String of quakes near Concord rattles Bay Area

Photo: Courtesy, USGS Photo: Courtesy, USGS Image 1 of / 1 Caption Close String of quakes near Concord rattles Bay Area 1 / 1 Back to Gallery

A swarm of small earthquakes — the strongest measuring magnitude 3.6 — centered just south of Concord jolted the Bay Area on Sunday afternoon.

The U.S. Geological Survey reported the strongest quake occurred about 3:13 p.m., with an epicenter a half-mile south of Concord. The quake, initially set at magnitude 4.0 but later downgraded, was about 9 miles deep and occurred on the Concord-Green Valley Fault, the USGS said.

Concord police said there were no reports of damage or injuries.

East Bay Municipal Utility District crews were repairing a minor break to a 12-inch steel water main at North Main Street and Geary Road in Walnut Creek on Sunday night. The break was reported at 4:15 p.m., but repair crews could not immediately determine if the shaking caused the underground break to the 52-year-old water main, said EBMUD spokeswoman Tracie Morales-Noisy.

The swarm started with a magnitude 2.5 quake recorded in the same area about 3:01 p.m. A 2.7 quake occurred about a minute after the strongest shaker, while a 2.4 quake shook the area about 3:28 p.m. Another one measuring 1.6 was recorded before 4 p.m., followed by a 2.3 quake shortly before 7 p.m.

The strongest quake did not produce much ground motion and was “a bit puny” as shakers go, said Jack Boatwright, the USGS coordinator for earthquake hazards in Northern California.

Still, the quake was felt throughout region. One Concord resident posted on SFGate.com’s Facebook page that the quake “shook my whole house,” while the earlier 2.5 quake felt like “a large truck going by fast.”

In Pleasant Hill, Oakland and San Francisco, people reported the quake felt like a quick but strong jolt, while others said it “felt like a boat on a wave.”

Seismologists will be examining whether the quakes were related to August’s magnitude 6.0 South Napa quake, Boatwright said.

“Even though we’re nine months after the South Napa quake, all of this activity is interesting to us,” he said. “The difficulty is we don’t know how the faults are connected.”

The last big earthquake along the Green Valley Fault occurred before 1770 “as far as we can tell,” Boatwright said. But it’s part of a fault system that starts in Concord and “runs all the way past Lake Berryessa,” he said.

Benny Evangelista and Rachel Swan are San Francisco Chronicle staff writers. E-mail: bevangelista@sfchronicle.com, rswan@sfchronicle.com Twitter: @ChronicleBenny, @RachelSwan