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Uncovering hidden Structures Beneath Northern California: New Insights into earthquake Hazards
Recent research utilizing a dense network of seismometers is revealing a more complex geological structure beneath the Mendocino Triple Junction off the coast of Northern California. This region,where the San Andreas Fault and the Cascadia Subduction Zone meet,is prone to powerful earthquakes,and understanding its underlying processes is crucial for accurate seismic hazard assessment. Published in Science on January 15, 2026, the findings highlight the existence of previously unknown plate fragments and a more intricate interaction between tectonic plates than previously understood.
The Mendocino Triple Junction: A Tectonic Crossroads
the Mendocino Triple Junction, located offshore from Humboldt County, California, is a convergence zone where three major tectonic plates interact. south of the junction, the Pacific Plate moves northwest alongside the North American Plate, creating the well-known San Andreas Fault. North of the junction, the Gorda Plate (part of the larger Juan de Fuca Plate) subducts – slides – beneath the North American Plate, descending into the earth’s mantle. The Cascadia Subduction Zone is known for its potential to generate magnitude 9.0+ earthquakes.
Scientists have long recognized the complexity of this region. A magnitude 7.2 earthquake in 1992, occurring at an unexpectedly shallow depth, initially hinted at hidden structures. Traditional models assumed a relatively simple plate configuration, but this event suggested a more nuanced reality.
Mapping the Hidden structure with Low-Frequency Earthquakes
Researchers, led by David Shelly of the U.S. Geological Survey (USGS), employed a network of seismometers to detect extremely small, “low-frequency” earthquakes.these tiny tremors, thousands of times weaker than those felt by humans, occur as tectonic plates slowly slide against each other. By analyzing these events, scientists can create a detailed picture of the subsurface structure, much like mapping an iceberg by observing what’s visible above the water. David Shelly’s USGS profile provides further details on his work.
The team also examined how these small earthquakes respond to tidal forces. Just as the moon and sun influence ocean tides, they also exert subtle stress on tectonic plates. An increase in the frequency of these small earthquakes when tidal forces align with plate movement confirms the accuracy of the underground model.
Five Moving Pieces: Discovering Hidden Plate Fragments
The research revealed that the region isn’t defined by just three major plates, but by five distinct moving pieces. Two of these pieces are hidden deep beneath the surface.
- Subducting North American Plate Fragment: At the southern end of the Cascadia Subduction Zone, a portion of the North American Plate is breaking away and being dragged downward along with the Gorda Plate.
- The Pioneer Fragment: South of the triple junction, a mass of rock called the Pioneer fragment is being pulled beneath the North American Plate as the Pacific Plate moves northward. This fragment, a remnant of
