For decades, seismologists have warned that a massive rupture of the Cascadia subduction zone — the so-called “Big One” — would be the worst earthquake disaster the West Coast has ever faced. The assumption was simple: Cascadia strikes, and the region copes.
A new study is challenging that assumption. Researchers analyzing nearly 3,100 years of ancient ocean sediments have found evidence that Cascadia may not act alone — and that what lies buried in those seafloor layers is prompting scientists to reconsider the full scope of the region’s earthquake risk.
A navigational mistake that changed earthquake science
In 1999, Chris Goldfinger and his team were collecting sediment cores from the Cascadia subduction zone off Oregon and northern California. A navigational error sent their vessel roughly 55 miles off course — placing them squarely in San Andreas fault territory near Cape Mendocino, California.
Rather than turn back, the team drilled a core at the unplanned location.
That accidental sample sat in storage for years. When researchers finally examined it closely, it contained something that would reshape how scientists think about earthquake risk along the entire West Coast.
Reading 3,100 years of earthquake history from the ocean floor
The study analyzed deep-sea sediment cores capturing roughly 3,100 years of geologic history. These cylindrical samples preserve layers of material that accumulated on the ocean floor over centuries — a slow-motion archive of the Earth’s activity. The key evidence came from deposits called turbidites, which form when underwater landslides send sediment rushing downhill, often triggered by powerful earthquakes.
By comparing turbidite timing and structure across cores from both fault zones, researchers identified striking patterns. Three events in the past 1,500 years showed the Cascadia subduction zone and the northern San Andreas appearing to rupture within minutes to hours of each other. The most recent occurred around 1700.
Upside-down sediment: the clue hiding in plain sight
A normal turbidite layer follows a predictable logic: coarser material sinks first and settles at the bottom, with finer sediment drifting down above it. The accidental Cape Mendocino core broke that rule entirely. Fine, silty material appeared at the bottom. Coarse, sandy sediment sat on top.
Researchers interpreted this inverted structure as evidence of two separate seismic events in rapid succession — the bottom layer a deposit from a major Cascadia rupture, the upper layer reflecting subsequent movement on the nearby San Andreas. To confirm this, the team used radiocarbon dating on turbidite deposits from that core and from others collected north and south of Cape Mendocino. The analysis held up across multiple samples. Researchers call these paired layers “doublets,” arguing that two separate fault systems — not aftershocks or a single event — best explain how these structures formed.
The 1700 earthquake and a rare global parallel
The most recent doublet in the record dates to around 1700 — the same period as the last known great Cascadia megathrust earthquake. That event is already well-documented in Japanese tsunami records and in Indigenous oral histories along the Pacific Northwest coast. The sediment record now suggests it may have been accompanied by a near-simultaneous rupture on the northern San Andreas.
Documented fault synchronization is extraordinarily rare. The only previously observed example involved two major Sumatra earthquakes that struck three months apart in 2004 and 2005. A potential parallel preserved across multiple North American sediment cores makes the Cape Mendocino findings particularly significant. The location itself matters, too — Cape Mendocino sits near where the two fault systems converge, making it a critical zone for understanding how they might interact.
What a double rupture would mean for West Coast cities
Lead researcher Goldfinger put the stakes plainly. “We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it,” he said. A closely timed double rupture would leave San Francisco, Portland, Seattle, and Vancouver all managing severe emergencies within the same compressed window.
None of this suggests that every large earthquake on one fault will trigger the other. The sediment record simply shows that paired ruptures have happened before — and that the possibility deserves serious attention. Emergency preparedness frameworks have largely been built around single-fault, single-city scenarios, and this research suggests that planning may need to expand considerably.
As more sediment cores are analyzed and dating techniques improve, scientists may be able to better define how often these doublets occur and what conditions make them more likely. That knowledge won’t eliminate the risk. It could, however, give emergency planners the framework they need to prepare for a worst-case scenario that’s no longer purely theoretical.
The complete version of the study can be found here: C. Goldfinger, J. Beeson, B. Black, A. Vizcaino, C.H. Nelson, A. Morey, J.R. Patton, J. Gutiérrez-Pastor, C. Romsos, M.D. Walzcak. Unravelling the dance of earthquakes: Evidence of partial synchronization of the northern San Andreas fault and Cascadia megathrust. Geosphere, 2025; 21 (6): 1132 DOI: 10.1130/GES02857.1
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