Researchers Uncover Hidden Fault Lines at Mendocino Triple Junction

New findings suggest seismic threat in Northern California may be greater than previously assessed.

Jan. 27, 2026 at 2:07pm

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Researchers examining Northern California have identified previously undetected fault lines at the Mendocino triple junction, suggesting that the seismic threat in the area might be greater than previously assessed. The junction, where the San Andreas Fault, Cascadia Subduction Zone, and Mendocino Fault intersect, is now believed to consist of at least five tectonic plates or fragments lying deep beneath the Earth's surface, rather than just three as previously thought.

Why it matters

The Mendocino triple junction is one of the most seismically active regions in the United States, with the potential to generate earthquakes of magnitude 8.0. This new discovery of hidden fault lines implies that existing models may be underestimating the actual earthquake risk, which could have major implications for millions of people living along the West Coast.

The details

Recent findings from researchers reveal that the Mendocino triple junction is more intricate than once believed, consisting of at least five tectonic plates or fragments lying deep beneath the Earth's surface. This discovery suggests that the earthquake hazard in this region might be greater than accounted for, as the hidden faults could suddenly release energy in ways the old models didn't predict. The team tracked tiny 'low-frequency' earthquakes deep underground to verify their results and found that the Mendocino triple junction is not just a three-plate intersection, but a far more complicated system of five moving pieces, two of which are hidden deep below the Earth's surface.

  • In 1992, a large magnitude 7.2 earthquake occurred at a much shallower depth than expected in the Mendocino triple junction region.
  • The new research findings were published on January 27, 2026.

The players

Amanda Thomas

A geophysicist at the University of California, Davis.

David Shelly

The first author of the study and a researcher at the US Geological Survey Geologic Hazards Center in Golden, Colorado.

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What they’re saying

“If we don't understand the underlying tectonic processes, it's hard to predict the seismic hazard.”

— Amanda Thomas, Geophysicist

“You can see a bit at the surface, but you have to figure out what the configuration is underneath.”

— David Shelly, Researcher

What’s next

Scientists plan to continue studying the complex fault structure at the Mendocino triple junction to better understand the seismic risks and improve earthquake hazard models for the region.

The takeaway

This discovery of hidden fault lines at the Mendocino triple junction highlights the importance of using advanced techniques to fully map the underground tectonic structure in seismically active regions, as existing models may be underestimating the true earthquake risk to millions of people living along the West Coast.