Seismologists call the sequence of small earthquakes a “swarm” and say it is normal for this part of the Bay Area, which lies atop a spiderweb of active faults.
They don’t know exactly how or why they’re happening, or whether they are occurring on a major fault — which may hint at the risk of a larger earthquake coming — or on one of the many smaller cracks nearby.
San Ramon sits atop a complex network of faults, making it prone to earthquake swarms. (Anna Vignet/KQED)
That’s why in mid-March, a group of United States Geological Survey seismologists and volunteers buried a network of 78 blue and grey toaster-sized seismometers across San Ramon and Danville.
“I don’t have any preconceived notions, I just want to find out what’s going on,” said Rufus Catchings, a research geophysicist with USGS, who is leading the work. “The data will hopefully tell us a lot more detail, like which faults are involved and whether earthquakes are happening on major faults.”
Catchings said a swarm on a smaller fault is “unlikely to generate a very large earthquake. But if it is connected, say, to the Calaveras or to one of the thrust faults, it could be very significant, and we need to know that.”
Rufus Catchings, a seismologist from USGS, installs a seismometer at a home in San Ramon on March 17, 2026. (Tâm Vũ/KQED)
San Ramon sits in a part of Contra Costa County that is prone to swarms and has experienced them a handful of times since the 1970s due to a complex system of faults in the region, including the Calaveras, Concord-Green Valley, Pleasanton, Mt. Diablo Thrust, Greenville and Sherburne Hills Thrust.
Epstein has lived in San Ramon for 12 years and experienced multiple swarms. The latest quakes have left her with a mountain of unanswered questions.
“I read as much as I could to understand whether or not this means that [a big one] is coming,” Epstein said. “I don’t think there is any solid answer. That’s why they’re putting all those little sensors everywhere.”
A web of sensors
The grid of sensors will track movement underground for the next six months, and afterward, seismologists will dig them up and analyze their readings.
“We know for sure that we’re going to catch some really small earthquakes because they’re just going on all the time, and if the swarm does pick up again, then we’ll definitely catch that,” said Annemarie Baltay, a research geophysicist with USGS.
Annemarie Baltay (left), a geophysicist researcher from USGS, and Rufus Catchings (right), a seismologist from USGS, install a seismometer at a home in San Ramon on March 17, 2026. (Tâm Vũ/KQED)
The battery-operated seismometers gather data around 200 times per second, Catchings said.
“We know that eventually there’s gonna be a big earthquake here,” Catchings said. “You can look at these mountains and see how they popped up through the tectonic forces.”
The researchers theorize that the swarm occurred along smaller sub-faults or due to liquid moving around these marginal cracks. But it’s hard to know exactly what’s happening five to 10 miles underground, Baltay said.
