Next Cascadia Quake Could Trigger One in California

Two of North America's most notorious earthquake faults may be more closely linked than scientists once thought, raising fears that a massive quake in the Pacific Northwest could set off another disaster along California's San Andreas fault. The Cascadia subduction zone, which runs from Vancouver Island in Canada to Northern California, has long been seen as the source of a possible "Really Big One," the New York Times reports. A new study in Geosphere suggests that earthquakes on Cascadia's fault have historically set off quakes on the San Andreas in a "possible stress triggering relationship."

Research led by Oregon State University professor emeritus Chris Goldfinger uncovered sediment evidence off the California coast indicating that the eight largest Cascadia events over the past 2,500 years all appear to be closely followed by major San Andreas earthquakes. The findings are based on analysis of "turbidites"—layers of sediment shifted by undersea landslides during earthquakes. Unusual layering in these samples, first noticed after an accidental detour in a 1999 field study, suggested back-to-back quakes: a lighter sediment from a Cascadia quake, then a heavier deposit likely from a nearby San Andreas event.

"When a fault ruptures, it's relieving stress locally, but it's transferring stress to areas nearby," Goldfinger says, per the Oregonian. "So when Cascadia ruptures, it transfers stress to Northern California. "Cascadia is often talked about as, 'When it goes, it'll be the biggest disaster in North American history,'" Goldfinger says. "But what if the San Andreas goes at the same time? It's still going to be the biggest disaster in North American history, but it's quite a bit bigger." If the theory holds true, it means the Pacific Northwest and California could face not just one catastrophic earthquake, but a rapid double punch—potentially compounding the damage and complicating emergency response.

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"It would be a very bad day on the West Coast of the US, that's for sure," Goldfinger says. Goldfinger has been investigating the connection between the faults for decades. "In the late eighties and early nineties, the paradigm shifted to 'uh-oh,'" he told the New Yorker in 2015. Goldfinger says the geological record shows there was a San Andreas quake in 1700 within minutes of hours of that year's better-documented Cascadia quake, the Oregonian reports. But it's not clear whether it will be so immediate next time, meaning a Cascadia quake could provide plenty of advance warning of a San Andreas quake. "It could be weeks. It could be a couple of decades. It could be 50 years even," he says.