The looming threat of earthquakes is nothing new for Californians. But a new discovery in the heart of the San Francisco bay has upped the risk of a historically disastrous quake.

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 longer a fault stretches, the bigger the earthquake it can produce. And here we've just doubled the length of this fault."

"That's a big deal," says David Ponce, a geologist with the research group. "Because the longer a fault stretches, the bigger the magnitude of an earthquake it can produce. And here we've just doubled the length of this fault." Ponce explains that seismologists consider these combined fault zones "to constitute one of the most dangerous earthquake risks in the nation."

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.

How could this new chunk of fault line remain hidden for so long under such a highly populous region?

"You have to understand that there are over 2.4 million people living right along this fault."

One big reason is that the traditional tools and methods used to discover fault-zones underwater have been stymied by the San Francisco Bay's mud. Geologists typically use sound-waves to ping and probe underwater, collecting data about what lies under the sea floor. But San Pablo Bay—the northern section of the greater San Francisco Bay that conceals this fault line—is heavily layered with mud containing tiny pockets of gas. That gassy mud deflects sound-waves and makes it exceedingly hard to probe deep into the rock where the fault zone would lie.

To bypass this issue, Watt and her colleagues used a fairly ingenious trick of deduction. They just looked for clues in the mud itself.

Using a boat-towed soundwave tool called a chirp subbottom profiler, the geologists put together a map of the various mud layers in the bay. The scientists found that the layers of mud themselves were offset, suggesting a fissure in the rock below.

The scientists also compared their mud data to a map of the magnetic properties of the bay floor. According to Ponce, because rocks at different layers in the Earth have different magnetic properties, a shift in the layers of rock below the earth's surface should be detectable. Just as expected, a weird magnetic anomaly clued the scientists in to the fact that the rocks below were—just like the mud—also offset. The fault-zone was found.

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.

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