90 earthquakes hit California’s Bay Area as experts weigh risk of larger one

The Bay Area of California has been struck with around 90 small earthquakes this month, as experts try and figure out why. Most recently, San Ramon in the East Bay has been the epicenter of some seismic activity. It sits on top of the Calaveras Fault, which is an active branch of the San Andreas Fault system. The Calaveras Fault has the potential to produce an earthquake with a magnitude of 6.7 which would have an impact on millions of people in the area. (Picture: Getty)

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What is the San Andreas Fault system?

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San Andreas Fault system runs around 800 miles between the Pacific tectonic plate to the west and the North American plate to the east. As the plates grind against each other, at a rate of around 2 inches per year, the motion causes smaller shakes and tremors. The system stretches across California, including Los Angeles and San Bernardino. The last major quake in the San Andreas fault zone was in 1906, where a 7.9 magnitude earthquake and fire levelled parts of San Francisco and killed around 3,000 people. (Picture: Google Maps)

Regarding the 6.7 magnitude earthquake the Calaveras Fault is capable of producing, the US Geological Survey (USGS) estimates an 18% chance of this happening by 2030. This month’s seismic activity began on November 9, with a 3.8 magnitude, and the tremors have not stopped since. However, although smaller quakes can be a sign a larger one is looming, researchers say this swarm is not a tell-tale sign. (Picture: USGS)

Geologist with the USGS Sarah Minson told SFGate: ‘This has happened many times before here in the past, and there were no big earthquakes that followed. We think that this place keeps having earthquake swarms due to a lot of fluid-filled cracks, thanks to very complex fault geometry – unlike, say, the San Andreas Fault, which is this nice clean edge.’ She added: ‘What may have happened this time is that there was a 3.8, and it kicked off some aftershocks that were very swarmy because it’s a very swarmy spot that has swarms.’ (Picture: USGS)

The last major earthquake on the Calaveras Fault was a magnitude 5.1 event in October 2022, which was near Mount Hamilton. This was the biggest earthquake on the fault since 2007, and the largest in the Bay Area since 2014. The largest quake on the fault line was a magnitude 6.6 in 1911. But this month’s activity marks at least the sixth swarm in the area since 1970, with the most recent one shaking in 2015. (Picture: Getty)

However, researchers studying the 2015 San Ramon earthquake swarm revealed the area has several small, closely spaced faults rather than a single big one, which suggests that quakes moved along these faults in a complex pattern and that they interact with each other. The research may have suggested that underground fluids have helped trigger the tremors. No other possible causes, like tidal forces, where found in connection. But this shows that the fault system under San Ramon is more complicated than researchers previously thought. (Picture: Getty)

California has been subjected to a number of quakes this week, including a 4.1 magnitude near Templeton which triggered an alert for residents. There is no cause for concern, as Minson says: ‘This week is a bit busy. But earthquakes are not uniformly spaced out in space and time, so this is all within the normal variation for earthquakes.’ (Picture: USGS)

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90 earthquakes hit California’s Bay Area as experts weigh risk of larger one

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