A major earthquake on the Cascadia subduction zone could trigger the San Andreas fault line, according to a study highlighted by ScienceDaily and led by Oregon State University scientist Chris Goldfinger. If both fault systems ruptured, Goldfinger said, it could become a multi-state and international emergency spanning San Francisco, Portland, Seattle and Vancouver.
Goldfinger said people are used to hearing Cascadia described as the catastrophic “Big One,” but the new work suggests that is not necessarily the worst-case scenario. He said an earthquake on either fault alone would already pull on the nation’s emergency resources, while a combined event would compress that strain into a much shorter window.
The study, published in September 2025 and titled “Unravelling the dance of earthquakes: Evidence of partial synchronization of the northern San Andreas fault and Cascadia megathrust,” is based on ancient sediment layers known as turbidites from both systems. Goldfinger and his team compared the timing and structure of the layers and found similarities that pointed to seismic synchronization between the two faults. They also used carbon dating to support the idea, identifying unique layering in the cores that lined up with the double-quake theory.
The finding traces back to a mistake in 1999, when Goldfinger was 55 miles south of Cape Mendocino in California and ended up in the San Andreas zone instead of Cascadia because of a navigational error. Drilling there produced the sediment core that eventually led to the simultaneous quake finding.
Goldfinger said there have been very few examples of synchronized quakes in the past 1,500 years, and that the most recent one was in 1700, when the ruptures were minutes to hours apart. He said the only other observed example of synchronized earthquakes was in Sumatra, where events in 2004 and 2005 happened three months apart. The broader warning is not that a paired rupture is certain, but that Cascadia and the San Andreas fault may be more connected than emergency planners have assumed.
