San Francisco Bay Area rattled by series of early morning earthquakes

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SAN FRANCISCO (AP) — A series of small earthquakes rattled the San Francisco Bay Area on Monday morning in an area that has had a lot of seismic activity in recent months.

The most powerful of Monday’s quakes was a magnitude 4.2 that struck shortly after 7 a.m. just south of San Ramon, according to the U.S. Geological Survey.

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.