Three giant tectonic plates meet at the Mendocino Triple Junction off the coast of northern California, and a new study suggests the underlying geology is much more complex than current models suggest.
US researchers have conducted a new analysis of small, low-frequency earthquakes recorded by seismometers in the Pacific Northwest, uncovering previously hidden faults.
Their results show that the triple junction does not consist of just three plates, but actually consists of five moving parts.
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This means earthquake prediction models may need to be updated to give experts a better estimate of when the next big earthquake will occur. Researchers say the discovery is similar to analyzing the underwater portion of an iceberg.
“You can see something at the surface, but you have to figure out what the structure is underneath,” said David Shirley, a seismologist with the U.S. Geological Survey’s Geological Hazards Center.
An updated model of the Mendocino triple junction. (Shirley et al., science2026)
In addition to studying data from seismometers, which pick up subtle ground vibrations from very small earthquakes that cannot be felt at the surface, the team also validated their field records using a tidal sensitivity model.
The daily push and pull of the tides creates tiny stresses on the underlying rocks. By modeling these stresses to test the rock’s response, scientists can check whether their interpretation of small, low-frequency earthquakes is correct, which in this case it was.
A large chunk of the North American plate has broken off and is being pulled down along with Golda (Juan de Fuca) Researchers determined. The team also confirmed the previously theorized existence of a pioneer fragment – a piece of older rock that was dragged beneath the North American plate.
The North American Plate, the Gorda Plate, and the Pacific Plate form the Mendocino Triple Junction, with the Gorda Plate being subducted (pushed) beneath the North American Plate and absorbed by the Earth’s mantle. Crucially, the subduction surface is not as deep as previously thought.
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This study changes the most likely locations of plate boundaries. The model is supported by a 1992 magnitude 7.2 earthquake in California, which originated at a much shallower depth than contemporary models predict.
“People have always thought that faults were along the leading edge of the subducting slab, but this example is different,” said Kathryn Materna, a structural geodesist at the University of Colorado, Boulder.
“The plate boundaries don’t seem to be where we thought they were.”
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Accuracy is crucial in predicting earthquakes, which is where this study is most useful. Both the San Andreas Fault (where the North American and Pacific Plate meet) and the Cascadia Subduction Zone (where the Gorda and North American Plate meet) can produce devastating earthquakes.
Tectonic faults and earthquake zones in California and along the western U.S. coast have many moving parts to consider, and scientists are working to gain the most complete understanding possible so we can be as prepared as possible for what might happen next on the ground.
“It’s difficult to predict earthquake hazards if we don’t understand the underlying tectonic processes,” said geophysicist Amanda Thomas of the University of California, Davis.
The study was published in science.
