SAN RAMON — A calm from the recent weeks of shaking ground was interrupted Monday morning when eight more earthquakes struck the Tri-Valley area in less than hour, according to the U.S. Geological Survey.
The most violent of the earthquakes was a magnitude 4.2 shaker that was centered about 2½ miles southeast of downtown San Ramon. Two of the quakes measured 2.5 while the other three were were 3.9, 3.3 and 2.8.
The first one shook at 6:27 a.m., while the last of the five happened at 7:07 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.
There were no immediate reports of injuries or damage.
The Mercury News
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