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At least a dozen other smaller quakes struck in the same area
starting around 6:30 a.m. and continuing for more than an hour.
The area east of San Francisco has experienced earthquake swarms
— when multiple small magnitude earthquakes strike over a short
period of time — for decades, according to seismology experts.
The earthquake swarm that struck Monday is the 10th sequence
since scientists began documenting them in 1970, said Lucy
Jones, a veteran seismologist in Southern California. Big quakes
in the Bay Area are typically due to major faults, including the
Hayward and San Andreas, which runs along the Northern
California coast before moving inland.
The earthquake swarms could be an indication of a new fault
starting to form — though it would take tens of thousands of
years to prove that theory, Jones said.
“It’s a gradual creation of a fault that’s going to take 100,000
years,” she said.
There were no immediate reports of major damage from Monday’s
tremors. Bay Area Rapid Transit said delays could result from
trains briefly running at reduced speeds during routine track
safety inspections.
Shaking was felt more than 30 miles (48 kilometers) away in San
Francisco and across the East Bay cities of Oakland and
Richmond.
Earthquake swarms have hit the area multiple times in recent
months. The Contra Costa County area recorded 87 quakes at
magnitude 2 or above in November and December, according to a
San Francisco Chronicle analysis of USGS data last month.
Jones said several areas in California get earthquake swarms,
which repeatedly strike a small area and don’t follow a typical
mainshock-aftershock sequence.
Besides a new fault trying to form, other quake swarms happened
when there are fluids or magma involved, like in California’s
Mammoth Mountain, where there is a volcano.
Jones said a swarm of quakes is not necessarily a precursor for
a bigger quake.
“We’ve had swarms ten times in the last 50, 55 years in the (San
Ramon) area and none of those were followed by a bigger
earthquake,” she said.
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