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Geologists have found a new fault line under the San Francisco Bay. It could produce a 7.4 quake, effecting 7.5 million people.

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A team of geologists led by Janet Watt at the United States Geological Survey just made the unfortunate discovery underneath the murky waters of the San Francisco Bay: a hidden connection between two earthquake fault zones. The scientists outline their discovery today in the journal Science Advances.


The two now-connected fault zones—the Rodgers Creek and Hayward faults—stretch more than 115 miles together. They reach from the wine country northwest of the bay, continue under the bay waters directly into Oakland in the east bay, and terminate just north of San Jose. Watt and her team estimate that when this connected fault next slips, it could produce untold disaster: Up to a magnitude 7.4 earthquake, which would be California's 5th largest earthquake and very likely its most deadly. And it could happen soon; Ponce says there's a 32 percent chance such a quake could occur in the next 30 years.

"You have to understand that there are over 2.4 million people living right along this fault, and the population of this whole area is around 7.5 million. It also turns out that major transportation, gas, water and electrical lines cross this fault. So when it goes, it's going to be absolutely disastrous," Ponce says.


For residents of the San Francisco Bay Area, Ponce says the takeaway from this new discovery is clear. "Everyone in this region should be prepared for an earthquake along this fault," he says. A potential 7.4 magnitude quake with such a high risk of occurring soon is no laughing matter.

"It will impact communities on both sides of the bay and there's going to be significant damage. So folks should do the standard things, like make sure you have food, supplies, and water for a couple of days, make sure you have a contact person to call, make sure your house is bolted to the foundation, and that your water heater is properly secured," he says.

Top Reddit comment:

There are new faults being discovered all over the world as we install more seismometers to record earthquakes and develop new techniques, but the fault identified and mapped in this new paper is in a particularly important location. This new fault connects the Hayward and Rodgers Creek, two faults that are most likely to have a M6.7+ that will affect the Bay Area in the next thirty years. Before this work, the section between the two faults beneath San Pablo Bay was a bit of a mystery. Researchers didn't know if the two Hayward and Rodgers Creek faults connected here under the layers and layers of mud with a bend, or if they were disconnected by a several kilometer gap or "step-over". There is a lot of research trying to figure out if an earthquake could jump that gap and rupture both faults in one go. Rupturing both together would result in a much larger and more damaging earthquake than if only one fault ruptured at a time. However with these new observations showing that the faults are connected, there is no gap to jump and a rupture through both the Hayward and Rodgers Creek is more likely.