Scientists studying Northern California have uncovered previously hidden fault lines, raising alarms that seismic risk in the region may be underestimated.
For decades, the Mendocino triple junction was believed to be where three tectonic plates meet: the San Andreas Fault ending in the north, the Cascadia Subduction Zone in the south, and the Mendocino Fault in the east.
Because three major fault systems converge there, the area is one of the most active earthquake zones in the US and could produce a magnitude 8.0 quake.
Now, researchers have discovered that the junction actually contains at least five tectonic plates or fragments deep below the surface, making the region far more complex than previously thought.
That means there may be an unaccounted earthquake hazard in the area, and current models could be underestimating the true risk.
Because the junction lies off the coast and influences both the San Andreas and Cascadia systems, revisions to the model could affect risk calculations for millions of people along the West Coast.
Scientists compared the new findings to an iceberg, where most of the structure remains hidden below the surface.
Geophysicist Amanda Thomas of the University of California, Davis, said: “If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard.”
This discovery shows that the underground fault structure is more complicated than scientists thought.
If the models don’t include these hidden faults, they may underestimate how much stress is building up underground.
That means a larger earthquake could happen unexpectedly, because the hidden faults could suddenly release energy in ways the old models didn’t predict.
The team suspected something more complex was happening at the Mendocino Triple Junction, as a large magnitude 7.2 earthquake in 1992 occurred at a much shallower depth than expected.
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