A swarm of earthquakes has struck California‘s Bay Area, just miles from some of the most populated cities throughout the area.

A magnitude 3.7 earthquake was detected at 12.08pm ET on Monday, less than three miles from the city of San Ramon, followed by a magnitude 2.9 quake at 1.06pm.

The US Geological Survey (USGS) has revealed that these were at least the sixth and seventh small earthquake to strike the since 5.33am this morning.

The quakes took place less than 40 miles from San Francisco, and less than 30 miles from major cities such as Oakland, Concord, and San Jose. 

All of the tremors have been stronger than magnitude 2.5, meaning residents likely felt all of the quakes but no severe damage was caused.

More than 1,000 people reported feeling the 3.7 magnitude quake, which USGS said sent shockwaves across the San Francisco Bay, into the city of more than 800,000 people.

Monday’s swarm was the latest seismic outbreak in the Bay Area, coming just days after residents in the region were shaken by three straight days of earthquake swarms rattling nerves along the state’s Central and Northern Coasts.

The recent swarms have also taken place along several branch faults in the broader San Andreas Fault System, sparking fears the main fault line could be closer to a major rupture that locals refer to as ‘the Big One.’

The seven earthquakes occurred near a collection of fault lines in California, including the Greenville Fault, the Calaveras Fault, and the Concord–Green Valley Fault.

All of these are part of the same big family of cracks in the Earth’s crust that make up the San Andreas Fault System.

It’s a long zone where the Pacific Plate and the North American Plate slide past each other and cause much of the state’s seismic activity.

In the Bay Area, the 80-mile-long San Andreas doesn’t work alone, as it connects to a whole family of parallel and branching faults, including the ones Monday’s earthquakes struck near.

They take some of the San Andreas’ plate motion and spread the earthquake risk across the entire region. 

USGS has previously warned that the San Andreas Fault could generate a mega earthquake potentially reaching up to 8.2 in magnitude.

Angie Lux, project scientist for Earthquake Early Warning at the Berkeley Seismology Lab, told the Daily Mail that she was ‘fairly confident’ a large earthquake will strike California within the next 30 years.

The last major quake to strike this part of the California was the 1906 San Francisco earthquake, which is believed to have reached magnitude 7.9. That quake left 80 percent of the city destroyed and more than 3,000 people dead.

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