SAN FRANCISCO (KPIX 5) — Scientists have discovered an alarming connection underwater: Two of the most dangerous earthquake faults in the Bay Area.

Experts long debated whether the Hayward Fault and the Rodgers Creek Fault connected, but now there’s strong evidence they do.

US Geological geophysicist Janet Watt made the discovery in an underwater survey using an acoustic instrument over the San Pablo Bay. She says the data is clearer than ever.

“We now have direct evidence that the faults come closer together in the bay and may be directly connected,” says Watt. “It would be devastating for an earthquake to rupture at both those faults — it’d be a very strong earthquake.”

Two Bay Area fault lines could potentially trigger a 7.3 magnitude earthquake if they ruptured together. That’s stronger than the 1989 Loma Prieta quake.

“If these two faults are connected then that means it would be easier for an earthquake to rupture from the Hayward to Rodgers or vice versa,” says Watt.

Old maps showed a gap of at least 2.5 miles between the faults. Watt says the 60-mile long Hayward Fault, which runs from San Pablo Bay to Alum Rock, and the 39-mile Rodgers Creek Fault that runs from San Pablo Bay to Healdsburg, are likely linked by a strand.

Watt’s discovery may help determine whether a big quake is ahead of us. After all, the Hayward Fault is the most populated fault in the world. A 7.3 magnitude quake would be devastating.

“People in the Bay Area just need to be prepared that they live in earthquake country,” she said.

Now scientists will work to confirm the new data. Watt’s survey will go from underwater to land to see if there is a connection above ground, as well.