More than 200 quakes, most measuring under 4.0 on the Richter scale, have been measured in the area of San Ramon since the swarm began, but no damage has been reported, according to the San Francisco Chronicle newspaper.
On Monday, residents of the San Ramon Valley were jolted by a 2.8 quake at 4:02 p.m. followed at 4:21 p.m. by a 3.5 and another 2.8, which could be felt strongly in Contra Costa and eastern Alameda counties.
Seismologists said the swarm could continue for days or even weeks before it ends, probably without notice, the newspaper said.
“The good news is that most of these earthquakes stay under magnitude 4,” said Susan Garcia of the U.S. Geological Survey’s Earthquake Science Center in Menlo Park, Calif.
“An earthquake swarm is not unusual in that area,” she said.
In fact, a smaller swarm of quakes hit the area last week, including a 3.4 temblor that could be felt as far north as Vallejo and as far south as Pleasanton, the newspaper said.
No major damage was reported then, either.
The quakes took place along the Hayward Fault, which stretches 74 miles from San Jose to Richmond, and is thought to be the most likely candidate for the next major California earthquake.
San Ramon is a suburban city of 76,000, located 35 miles east of San Francisco.
