What did scientists discover?

Scientists have identified previously hidden fault lines off California’s north coast, raising fresh concerns that earthquake risk in the region may be significantly underestimated, according to a new seismic study published in the journal Science.

The findings suggest that one of the most seismically active regions in the United States—the Mendocino Triple Junction—is far more complex than scientists once believed.

“If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard,” said study co-author Amanda Thomas, a professor of earth and planetary sciences at the University of California, Davis.

Where is the Mendocino Triple Junction, and why is it dangerous?

The Mendocino Triple Junction sits offshore along California’s far north coast, near Humboldt County. It is traditionally described as the meeting point of three major tectonic systems:

• The San Andreas Fault, ending in Northern California
• The Cascadia Subduction Zone, extending north toward Oregon and Washington
• The Mendocino Fault, running offshore into the Pacific

Each of these systems is capable of producing powerful earthquakes on its own. Where they intersect, stress builds quickly—and unpredictably.

The region was responsible for the magnitude 7.2 earthquake that struck Humboldt County in 1992, causing widespread damage and underscoring the area’s seismic volatility.

Why this study changes what scientists thought they knew

The new research challenges the long-standing idea that the Mendocino Triple Junction involves only three tectonic plates.

Instead, scientists found evidence of five distinct moving plates, two of which are hidden deep underground and invisible at the surface.

That means the area is not a triple junction at all—but a far more complicated tectonic puzzle.

“In some ways, what we see at the surface is just the tip of the iceberg,” said lead author David Shelly, a geophysicist at the US Geological Survey’s Geologic Hazards Center in Colorado.

How researchers uncovered the hidden plates

The discovery was made using a dense network of seismometers across the Pacific Northwest. These instruments detected low-frequency earthquakes—slow, subtle tremors that occur deep underground where tectonic plates grind against each other.

Unlike traditional earthquakes, these tremors:

• They are too weak to be felt at the surface
• It occurs deep within the Earth
• Reveal how plates move and interact below ground

The findings were further confirmed using tidal-sensitivity models, which analyze how Earth’s tides influence seismic signals.

The newly identified tectonic threats

The study highlights two major hidden components reshaping scientists’ understanding of the region.

A broken piece of the North American plate

Researchers found that a fragment of the North American plate has broken off and is being dragged downward alongside the Gorda plate at the southern edge of the Cascadia Subduction Zone.

This process increases internal deformation, creating new fault zones capable of generating earthquakes.

The long-lost Pioneer fragment

The study also confirms the existence of the Pioneer fragment, a remnant of the ancient Farallon plate—a massive tectonic plate that once dominated the western edge of North America.

Although most of the Farallon Plate has disappeared beneath the continent, this surviving fragment is now being subducted under the North American plate, adding yet another layer of tectonic complexity.

Why earthquake depths matter

One of the most striking findings is that the subducting plates are much shallower than previously assumed.

This could explain why the 1992 Humboldt County earthquake originated closer to the surface than models predicted—making it more damaging.

“It had been assumed that faults follow the leading edge of the subducting slab,” said Kathryn Materna, a tectonic geodesist at the University of Colorado Boulder. “But this example deviates from that. The plate boundary seems not to be where we thought it was.”

Shallower earthquakes tend to cause stronger ground shaking, increasing the risk to communities even when magnitudes are similar.

Why this matters for people living in the region

Millions of people live in Northern California and the Pacific Northwest, and critical infrastructure—including highways, bridges, ports, and energy systems—runs through or near this tectonic zone.

If seismic hazard models are based on incomplete plate maps, they may underestimate:

• The likelihood of large earthquakes
• Where strong shaking could occur
• Which faults pose the greatest threat

This has implications for building codes, emergency planning, and insurance risk assessments.

A broader warning for North America

The findings also echo recent warnings from other researchers that overlooked or poorly understood fault lines could trigger large earthquakes across parts of western North America, from California into Canada.

As seismic monitoring improves, scientists are increasingly discovering that Earth’s tectonic systems are more fragmented—and more active—than surface maps suggest.

What comes next

Researchers say the next step is to integrate the newly identified plates into updated seismic hazard models.

That process could take years, but the implications are immediate: understanding what lies beneath the surface is essential for preparing for what may come next.

As Thomas put it, predicting earthquakes remains incredibly difficult—but understanding the architecture of Earth’s crust is a necessary first step.

TL;DR

• Scientists have discovered hidden fault lines off California’s north coast
• The Mendocino Triple Junction actually involves five tectonic plates, not three
• Two of the plates are buried deep underground and were previously unknown
• Shallower subduction could mean stronger, more dangerous earthquakes
• The findings suggest earthquake risk in the region may be underestimated