The East Bay experienced a "staccato" swarm of dozens of small earthquakes near San Ramon, peaking at magnitude 3.8 while most events remained under M2.0. The quakes occurred on the Calaveras Fault, which can produce at least M6.7 and has produced damaging shocks in 1911, 1979, 1984 and 2007. UC Berkeley research suggests the Hayward Fault may connect to Calaveras, raising the combined rupture potential to about M7.3. Experts say swarms may be linked to fluid movement and estimate a roughly 1%–9% chance the M3.8 could be a foreshock.
Calaveras Fault Fueled East Bay 'Staccato' Swarm — What It's Capable Of
The East Bay experienced a "staccato" swarm of dozens of small earthquakes near San Ramon, peaking at magnitude 3.8 while most events remained under M2.0. The quakes occurred on the Calaveras Fault, which can produce at least M6.7 and has produced damaging shocks in 1911, 1979, 1984 and 2007. UC Berkeley research suggests the Hayward Fault may connect to Calaveras, raising the combined rupture potential to about M7.3. Experts say swarms may be linked to fluid movement and estimate a roughly 1%–9% chance the M3.8 could be a foreshock.
02:50 PM, 11/11/2025Science
East Bay 'Staccato' Swarm Centered on the Calaveras Fault
On Sunday morning the East Bay experienced a cluster of dozens of small earthquakes concentrated near San Ramon, a sequence experts described as a distinctive "staccato" of events. The largest reached magnitude 3.8; most shocks were under magnitude 2.0 and the sequence continued into Monday morning.
Where the Calaveras Fault Runs
The quakes occurred on the Calaveras Fault, a lesser-known strand of the regional fault system that also includes the San Andreas and Hayward faults. Geologists trace the Calaveras from the Danville area (near the recent swarm), through or near Dublin, Pleasanton, Alamo and Sunol, then south through San Jose and Morgan Hill and past Hollister in San Benito County where it splays from the San Andreas.
Historical Activity and Hazard
The Calaveras Fault has produced damaging earthquakes in the past: magnitude 6.2 events near Morgan Hill (1984 and 1911), a magnitude 5.9 at Coyote Lake (1979), and a magnitude 5.6 near Alum Rock (2007), which at the time was the Bay Area's largest quake since the 1989 Loma Prieta event.
In 2015 the U.S. Geological Survey estimated about a 7.4% probability that the Calaveras would produce a magnitude 6.7 earthquake by 2045. Later work at the UC Berkeley Seismology Lab suggests the Hayward Fault — which runs roughly parallel to the west — may connect with the Calaveras; if both ruptured together, models indicate a combined event could reach roughly magnitude 7.3.
Nature and Cause of the Swarm
UC Berkeley's Seismology Lab counts at least ten prior swarm episodes in the San Ramon area since 1970. Geophysicist Ross Stein, CEO of Temblor, described the recent sequence as a "staccato" — many small, closely spaced quakes rather than a single mainshock followed by classic aftershocks. Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington, noted such swarms can reflect movement of fluids in the Earth's crust.
Could the M3.8 be a foreshock? According to Stein, the odds are fairly low. Based on the area's swarm history he estimated roughly a 1%–9% chance that the magnitude 3.8 could be followed by a substantially larger earthquake.
Practical note: For residents near these faults, basic preparedness is wise — secure heavy objects, identify safe spots in your home, maintain an emergency kit, and follow updates from local authorities and the U.S. Geological Survey.