Scientists have uncovered startling evidence of a catastrophic earthquake chain reaction that could potentially affect millions of people along the US West Coast.
The chilling scenario involves the Cascadia Subduction Zone, which stretches from Northern California through Oregon and Washington, and California’s notorious San Andreas Fault.
Researchers examined sediment layers in seafloor cores from Noyo Canyon, off Fort Bragg in Northern California, and farther north along the Cascadia.
The deposits were left by underwater landslides triggered by powerful earthquakes over the past 3,000 years.
Many of the layers appeared in unusual pairs and dated to approximately the same periods, suggesting two major earthquakes may have struck in rapid succession.
The team believes a Cascadia megaquake first shook the seafloor near the San Andreas, producing the initial deposit. A second, coarser layer may have formed when the San Andreas ruptured minutes or hours later, raising the terrifying possibility that one megaquake could trigger another.
Researchers warned that when the massive subduction zone off the Pacific Northwest finally shifts, the results can be catastrophic. A magnitude 9 or greater earthquake would unleash intense ground shaking, along with tsunamis and landslides that amplify the destruction.
The US Geological Survey estimates a 10 to 15 percent chance of a magnitude 9 earthquake occurring on the Cascadia Subduction Zone in the next 50 years.
The chilling scenario involves the Cascadia Subduction Zone, which stretches from Northern California through Oregon and Washington, and California’s notorious San Andreas Fault
Many of the layers appeared in unusual pairs and dated to approximately the same periods, suggesting two major earthquakes may have struck in rapid succession
The study was originally published in the journal Geosphere in October 2025, but its findings resurfaced this month after the Geological Society of America highlighted the research in a new release on ScienceDaily this month.
Lead author Dr Chris Goldfinger, a paleoseismologist at Oregon State University, said: ‘It’s kind of hard to exaggerate what a M9 earthquake would be like in the Pacific Northwest.
‘And so the possibility that a San Andreas earthquake would follow, it’s movie territory.’
A magnitude-9 Cascadia quake could unleash violent shaking across Seattle and Portland, collapsing buildings, severing highways and bridges, and triggering a tsunami along low-lying coastal communities.
If the San Andreas ruptured soon afterward, San Francisco and other parts of California could then face a second wave of destructive shaking, fires, power failures and transport disruption, stretching emergency services across nearly the entire West Coast at once.
The western edge of the US sits atop a complex system of tectonic boundaries.
North of Cape Mendocino, California, the Juan de Fuca plate is being forced beneath the North American plate, forming the Cascadia megathrust.
South of that point, the Pacific and North American plates slide past one another along the San Andreas Fault, periodically producing major earthquakes such as the devastating 1906 San Francisco event.
If the San Andreas (pictured) ruptured soon afterward, San Francisco and other parts of California could then face a second wave of destructive shaking, fires, power failures and transport disruption
Experts say that if these two systems were to rupture close together, it would significantly change how scientists assess earthquake risk along the West Coast.
The core samples collected from Noyo Canyon featured ancient repeated layers called turbidites, which form when underwater landslides, known as turbidity currents, rush down the seafloor and deposit material.
Typically, these layers show a clear structure, with heavier grains settling first and finer particles resting on top.
But Goldfinger and his team found that many of the deposits appeared in pairs, with core samples from Noyo Canyon and Cascadia showing the strange double-layer pattern.
‘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,’ Goldfinger said.
‘And we were just scratching our heads.’
The team then concluded that each pair of deposits likely recorded two separate but unrelated earthquake events.
The first layer appeared to come from a major Cascadia megathrust earthquake, while the second reflected movement along the nearby San Andreas Fault.
‘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,’ said 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.’
Scientists still do not know exactly how much time separated the suspected earthquakes because later deposits may have erased evidence of the gap.
However, several samples suggest the San Andreas rupture may have followed the Cascadia megaquake within minutes or hours.
If the team’s interpretation is correct, the US West Coast could be struck by two catastrophic earthquakes in rapid succession, leaving emergency crews battling widespread destruction across several states at once.
