Over the last month, the ground beneath the San Francisco Bay Area has stirred with unnerving frequency — dozens of small earthquakes have rattled residents from San Ramon to Oakland, raising anxiety and fresh questions about what, if anything, these tremors might be signaling.
The most recent burst came Monday, when no fewer than 19 quakes of magnitude 2.0 or greater struck near San Ramon in Contra Costa County. The strongest — a magnitude 3.6 — hit just after 9 a.m., strong enough to cause startled glances among customers in a Safeway checkout line in Dublin, though not powerful enough to cause damage.
Earlier that morning, six smaller quakes shook the East Bay during the 5 a.m. hour — a rude awakening for many. On social media, reactions ranged from lighthearted to anxious: “San Ramon is basically a massage chair today … but like, the stressful kind,” one user joked. Others expressed concern, with one poster vowing to stock up on emergency water and supplies.
This swarm of tremors, clustered around the Calaveras fault, is not without precedent. The region experienced similar seismic clusters in 2002, 2003, and again in 2015. None of those episodes preceded a major quake, and so far, seismologists are cautious not to suggest this one will either. But as USGS seismologist Annemarie Baltay noted, “It may be unsettling for folks who live there.”
Adding to the tension, a magnitude 2.9 quake struck Monday afternoon in Oakland’s Montclair neighborhood — this time, closer to the Hayward fault, a geological feature scientists regard as one of the most dangerous in California.
While most earthquake swarms fizzle out harmlessly, history offers sobering counterexamples. The 1989 Loma Prieta earthquake, which killed 63 and caused the collapse of a portion of the Bay Bridge, was preceded by years of smaller seismic activity. And in 1857, a magnitude 7.8 quake on the San Andreas fault followed a modest foreshock of 5.6 just an hour earlier.
The Calaveras fault, which runs from Contra Costa County through Dublin and Pleasanton and connects to both the Hayward and San Andreas faults, is considered capable of producing a magnitude 6.7 or greater quake. The last major temblor on the Calaveras — the 1984 Morgan Hill earthquake — was a 6.2 and caused injuries and widespread damage. A more recent 5.1 quake in 2022 reminded residents that the fault remains very much alive.
But the real monster in the Bay Area’s seismic system may be the Hayward fault. Running through Richmond, Berkeley, Oakland, Fremont, and San José, the Hayward fault has remained ominously quiet since 1868 — when it produced a quake estimated between magnitude 6.8 and 7.0, one of the most destructive in state history. A repeat event today, scientists estimate, could kill 800 people, injure 18,000, and trap tens of thousands in elevators and collapsed buildings.
The USGS currently estimates a 72% probability that the Bay Area will experience a magnitude 6.7+ earthquake by 2043. The swarm in San Ramon may not be the event that triggers it, but experts agree it’s a timely reminder.
Preparation remains the most effective defense: secure tall furniture, mount TVs, and retrofit older buildings where possible. Emergency kits, flashlights, and family communication plans may seem like overkill — until the moment they’re not.
