It hasn't been felt very much in San Francisco, but the earthquake swarm that has been rocking San Ramon for the past three months had its most active day on Monday, with a barrage of 75 small and mid-size earthquakes. And residents in the area are pretty on edge.

Monday's earthquake swarm centered near the East Bay city of San Ramon was not the first to hit the area in recent weeks and months, but it was probably the most sustained, after a few other earthquakes hit there on Friday.

SFist initially reported on the series of earthquakes around 8 am, at which point 17 earthquakes had struck with magnitudes measuring between 2.0 and 4.2. The swarm continued throughout the day, with some of the quakes around 1.0M being less noticeable, ultimately totaling 75.

The most recent of the quakes, which continued into Tuesday, was a 2.5M temblor that struck at 1:45 pm today. There was also a 2.8M quake recorded at 9 am today, according to the US Geological Survey.

As one area resident, Sue Wild, tells the Chronicle today, the first quake to hit Monday morning, a 3.8M that struck at 6:27 am, was no joke if you were in the immediate vicinity. "It literally felt like a freight train was coming through the middle of the bed."

Area residents admit that they're pretty rattled at this point, nerves on edge, etc., anticipating that all these mini-quakes are leading up to a Big One in the near future, where more than just a few trinkets may fall off some shelves.

"Obviously, mentally it’s kind of taxing and you are always thinking about, ‘Is this the big one?'" says 45-year-old recent San Ramon transplant Lynn Tao, speaking to the Chronicle, after relocating there in 2024.

The Chronicle asks the question of whether San Ramon has now become the most seismically active area in the world, but experts say that's actually hard to pin down. Over time, for instance, The Geysers, the area in northern Sonoma County and Lake County that is home to geothermic springs, geysers, power plants, and thus seismic shaking, is extremely active, though many of the quakes there are just little 1.0M jiggles.

And, as Angie Lux, a seismologist at Berkeley Seismology Lab, tells the paper, there are regions all over the world with either no seismic sensing technology or very few nearby residents — Alaska being an example where plenty of shaking is probably going on in places where very few people exist nearby.

But San Ramon certainly takes the cakes in terms of significant swarms that we know about in the last few months. The latest seismic activity, which the area is not unfamiliar with, began in mid-November with a swarm of 83 quakes spread over 10 days. Seimologists noted there have been five such swarms, some lasting more than a month, recorded in the San Ramon area since 1970.

Experts don't have a definitive answer for why this area is so seismically active, but one theory suggests that these swarms are triggered by fluid moving between subfaults in the Calaveras Fault zone.

Previously: Another Series of San Ramon Earthquakes Rumbles Under East Bay