They were drilling off Oregon. What they find could shake the entire state of California

When the massive subduction zone beneath the Pacific Northwest moves, it does so violently. An earthquake of magnitude 9 or higher in this region would unleash catastrophic tremors, followed by tsunamis and landslides that would multiply the devastation. Now, new research published in the journal Geosphere suggests that this “really big earthquake” may not stop there, but could also trigger a major earthquake in California.

“It's hard to overstate what an M9 earthquake would be like in the Pacific Northwest,” says Dr. Chris Goldfinger, a paleontologist at Oregon State University and lead author of the new study. “So the possibility of an earthquake occurring in San Andreas is cinematic territory.”

Two powerful systems for on-the-go breakdowns

The Pacific Coast of the United States lies at the boundaries of several massive tectonic plates. North of Cape Mendocino, California, the Juan de Fuca plate is subducting beneath North America along a massive subduction zone known as Cascadia. To the south, the Pacific and North American plates collide with each other along the San Andreas Fault, sometimes triggering devastating earthquakes such as the 1906 San Francisco event.

If these two massive fault systems ruptured in succession, it would redefine the seismic threat facing the entire western United States.

Lucky mistake at sea

The discovery connecting the two fault zones occurred by chance. During a 1999 research cruise, scientists planned to collect sediment samples off the Pacific Northwest to study ancient Cascadia earthquakes. However, one of the graduate students accidentally entered an incorrect latitude, sending the ship approximately 90 kilometers south of the intended location – beyond the Cascadia Margin and into the San Andreas region.

“We ended up off Northern California,” Goldfinger says. “When I woke up, I was so hot. But once we got there, I thought, 'Okay, let's take a closer look here.'

Strange layers in sediments

A core recovered from the Noyo Canyon submarine near Fort Bragg revealed something unexpected. Over the past 3,000 years, it has contained a recurring pattern of turbidites, sedimentary layers deposited by underwater landslides known as turbidites. Typically, these sediments show a simple pattern with coarse grains at the bottom and fine grains at the top. However, many layers in the Noyo Valley core and Cascadia appeared in distinct pairs.

“There were these big, thick sand events where there was a fine-grained component, and on top of that there was a very coarse-grained sand unit. We were just scratching our heads,” Goldfinger says.

Evidence of associated massive earthquakes

Radiocarbon dating revealed that many of these paired deposits north and south of Cape Mendocino formed at approximately the same time, within the limits of dating accuracy. This coincidence was so frequent that it was not a coincidence. After ruling out other possible explanations, the researchers concluded that the first deposit in each pair was likely caused by a large earthquake on Cascadia, while the second deposit was caused by movement along the nearby San Andreas Fault.

“A light bulb went on and we realized that the Noyo Channel was likely recording the Cascadia earthquakes, and that at a similar distance, the Cascadia sites were likely recording the San Andreas earthquakes,” Goldfinger says. “Well, what if? What if Cascadia exploded and triggered a weak turbidity current near the San Andreas, and then the San Andreas later exploded and caused very coarse sand deposits to collapse. That would create these inverted double layers.”

Can two earthquakes strike within hours?

It remains unclear how much time may have separated these linked events, as subsequent activity may have eroded some of the sediments between the layers. However, in many cores, the second deposit appears to have formed within minutes or hours of the first. If this interpretation is correct, it suggests that a powerful Cascadia earthquake could quickly trigger a massive San Andreas event, shaking almost the entire Pacific coast in quick succession.

Such a cascade would pose a serious threat to lives, infrastructure and emergency preparedness throughout the region.

“I'm from the Bay Area originally,” Goldfinger says. “If I were in my hometown of Palo Alto, and the Cascadia explosion happened, I think I'd drive east. It seems to me there's a very high risk that the San Andreas will blow up next.”

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