earthquakes that the existing network would miss entirely.
What Is Causing the Swarm?
That question is exactly what scientists are trying to answer. Several theories have been proposed. One possibility is that the earthquakes are occurring along a network of smaller sub-faults that branch off from the main Calaveras Fault system — a major active fault that runs through the eastern East Bay. Another is that fluid movement in the rocks at depth is triggering the seismicity, a mechanism that has been associated with swarm activity in other parts of the world.
What seismologists can say with some confidence is that the swarm is occurring at depths of approximately 5 to 10 miles beneath the surface, and that it is concentrated in a relatively compact geographical area. The cluster does not appear to have expanded significantly over time, which some researchers say is mildly reassuring — swarms that stay geographically confined tend to be less likely to escalate to a large damaging earthquake than those that grow in extent.
Impact on San Ramon Residents
For the approximately 80,000 people who live in San Ramon, the ongoing earthquake sequence has become an unavoidable part of daily life. Some residents describe becoming desensitised to the smaller events, noticing only the stronger shakes. Others report disturbed sleep and heightened anxiety — particularly among those who moved to the area from outside California and are less accustomed to seismic activity.
No significant damage to homes or infrastructure has been reported as a result of the swarm, and emergency services say they are monitoring the situation but have not issued any specific public advisories beyond standard earthquake preparedness guidance. The relatively modest magnitudes involved mean that well-constructed buildings in the area should have no difficulty withstanding the forces generated.
What Happens Next?
Earthquake swarms are notoriously difficult to predict. They can continue for weeks, months or years, or they can stop abruptly for no obvious reason. They can escalate to a larger mainshock — though this is the minority outcome — or they can simply fade away as the stresses driving them are gradually released through the accumulation of small events.
USGS scientists say the new sensor network should help them determine whether the swarm is showing any signs of migration or escalation, and whether any particular fault segment is accumulating stress at a worrying rate. Until those results are available, the advice to San Ramon residents is straightforward: stay prepared, have an emergency kit ready, and be aware of what to do if a larger earthquake strikes.