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East Bay fault is more powerful than scientists ever imagined

Turns out two nearby faults are joined at the hip

The anniversary of the Loma Prieta earthquake a few days ago and today’s statewide earthquake drill probably took up most of your mental bandwidth for earthquakes this week. But we’re afraid this news is too important to wait, because it turns out that one of the Bay Area’s largest and most dangerous fault lines is even larger and more dangerous than anyone thought.

A paper in the scientific journal Science Advances by Janet Watt of the US Geological Survey titled "Missing Link Between Hayward, Rodgers Creek Faults" bears some bad news for the East Bay in particular. Quake watchers have long worried about the Hayward fault, which runs right under Oakland and which, statistically speaking, is the most likely geological structure to have a major hemorrhage in the Bay Area sooner rather than later.

The Rodgers Creek fault to the north has been less of a concern, until Watt and her partners took a closer look at the relationship between the two seams. Previously, the pair were thought to be "separate segments that are capable of rupturing together." Which is bad enough already. But now Watt writes that the two are actually physically connected by a fault "strand."

This is a problem, because the length of a fault is one of the things that determines how large of an earthquake it can produce. It also means that the two are more likely to work in tandem. "The discovery of a strand directly linking the faults facilitates a simultaneous rupture," Watt writes.

The two faults together run nearly the entire length of the Bay Area, 115 miles from Wine Country down to almost the outskirts of San Jose. We now know they're also more powerful, capable of rolling out a 7.4 magnitude event—probably within the next 30 years or so.

In the past, the deep mud and bubbling gases at the bottom of the San Pablo Bay made it hard for researchers to suss out precisely how the two faults might be connected. This paper lays out the imaging techniques used to spot fissures in the mud layers that pointed to a previously undiscovered seam.

Now, there’s no use in panicking. All this means is that the 2.4 million people living on top of the fault line, and all of us nearby, need to prepare. Living in the Bay Area, nobody knows precisely when the bell is going to toll, but we can always be confident that it’s not going to wait forever. Stay safe.