They were drilling off Oregon. What they find could shake the entire West Coast

When the massive subduction zone off the Pacific Northwest finally shifts, the results could be catastrophic. An earthquake of magnitude 9 or more could unleash severe earthquakes, along with tsunamis and landslides that exacerbate the devastation. Now, a study published in the journal Geosphere suggests that this long-feared “really large organism” may not be acting alone. It could also cause a strong earthquake along the San Andreas Fault 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 study. “So the possibility of an earthquake occurring in San Andreas is cinematic territory.”

Tectonic forces shape the Pacific coast

The western edge of the United States lies above a complex system of tectonic boundaries. North of Cape Mendocino, California, the Juan de Fuca plate is thrusting beneath the North American plate, forming the massive Cascadia thrust. South of that point, the Pacific and North American plates slide past each other along the San Andreas Fault, periodically triggering major earthquakes such as the devastating San Francisco event of 1906.

If these two systems exploded close to each other, it would dramatically change how scientists assess earthquake risk along the West Coast.

My research mistake changed everything

The idea of ​​a connection between these defects came from an unexpected place. In 1999, scientists set out on a research expedition to study ancient Cascadia earthquakes by collecting sediments from the sea floor. Their goal was to reconstruct past events along the edge of the Pacific Northwest.

However, a simple navigation error changed the course of the study. One of the graduate students entered the wrong latitude overnight, sending the ship about 90 kilometers south of its intended location. By morning, the team had drifted out of Cascadia and into an area affected by the San Andreas Fault.

“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.’

Unusual sediment layers reveal a pattern

The core sample they collected from Noyo Canyon near Fort Bragg holds a surprising record of past activity. These sediments date back about 3,000 years and contain repeated layers called turbidites. These form when underwater landslides, known as turbidity currents, push down the seafloor and deposit material. Typically, these layers show a clear structure, with the heavier grains settling first and the finer particles settling on top.

But in this case, many deposits appeared in pairs. Both the Noyo Canyon core and samples from Cascadia showed this unusual double pattern.

“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 earthquakes associated with the Cascadia and San Andreas earthquakes

Radiocarbon dating of these layers revealed another important piece of evidence. Many of the paired deposits from sites north and south of Cape Mendocino formed at about the same time, within the limits of dating accuracy. This level of overlap suggests a common cause and not mere coincidence.

After considering and ruling out other possibilities, the researchers concluded that each pair was likely recording two separate but related events. The first layer appears to have come from the Great Cascadia Earthquake, while the second layer reflects 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.”

Possible chain reaction along the coast

The amount of time that may pass between these associated earthquakes remains uncertain. In some cases, subsequent deposits may have erased evidence of any gap between events. However, many samples indicate that the second layer formed within minutes or hours of the first layer.

If this interpretation is correct, a strong Cascadia earthquake could quickly trigger a large rupture along the San Andreas, sending strong tremors across much of the Pacific coast in quick succession.

Such a scenario raises serious concerns about preparedness. A series of major earthquakes could strain emergency response systems and infrastructure across multiple states simultaneously.

“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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