🌍 The “Big One” may not come alone: Cascadia and San Andreas can strike together

For decades, the nightmare scenario on the U.S. West Coast was “the Big One” — a massive Cascadia megathrust earthquake.

But research from Oregon State University suggests something even more dangerous: the Cascadia subduction zone and the northern San Andreas Fault may be partially synchronized — meaning one major quake could trigger another within minutes or hours.

🔹 Marine geologist Chris Goldfinger and his team analyzed 3,100 years of deep-sea sediment cores, looking at turbidites — underwater landslide deposits often triggered by earthquakes.

🔹 In several cores, they found unusual “doublets”: reversed sediment layers, with fine silt below and coarse sand above. The pattern suggests two large quakes happened back-to-back — not simply one quake followed by aftershocks.

🔹 Over the past 1,500 years, researchers identified three cases where Cascadia and northern San Andreas ruptures may have occurred just minutes to hours apart. The most recent was in 1700.

🔹 The discovery began almost by accident. During a 1999 research cruise, the team drifted about 55 miles off course near Cape Mendocino — exactly where the two fault systems meet — and collected a core that showed the strange upside-down layering.

🔹 A dual event would be a disaster-response nightmare: San Francisco, Portland, Seattle, and Vancouver could all face emergencies within the same compressed timeframe.

As Goldfinger put it, one major fault rupture alone could draw down the resources of the whole country. If both systems go together, it is not just the worst case — it is worse than the worst case.

The uncomfortable takeaway: the real question may not be whether the Big One will happen, but whether it comes as a single blow — or as a one-two punch.

@science