SAN RAMON (CBS SF) -- A swarm of earthquakes rumbled under the San Ramon-Danville area Wednesday, measuring as high as 3.9 in magnitude, giving local residents a subtle reminder of the seismic activity deep under the region.
The 3.9 magnitude quake struck at 11:43 a.m., followed by an aftershock measuring 2.6 magnitude at 11:46 a.m., according to the U.S. Geological Survey. A third aftershock with 3.0 magnitude struck at 11:58 a.m. More than a dozen smaller quakes have followed over the last several hours.
Preliminary data from the USGS showed the quake struck an area off El Capitan Drive just west of Crow Canyon Country Club in Danville close to the border with San Ramon.
It was felt as far east as San Francisco, as far north as Vallejo, and as far south as Santa Cruz, according to USGS data.
"Yes, here in the border of Oakland/Emeryville," tweeted Kimberly Huntimer. "Totally startled me."
"Thought I felt an earthquake here in San Francisco," tweeted Mary French.
Closer to the epicenter, the shaking was felt strongly.
"Yes, a lot of shaking in San Ramon," tweeted M.Everest.
"It was very scary here in San Ramon, tweeted Shak. "Probably was for 2 seconds ... felt a lifetime."
"A strong jolt in Alameda," tweeted Kimberlee MacV.
Following the quake, BART trains were held briefly and tracks were inspected, which is standard operating procedure following earthquakes.
There were no reports of any damage or injuries.
At San Ramon Valley Christian Academy, student Harper Skaggs put the skills she learned during The Great Shake Out to use as she and her classmates spent five minutes under their desks.
"It was strong, because I was nearby. I was at school in social studies and I just felt the whole thing and we had to like get under out desks," Harper told KPIX 5.
The USGS says the quakes and subsequent aftershocks were likely part of the Calaveras Fault system, possibly centered on the Pleasanton Fault and that this could be the beginning of a long period of shakes.
"The San Ramon Valley does experience what are known as earthquake swarms - where there can be dozens of earthquakes over days and weeks," said USGS geophysicist Brian Kilgore.
The USGS says there is a 26% chance of a magnitude 6.7 or higher earthquake on the Calaveras Fault in the next 30 years, only a slightly lower chance the anticipated "Big One" on the Hayward Fault.
Andria Borba contributed to this report.
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