When a massive subduction zone off the coast of the Pacific Northwest eventually moves, the consequences could be catastrophic. When an earthquake with a magnitude of 9 or higher occurs, the ground shakes violently, causing tsunamis and landslides, which spread the destruction. Now, a study has been published earth sphere It suggests that this long-feared “really big thing” may not be acting alone. It could also trigger powerful earthquakes along the San Andreas Fault in California.
“It’s hard to overstate what a magnitude 9 earthquake would be like in the Pacific Northwest,” said Dr. Chris Goldfinger, a paleoseismologist at Oregon State University and lead author of the study. “And then the possibility of a San Andreas earthquake is the realm of movies.”
Crustal deformation forces shaping the Pacific coast
The western edge of the United States sits on a complex tectonic boundary system. North of Cape Mendocino, California, the Juan de Fuca plate is being pushed beneath the North American plate, forming the Cascadia superthrust. South of that point, the Pacific and North American plates slide against each other along the San Andreas Fault, periodically causing large earthquakes like the devastating San Francisco earthquake of 1906.
If these two systems ruptured nearby, it would significantly change the way scientists assess seismic risk along the West Coast.
The research mistake that changed everything
The idea that these faults might be related came from an unexpected source. In 1999, scientists set out on a research cruise to collect sediment cores from the ocean floor to study ancient Cascadia earthquakes. Their goal was to recreate past events along the rim of the Pacific Northwest.
However, a simple navigation error changed the course of the study. A graduate student entered the wrong latitude overnight, sending the ship about 90 kilometers south of its planned location. By morning, the team had drifted out of the Cascadia region and into the area affected by the San Andreas Fault.
“It ended up off the coast of Northern California,” Goldfinger said. “When I woke up, I was pretty hot. But once I got there, I thought, ‘Okay, let’s core here.’
Unusual sediment layers reveal patterns
The core samples they collected from Noyo Valley near Fort Bragg contained a surprising record of past activity. Dating back about 3,000 years, the deposits contained repeating layers called turbidites. They form when underwater landslides, known as turbidity currents, push onto the ocean floor and deposit material. Typically, these layers exhibit a well-defined structure, with heavier particles settling first and finer particles remaining above them.
However, in this case, many of the deposits appeared in pairs. Both the Noyo Valley core and the Cascadia sample exhibited this unusual bilayer pattern.
“There was a big, thick sand doublet event, with a fine-grained element, and then a very coarse sand unit on top of that, and we were just scratching our heads,” Goldfinger says.
Evidence of a connection between the Cascadia and San Andreas earthquakes
Radiocarbon dating of these layers revealed another important clue. Many of the paired deposits from sites north and south of Cape Mendocino were formed at about the same time, within dating accuracy. This level of overlap suggests a common cause rather than coincidence.
After considering and eliminating other possibilities, the researchers concluded that each pair was likely recording two separate but related events. The first layer appears to be from the massive Cascadia megaquake, while the second layer reflects movement along the nearby San Andreas Fault.
“The light bulb went on and we found that the Noyo Strait was probably recording the Cascadia earthquake, and the Cascadia ruins at a similar distance were probably recording the San Andreas earthquake,” Goldfinger says. “What would happen if Cascadia exploded, causing a weak turbidity current near the San Andreas, and then some time later the San Andreas disappeared, triggering a descent of very coarse sandy sediments? That would create this upside-down double stratigraphy.”
Potential chain reactions along the coast
Exactly how much time passes between these linked earthquakes is still unknown. In some cases, subsequent deposition may have erased evidence of gaps between events. Still, some samples suggest that a second layer formed within minutes or hours of the first layer.
If that interpretation is correct, the Cascadia earthquake could soon trigger a massive fault along the San Andreas, triggering a series of powerful tremors across much of the Pacific Coast.
Such a scenario raises serious preparedness concerns. A series of major earthquakes could strain emergency response systems and infrastructure across multiple states at the same time.
“I’m originally from the Bay Area,” Goldfinger says. “If I were in my hometown of Palo Alto and the Cascadia broke down, I would drive east. It seems to me that the risk of the San Andreas breaking down next is very high.”
