When the massive subduction zone off the Pacific Northwest finally shifts, the result can be catastrophic. A magnitude 9 or greater earthquake would unleash intense ground shaking, tsunamis, and landslides. Now a study published in Geosphere suggests this long-feared "really big one" may not act alone. It could also set off a powerful earthquake along California's San Andreas Fault.
The discovery came from an unexpected navigation error. In 1999, a research cruise set out to study ancient Cascadia earthquakes by collecting sediment cores from the seafloor. But a graduate student entered the wrong latitude overnight, sending the ship about 90 kilometers south of its intended position — into an area influenced by the San Andreas Fault. The core sample they collected from Noyo Canyon near Fort Bragg held a surprising record.
The sediment contained repeated double-layer patterns called turbidites — deposits formed by underwater landslides. Radiocarbon dating revealed that many of these paired deposits at sites north and south of Cape Mendocino formed at roughly the same time. The researchers concluded that each pair likely records two separate but related events: a major Cascadia megathrust earthquake followed by movement along the nearby San Andreas Fault.
Dr. Chris Goldfinger, a paleoseismologist at Oregon State University and lead author, described the scenario: "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." The idea that these two fault systems might be linked changes how scientists assess earthquake risk along the entire West Coast.
The western edge of the United States sits atop a complex system of tectonic boundaries. North of Cape Mendocino, the Juan de Fuca plate is 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. If these two systems were to rupture close together, it would significantly expand the geographic scope of a major seismic event, potentially affecting nearly the entire U.S. West Coast within hours.
Knowledge takeaway: seafloor sediment cores spanning 3,000 years reveal paired turbidite layers that suggest Cascadia megathrust earthquakes have historically triggered San Andreas Fault ruptures, a finding that could transform earthquake hazard models for the Pacific Coast.