SAN FRANCISCO — A surprising 3.7 magnitude earthquake struck the San Francisco Bay Area Friday night, startling rush hour commuters as the second such tremblor in Northern California in just over a week.

The quake hit at around 6:38 p.m. local time, according to the U.S. Geological Survey, just as sunset was approaching. The USGS earthquake monitor initially put the quake at 4.1 on the Richter scale but then, as is common, the measurements slightly downwards to 3.9 and then at 3.7 as more data came in.

There have been no immediate reports of injuries or damage.

The tremblor appears to have been close to the San Andres Fault, with its epicenter documented in the town of Millbrae, near San Francisco International Airport, about 10 miles south of the city.

In San Francisco, the earthquake began as a slight shudder and then a violent jolt, as if a truck had run into a building. It was followed by a few slight rumbles and it was over in less than a minute. A few car alarms could be heard in parts of the city within half an hour of the quake.

Friday's quake was the second in Northern California in 9 days

Friday's quake in San Francisco was the second in less than nine days in Northern California.

A magnitude 4.2 earthquake rattled millions of residents in the region on Oct. 18, prompting an unexpected "ShakeAlert" on mobile devices.

The quake hit near the small community of Isleton in Sacramento County around 9:30 a.m., the USGS reported. There were no reports of injuries or damage. The earthquake was felt in several eastern San Francisco Bay Area suburbs including Antioch, Concord, Fairfield, and even Berkeley, the site of the University of California's main campus.

The Isleton earthquake came a day before the annual Great ShakeOut, a global drill during which emergency systems were tested for earthquake preparedness, including thousands of MyShake app users.

That quake also occurred one day after the 34th anniversary of the Loma Prieta earthquake that rocked the San Francisco Bay Area in 1989, killing 63 people and injuring nearly 3,800 others. The devastation caused up to $10 billion in damage.

San Francisco quake caused slight transit rail system disruption

Friday's quake prompted the San Francisco Bay Area subway system known as BART to hold trains at stations for a short time to check for structural damage along its tracks and most importantly in the tunnel that runs trains back and forth from San Francisco to Oakland under the Bay.  

The transit rail system resumed normal service shortly after 7 p.m. local time after completing safety track inspections.  

Two quakes in a short stretch can 'happen regularly' USGS director says

Two earthquakes occurring in the same region in a relatively short period is actually quite common, Christine Goulet, director of the USGS' Earthquake Science Center in Los Angeles, said Friday.

But are they precursors to the big one?

"These are small earthquakes and they happen regularly across seismically active regions," Goulet said. "In some cases, when there are a lot clustered in time in a very small region, we characterize them as a swarm."

But it's "rare" that small quakes or even swarms lead to larger earthquakes, Goulet added.

"The few we have seen in the Bay Area are not close enough to be, at first glance, related at all," Goulet said. "I consider these distinct unrelated events."

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