A swarm of earthquakes struck San Ramon, east of San Francisco, on Saturday, December 21, according to the U.S. Geological Survey (USGS). The seismic sequence included at least a dozen earthquakes, with the strongest reaching magnitude 3.9.
The swarm began at 11:06 a.m. Saturday and continued for several hours, with additional tremors recorded into the early morning hours of Sunday. The largest quake, a magnitude 3.9 event, occurred at 6:31 p.m. Saturday at a depth of approximately 6 miles. The remaining earthquakes ranged in magnitude from 1.3 to 3.0.
More than 2,000 people reported feeling the shaking, with reports coming from as far away as Sacramento and San Jose. Residents took to social media to share their experiences, especially as parts of San Francisco were simultaneously affected by an unrelated power outage.
A Pattern of Repeated Seismic Activity
This is not the first time San Ramon has experienced heightened seismic activity. The city was previously hit by a swarm of more than 90 earthquakes starting on November 9, followed by additional tremors earlier this month. Seismologists note that at least six earthquake swarms have been recorded in the area since 1970.
According to experts, these recurring swarms are not unusual for the region and are unlikely to lead to a major earthquake.
What Is an Earthquake Swarm?
An earthquake swarm is a series of small to moderate earthquakes occurring in a localized area over days, weeks, or even months, without a single dominant mainshock. Unlike typical aftershock sequences, the largest quake in a swarm may occur at any point during the activity.
Earthquakes in the magnitude 2.5 to 5.4 range are often felt by people but rarely cause significant damage, while those below magnitude 2.5 are usually not noticed.
What Causes Earthquake Swarms?
Scientists are still studying the exact causes of earthquake swarms, but research suggests they are often linked to fluids—typically water—interacting with fault lines. When faults become saturated with fluids, they can slip more easily, triggering repeated small earthquakes. In some cases, magma movement can also play a role, though there is no indication of volcanic activity in the San Ramon events.
Geologists believe San Ramon’s complex fault geometry and fluid-filled cracks make the area particularly prone to these recurring swarms.
What Happens Next?
Experts say the current swarm could continue for weeks or even months, fluctuating in intensity. However, similar past events in the area have not been followed by major earthquakes, offering some reassurance to residents.
Seismologists continue to closely monitor the activity and emphasize that earthquake swarms, while unsettling, are a natural part of California’s seismic landscape.
