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When the tectonic subduction zone beneath the Pacific Northwest moves, it does so in dramatic fashion. Not only is ground shaking from a magnitude 9+ earthquake incredibly destructive, the event triggers tsunamis and landslides to compound the damage. Now, a new in the Geosphere suggests the "really big one" could also trigger a major earthquake in California.

"It's kind of hard to exaggerate what a M9 earthquake would be like in the Pacific Northwest," says Dr. Chris Goldfinger, a paleoseismologist at Oregon State University and lead author of the new study. "And so the possibility that a San Andreas earthquake would follow, it's movie territory."

Tectonic setting

The United States' Pacific coast is defined by tectonic plate boundaries.

To the north of Cape Mendocino, California, the Juan de Fuca plate is plunging beneath the continent in what geologists call a megathrust subduction zone.

To the south, the Pacific and North American plates are grinding against each other, resulting in periodic earthquakes like the famously destructive 1906 San Francisco quake.

The prospect that both could move at the same time redefines the concept of earthquake hazard in the western U.S.

An accidental discovery

Coming to such a stunning conclusion wasn't the researchers' goal when they set out. In fact, the study came about because of a navigational error on a research cruise in 1999.

Originally, the plan was just to look at the offshore sedimentary record of Cascadia earthquakes by drilling sediment cores and conducting seismic surveys on the Pacific Northwest margin. But one night a graduate student entered the wrong latitude for their overnight destination and the drill ship ended up too far south. The researchers had left the Cascadia margin and were now 90 kilometers south of Cape Mendocino—in the realm of the San Andreas.

"We wound up off northern California," says Goldfinger. "When I woke up, I was pretty hot. But, once we were there, I thought, 'well, let's take a core here.'"

When they examined that core, taken from the submarine Noyo Canyon off the California coast near Fort Bragg, they noticed something strange. Throughout the core, going back some 3,000 years, were a series of turbidites, or deposits from fast-moving underwater landslides called turbidity currents. Turbidites have characteristic layering, with coarser sediment grains at the bottom and smaller grains at the top. But unusually, many of the turbidites in both the Noyo Canyon core and the Cascadia cores were deposited in pairs.

"There were these big, thick, sandy doublet events where it had a fine-grained element, and on top of it was a very coarse grained sandy unit. And we were just scratching our heads," says Goldfinger.

Once they'd used radiocarbon to date the turbidite events up and down the coast, they found another surprise. In cores taken both north and south of Cape Mendocino, more than half of the turbidites were deposited at the same time, within the error of radiocarbon dating. The researchers felt that was too many to be a coincidence and had to be due to a common cause.

After ruling out other conceivable explanations, they were left with the realization that the first unit of each doublet in Noyo Canyon was a turbidity current set off by a large earthquake on the Cascadia megathrust. The second unit of the doublet, then, had to have been caused by movement on the nearby San Andreas.

"A lightbulb went on and we realized that the Noyo channel was probably recording Cascadia earthquakes, and that at a similar distance, Cascadia sites were probably recording San Andreas earthquakes," says Goldfinger. "Well, what if? What if Cascadia went off and triggered a weak turbidity current near the San Andreas, and then the San Andreas went off some time later and triggered a very coarse, sandy deposit to come down. It would create this upside-down doublet stratigraphy."

Cascading hazards

The timing between the earthquakes is uncertain, as the top turbidite could have eroded away sediment between the doublets. But, in some of the turbidite deposits, the researchers saw evidence that the second unit of the doublet was deposited within minutes or hours of the first, raising the possibility that almost the entire US Pacific coast could experience a major earthquake nearly simultaneously.

The potential impact of such a series of earthquakes raises questions about preparedness for that scope of hazard to people's lives and infrastructure.

"I'm from the Bay Area originally," says Goldfinger. "If I were in my hometown of Palo Alto, and Cascadia went off, I think I would drive east. There looks to me like a very high risk the San Andreas would go off next."