Swarm of earthquakes jolt San Ramon area. Biggest is 4.2

SAN RAMON — An ongoing string of more than a dozen earthquakes in less than 90 minutes early Monday ended what had been some recent calm from recent weeks of shaking ground in the Tri-Valley area, according to the U.S. Geological Survey.

The most violent of the 19 earthquakes measured by the U.S. Geological Survey was a magnitude 4.2 shaker that was centered about 2½ miles southeast of downtown San Ramon. Quakes of 3.8, 3.3, 3.1 and 3.0 also were among the 19. The rest of the earthquakes all measured below a magnitude 3.0.

All but a pair of the 19 quakes were centered about 2½ miles east to southeast of San Ramon. A 3.8 quake and 2.6 quake were centered about 3.1 miles southeast of the city.

The first quake shook at 6:27 a.m., while the last of the flurry happened at 7:48 a.m., according to the USGS. The 4.2 shaker could be felt as far away as Concord and on the Peninsula.

Those quakes follow a 3.1 quake that happened Friday morning that was centered about 3 miles southeast of the city.

The quakes caused the BART commute to be delayed, as trains ran at reduced speeds while the agency completed track safety inspections. The delays were expected to be at least 20 minutes system-wide.

Until the quake Friday, an earthquake swarm that had delivered 91 earthquakes of magnitude 2.0 and above since Nov. 9 appeared that it might be over. A gap of about two weeks between earthquakes occurred, and scientists said it appeared the quakes might be over.

Earthquake swarms are collections of dozens or even hundreds of small, usually harmless quakes. Scientists are still learning about them and said they don’t have strict parameters for when swarms start or conclude.