East Bay Seismic Swarm: M3.7 Near San Ramon Highlights Calaveras Fault Activity

A magnitude 3.7 earthquake struck the Tri-Valley area of the East Bay shortly after 9 a.m. Monday, becoming the largest event in a cluster of tremors reported that morning, the U.S. Geological Survey said. The quake was centered near San Ramon at an estimated depth of about 6 miles. No immediate damage was reported, but the shaking was felt as far south as San Jose and as far north as Hercules.

What Happened

The 3.7-magnitude event occurred at approximately 9:07 a.m. and was part of a swarm that produced more than two dozen small quakes before 10:30 a.m. Other notable jolts that morning included a magnitude 3.0 around 5:20 a.m. and a magnitude 2.9 at about 2:30 a.m., both widely felt. USGS seismologists said it was too soon to know whether the 3.7 would remain the largest of the sequence.

“What we do know is this is very typical,” said Annemarie Baltay, a research geophysicist with the USGS. “It's just the earth saying, 'I'm doing my stuff.'”

Why It Matters

The activity is occurring along a segment of the Calaveras Fault, which runs from the Salinas area in Monterey County northward past Danville in the East Bay. While scientists have not pinpointed the exact mechanism behind this particular swarm, they say it likely reflects the fault releasing accumulated stress or the movement of fluids beneath the surface — processes that commonly produce sequences of small quakes without escalating into major events.

Historical Context and Risk

Geologists estimate the Calaveras Fault is capable of producing earthquakes up to about magnitude 6.7. Historically, the fault generated magnitude 6.2 events in Morgan Hill in 1984 and another similar-sized quake in 1911. Swarms on this part of the fault have been recorded since at least the 1970s, with notable episodes in 2002, 2003, 2015 and a recent swarm that peaked at magnitude 3.8 on Nov. 9.

What Residents Should Know

Swarm activity like this is generally not a sign of an imminent major quake, but it serves as a reminder to review earthquake preparedness: secure heavy items, know shut-off procedures for gas and water if applicable, and have an emergency kit and plan. For the latest details and any updates, consult the USGS and local emergency management agencies.