DANVILLE (CBS SF) — The earth continued to rumble beneath Danville Friday as five small earthquakes — the largest a magnitude 3.6 — rattled the area, according to the U.S. Geological Survey.
The 3.6 earthquake hit at 12:19 p.m. PST, according to the USGS. At a depth of 0.5 kilometers, it was shallower than the others and likely felt more widely. BART trains were temporarily halted for track inspection as a result.
The first quake struck along the Calaveras fault system at 2:32 a.m. and measured magnitude 2.7. It was followed by a 2.8 quake at 3:24 a.m. and a 3.3 at 5:28 a.m. The latest one was a 2.8 which struck at 12:22 p.m., the USGS said.
While the quakes were small and caused no major damage, they were shallow so strongly felt close to the epicenter.
Danville has been at the epicenter of what scientists are describing as an “earthquake swarm.” The series of small quakes haven’t caused any real damage, but they have certainly rattled nerves.
“I’m sound, sound asleep and just after midnight, it’s like somebody whacked the house,” said Danville resident Robin Soule.
“Most of them were a sudden jolt,” agreed Danville’s David Anderson. “Not very long. Not the roller kind. Just, “Boom!'”
There were at least 20 small quakes just on Friday, including the magnitude 3.6 temblor at around noon that caused BART to slow trains and inspect the tracks for damage.
“Anytime, there’s multiple earthquakes going on, it’s got to raise an eyebrow, admitted Anderson. How concerned should I be? What’s the next step? An 6.0? An 8.0?”
Scientists at the USGS say the quakes are centered on the Calaveras Fault.
They say there are common misconceptions about an earthquake swarm: that they indicates a bigger quake is on the way or — conversely — that it releases pressure, making a major earthquake less likely.
They say neither is true.
“We don’t expect this to grow into anything really large,” said USGS Seismologist Anne Marie Baltay. “This swarm is very typical of what we see in this region. It’s happened seven times in this area since 1970.”
BART officials said they also halted traffic on its Pittsburgh/Bay Point line for a few minutes following the earlier quakes to check for any track damage.
The swarm was just the latest as seismic activity was on an upswing in the area. On Tuesday, the ground near Danville’s Monte Vista High School was the epicenter of five small quakes.
“Looking in that general region, I’m counting 55 quakes just in the last week,” Amy Vaughan, a geophysicist, told the San Francisco Chronicle. “Of those, this 3.3 was the largest, but there were several in the two range and even one other at 3.0.”
The final quake in the Tuesday series took place at about 6:16 p.m.
The Tuesday quake swarm was about two kilometers west northwest of the town of Diablo with a depth of approximately 7.5 kilometers. The quakes were reported on social media by residents in San Ramon and other parts of the East Bay.
According to the USGS, the area near the Calaveras Fault was first struck by a 2.8 magnitude quake at 1:32 a.m. It had a depth of 6.8 kilometers. Three hours later, quakes measuring 2.6, 2.9 and 3.0 struck in rapid succession.
A 2.9 quake was reported in the same area early Monday.
Back in 2015, the Danville-San Ramon area was struck by dozens of small quakes also along the Calaveras Fault.
The quakes are a reminder that the Bay Area is earthquake country and the “big one” is never far away.
“Our forecast for the next 30 years is a magnitude 6.7 or larger,” said Baltay.