Researchers examining Northern California have identified previously undetected fault lines, suggesting that the seismic threat in the area might be greater than previously assessed.

For many years, the Mendocino triple junction was recognized as the meeting point of three tectonic plates: where the northern reach of the San Andreas Fault, the southern end of the Cascadia Subduction Zone, and the eastern stretch of the Mendocino Fault intersect.

Due to the convergence of these significant fault systems, this zone ranks among the most seismically active in the United States, with the potential to generate earthquakes of magnitude 8.0.

Recent findings from researchers reveal that the 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 implies that the earthquake hazard in this region might be greater than accounted for, suggesting that existing models could be underestimating the actual 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.”

Scientists once believed the Mendocino triple junction was simply where three major fault systems ended: the San Andreas Fault (PICTURED), the Cascadia Subduction Zone, and the Mendocino Fault 

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.

First author David Shelly of the US Geological Survey Geologic Hazards Center in Golden, Colorado, said in a statement: ‘You can see a bit at the surface, but you have to figure out what the configuration is underneath.’

Using a network of seismometers in the Pacific Northwest, the team tracked tiny ‘low-frequency’ earthquakes deep underground where tectonic plates grind and slide past each other. 

These events are far too weak to be felt on the surface, thousands of times smaller than a detectable quake.

To verify their results, the team compared the seismic activity to tidal forces. 

Because three major fault systems converge there, the area is one of the most active earthquake zones in the US and is capable of producing a magnitude 8.0 quake

The sun and moon’s gravity pulls on the Earth’s crust just as it does on the oceans. 

When those forces align with the direction a plate is moving, the researchers observed a rise in small quakes, Thomas said, confirming their model.

The new model reveals 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.

At the southern end of the Cascadia subduction zone, researchers say a chunk of the North American plate has snapped off and is being pulled down along with the Gorda plate as it sinks beneath the continent.

Further south, the Pacific plate is dragging a mass of rock called the Pioneer fragment northward under North America. The fault between the Pioneer fragment and the North American plate runs almost horizontally and is completely invisible from above ground.

The Pioneer fragment is a remnant of the ancient Farallon plate, which once ran along California’s coastline before mostly disappearing.

The updated model also explains the unusually shallow depth of the 1992 earthquake, because the subducting surface lies much higher than previously thought, Materna said.

‘It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that,’ Materna said. ‘The plate boundary seems not to be where we thought it was.’