Synced as tightly as two geological clocks, the dangerous Hayward and Rodgers Creek faults have ruptured together before — and will rupture together again.

Or will they?

No one knows, because the evidence is hidden in deep mud at the bottom of San Francisco Bay. But getting answers will greatly help us prepare for earthquakes, telling us whether the danger is distant and limited, or imminent and widespread. Related Articles 4.5 earthquake rattles Southern California

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To solve this underwater mystery, a U.S. Geological Survey research team last fall sailed out into San Pablo Bay in search of clues.

“What we are trying to understand is past events on these faults to forecast what will happen in the future,” said Janet Watt, the USGS research geophysicist who is leading the study.

For decades, scientists have wondered whether these two major Bay Area faults connect beneath San Pablo Bay. The Rodgers Creek Fault enters the bay from the north; the Hayward Fault enters the bay from the south.

Watt’s team announced earlier this year that they do indeed connect. Previously scientists believed that they ran parallel to each other.

This means that it’s easy for an earthquake rupture to continue from one fault to the next — and that the 118-mile-long rift from Healdsburg to San Jose could deliver a blow greater than the sum of its parts.

According to USGS, a break along the combined length of the two faults could produce a major earthquake of magnitude 7.4 — releasing more than five times the energy released by the 1989 Loma Prieta earthquake, which had a magnitude of 6.9, caused about $6 billion in damage and killed 63 people.

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But geometry is not the only predictor of catastrophe.

Earthquake history also matters — because that tells us when the Big One is due.

“Now that we know these two faults are essentially holding hands, we can ask if — or how often — they ruptured together in the past?” said Watt, of the USGS’ Pacific Coastal and Marine Science Center in Santa Cruz.

Scientists have long been fascinated by what’s called “the earthquake cycle paradigm” — the accumulation of stress on a fault followed by its abrupt release as slippage or rupture, and then slow re-accumulation.

Describing the 1872 Owens Valley earthquake, early American geologist Grove Karl Gilbert wrote: “Suddenly, and almost instantaneously, there is an amount of motion sufficient to relieve the strain, and this is followed by a long period of quiet, during which the strain is gradually reimposed.”

If understood, this pattern can help predict the timing of future earthquakes, wrote USGS scientist David P. Schwartz and his team in the 2014 paper “The Earthquake Cycle in the San Francisco Bay Region: A.D. 1600–2012.”

When, and how often, have the Hayward and Rodgers Creek faults ruptured? Were the ruptures together or separate?

That’s what the current study hopes to learn.

Newspaper accounts show that back in 1868 the Hayward Fault triggered a massive 6.8 earthquake. There are no such reports for the more rural Rodgers Creek Fault. It’s rumored to have ruptured in the mid-1700s, but no official records exist.

The answers, Watt said, are buried in the earth.

That’s because earthquakes leave patterns. Every time there’s a vertical crack in the ground — one block of earth rising or falling relative to another — it creates a scarp, or a very steep bank or slope. Slowly, over time, sediments collect in this scarp, creating a wedge. Eventually sediments pile up and the ground is again level. Until, that is, the next rupture.

A cross-section of this earth, using special dating techniques, reveals a timeline: each fracture, healing and refracture.

On land, it’s no big deal to get a profile of the earth. You just dig a trench.

But under water, that’s a trickier problem.

Last October, seeking samples of buried bay mud, Watt and her team went out on a barge for four days in the north end of San Pablo Bay. That part of the bay is shallow and rich with sediments, washed down rivers during Sierra hydraulic mining operations in the late 19th and early 20th centuries.

The team retrieved 20 cores of mud, sampling both sides of the merged faults with a 12-foot-long coring device, suspended from a large metal “A frame” in the back of the boat.

Now the cores are back in USGS’ Santa Cruz lab, where they’re being studied for traits like density, magnetization and grain size.

To date the historic ruptures, they use a technique called carbon dating. This is based on the ages of dead single-celled bay organisms called foraminifera, whose carbon-rich shells are trapped and preserved in the sea floor sediment. The age of the fossils reveals the age of the mud that surrounds it — and the timing of each fracture.

“It will help pinpoint when the segments of the faults last moved,” Watt said. “We will never know for certain if they went together. There’s always uncertainty. But the more points we have, we can reduce our uncertainty.”

The cores can also tell scientists how much the two faults are safely “creeping,” releasing stress, rather than suddenly rupturing.

It’s a time-consuming process. The answers aren’t expected until some time next winter.

“Stay tuned,” Watt said. “The more observations we have along this zone, the more confidence we can build in the earthquake history of these faults.”

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