What’s the Deal with the Recent Cluster of Earthquakes in the Bay Area?

A cluster of small earthquakes shook the San Ramon area over the weekend, marking the latest in a series of recurring seismic swarms in the East Bay, according to the U.S. Geological Survey.

At least six earthquakes were recorded between Saturday, Dec. 13, and Sunday morning, Dec. 14, with magnitudes ranging from 1.3 to 2.9. The strongest quake—a magnitude 2.9—struck Saturday afternoon about five miles below the surface. More than 40 people reported feeling shaking as far away as San Francisco and Oakley.

The tremors began late Saturday morning and continued intermittently for several hours, capping off a pattern that residents may find familiar. San Ramon experienced a much larger swarm starting in early November, when more than 90 small earthquakes were recorded over several days.

Earthquake swarms differ from typical aftershock sequences. Instead of following a single large quake, swarms consist of clusters of similarly sized earthquakes that can start, stop, and fluctuate over days or even months. In many cases, there is no clear “main” quake.

Seismologists say swarms in the San Ramon area are not unusual. The region has experienced multiple swarms dating back to at least 1970. Researchers believe these events are likely driven by fluids—such as groundwater—moving through complex, fractured fault systems beneath the area, rather than stress buildup along a major fault like the San Andreas.

While earthquakes of this size are often felt, they rarely cause damage. Still, officials remind residents to be prepared. If shaking occurs, the safest response is to drop to the ground, take cover under something sturdy, and hold on until the shaking stops.