Chris Goldfinger, a marine geologist at Oregon State University, said a Cascadia subduction zone quake could set off the san andreas fault. He said that would turn one major earthquake into a multi-state and international emergency spanning San Francisco, Portland, Seattle and Vancouver.
Goldfinger said, “We’re used to hearing the ‘Big One’ – Cascadia – being this catastrophic huge thing.” He added, “It turns out it’s not the worst-case scenario.”
Oregon State sediment cores
The study examined ancient sediment layers called turbidites from both fault systems. Goldfinger and his team found similarities in timing and structure, and used carbon dating to identify a layering pattern that fit the double-earthquake theory.
Goldfinger said there were very few instances of simultaneous quakes in the last 1,500 years. He said the most recent simultaneous quakes were in 1700 and were just minutes to hours apart.
1999 Cape Mendocino finding
Goldfinger said the work traces back to 1999, when a navigational error took him 55 miles south of Cape Mendocino in California and into the San Andreas zone instead of the Cascadia subduction zone. Drilling in that area led to a sediment core that helped produce the simultaneous quake finding.
ScienceDaily highlighted the Oregon State University study this week. The original study, titled “Unravelling the dance of earthquakes: Evidence of partial synchronization of the northern San Andreas fault and Cascadia megathrust,” was published in September 2025.
West Coast emergency response
Goldfinger said an earthquake on one of the faults alone would draw down the resources of the whole country to respond. He said the combined rupture could leave San Francisco, Portland, Seattle and Vancouver in an emergency situation in a compressed timeframe.
The only observed example of a synchronized earthquake has been seen in Sumatra, with events three months apart in 2004 and 2005. For people along the West Coast, the new finding points to a broader emergency scenario than a single-fault rupture, with response needs that could spread across several cities at once.
