Scientists have identified a concerning scenario where a powerful earthquake on the Cascadia Subduction Zone could trigger a subsequent rupture along California's San Andreas Fault. The research, which examined sediment layers from seafloor cores off the Northern California coast and further north along Cascadia, suggests that major earthquakes along these fault systems may have occurred in rapid succession in the past.
These sediment layers, deposited by underwater landslides, appeared in unusual pairs. Researchers theorize that a massive Cascadia megaquake could have initially shaken the seafloor near the San Andreas Fault, causing an initial deposit. A second, coarser layer might have formed when the San Andreas Fault ruptured minutes or hours later. This sequence raises the alarming possibility of one catastrophic earthquake triggering another, potentially affecting millions of people along the U.S. West Coast.
The Cascadia Subduction Zone extends from Northern California through Oregon and Washington. When it eventually ruptures, experts warn it could produce a magnitude 9 or greater earthquake, unleashing intense ground shaking, tsunamis, and landslides. The U.S. Geological Survey estimates a 10 to 15 percent probability of such an event occurring in the next 50 years.
Potential Cascading Effects
If a Cascadia megaquake were followed by a San Andreas rupture, the consequences could be amplified. A magnitude-9 Cascadia quake could cause widespread destruction in cities like Seattle and Portland, leading to building collapses, severed infrastructure, and coastal tsunamis. A subsequent San Andreas event could then subject San Francisco and other parts of California to a second wave of destructive shaking, fires, and transportation disruptions. Such a scenario would simultaneously strain emergency services across a vast area of the western United States.
Understanding the Tectonic Setting
The western edge of the U.S. is characterized by complex tectonic boundaries. North of Cape Mendocino, California, the Juan de Fuca plate is being subducted beneath the North American plate, forming the Cascadia megathrust. South of this region, the Pacific and North American plates interact along the San Andreas Fault, a boundary known for generating major seismic events, including the devastating 1906 San Francisco earthquake.
The study's lead author, Dr. Chris Goldfinger, a paleoseismologist at Oregon State University, explained that the unusual sediment deposits found in Noyo Canyon featured thick, sandy layers paired with finer-grained material. This "upside-down doublet stratigraphy" led the research team to conclude that two distinct earthquake events likely contributed to the deposits.
While the exact time separation between these suspected ancient earthquakes remains unclear, some evidence suggests the San Andreas rupture could have followed the Cascadia megaquake within minutes or hours. If this interpretation holds true, it significantly alters how scientists assess earthquake risks along the West Coast, highlighting the potential for interconnected seismic activity.