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.

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. 

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.'