Hidden Faults Uncovered at Mendocino Triple Junction Raise US Quake Risks
New research reveals five tectonic fragments instead of three at key California hotspot.
Scientists have discovered previously unknown fault structures hidden beneath Northern California by studying swarms of tiny, imperceptible earthquakes. The breakthrough, detailed in a study published January 15, 2026, in the journal Science, reshapes our understanding of the Mendocino Triple Junction—one of the most seismically complex and hazardous regions in the United States.
Located offshore near Humboldt County, the Mendocino Triple Junction marks the rare meeting point of three major tectonic plates: the Pacific, North American, and Juan de Fuca. It connects the notorious San Andreas Fault system to the south with the massive Cascadia Subduction Zone to the north. For decades, the area’s subsurface geometry has puzzled researchers because surface observations failed to align with deeper tectonic behavior.
A collaborative team from the US Geological Survey, University of California Davis, and University of Colorado Boulder analyzed low-frequency microearthquakes—events too small to be felt but detectable by sensitive instruments. These subtle signals acted as natural tracers, illuminating how plates interact underground. The findings revealed a far more intricate picture: rather than three plates converging, the region contains five distinct moving pieces.
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Two significant fragments lie buried deep below the surface. One broken section of the North American plate is being subducted and dragged downward, while the so-called Pioneer fragment—a detached piece of ancient crust—is being pulled beneath California by the descending Pacific plate. These newly identified structures explain anomalies in past seismic activity, including why the powerful 1992 magnitude 7.2 Cape Mendocino earthquake ruptured much shallower and closer to land than models had predicted.
Lead researchers emphasized the importance of this refined tectonic model. “If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard,” said co-author Amanda Thomas, a professor of earth and planetary sciences at UC Davis. Accurate mapping of these hidden faults is vital for updating earthquake hazard assessments in a region capable of generating devastating events, including potential megathrust quakes along Cascadia.
The discovery underscores how advanced seismic monitoring continues to uncover surprises even in well-studied areas, offering critical insights that could improve preparedness for millions living along the US West Coast.
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