What looked like a simple union of plates in California hides a much more chaotic system, and these small earthquakes have just revealed it

The earthquakes that most people notice are the ones that shake dishes and set off phone alerts. But a new study suggests that the smallest earthquakes, the ones you never feel, may be some of the most useful for understanding earthquake hazards in Northern California.

By tracking swarms of low-frequency deep-sea earthquakes, scientists found signs of hidden, shifting structures beneath the Mendocino triple junction, where the San Andreas fault system meets the Cascadia subduction zone.

The research points to at least five moving pieces at depth, not just the three tectonic plates that usually appear on maps, and this could change how seismic risk is estimated in this region.

Triple link

Mendocino Triple Junction is located off the coast of Humboldt County, at the meeting point of three major tectonic plates. Tectonic plates are giant slabs of the Earth’s outer shell, and they move slowly all the time, even when the Earth feels calm.

To the south, the Pacific plate slides nearly northwestward overtaking the North American plate, helping to form the San Andreas fault system. To the north, the Gorda plate moves northeast and dips beneath North America.

This sinking is a subduction process, and it occurs when one plate sinks under another and descends into the hot interior of the Earth. Subduction zones can store pressure for a long time, then release it in huge earthquakes, which is why this stretch of coastline has scientists on alert.

Small earthquakes, big clues

The new work focuses on low-frequency earthquakes, which are small events with a softer signal and less intense than traditional earthquakes. They often arrive in groups called swarms, and these swarms can act like breadcrumbs tracking where rocks are sliding far beneath the surface.

To find them, the team used a network of seismometers across the Pacific Northwest. Seismographs are sensitive instruments that can pick up motions thousands of times weaker than anything a person can feel.

Co-author Robert Skomal helped analyze how these tiny signals line up with the movement of the plates. An official summary of the study is available at the USGS, along with a technical description of how to interpret the signals.

Tectonic map highlights the Mendocino Triple Junction, where three plates meet and hidden fault systems are now being discovered. Sun and moon test

Researchers did not just count earthquakes and draw lines on the map. They also checked their model using tidal forces, the same gravitational force from the Sun and Moon that drives ocean tides.

These forces also stretch and compress solid rocks by small amounts, including buried plate boundaries. When this diameter lines up with the direction in which the plate tends to move, the study found that more low-frequency earthquakes tend to occur.

Amanda Thomas, a professor of Earth and planetary sciences at the University of California, Davis, outlined the practical reason for doing this work: “If we don’t understand the underlying tectonic processes, it’s difficult to predict seismic hazard.”

It’s a reminder that even small earthquakes can influence decisions that shape building design and emergency planning.

Hidden pieces in the rear

So, what did the team model add that wasn’t removed from the old maps? This suggests that two important moving pieces are buried out of sight, meaning they cannot be traced through surface cracks or obvious terrain.

David Shelley, a research geophysicist at the USGS, compared the problem to an iceberg, saying: “You can see a little bit on the surface, but you have to know what the formation is underneath.” Low-frequency earthquakes are part of the way scientists do this, because they tend to occur in places where rocks are sliding, rubbing, or overlapping.

One of the buried features appears to be a piece that broke off from the North American plate near the southern end of the Cascadia subduction zone and is being pulled down with the subducting Gorda plate. Another piece is the Pioneer Fragment, a mass of rock being pulled under North America as the Pacific Plate moves north, along a boundary described as nearly horizontal.

Why does 1992 still matter?

The Mendocino region is not just a theoretical problem. In 1992, a 7.2 magnitude earthquake struck this general area, and it occurred at a shallower depth than many scientists expected, a mismatch that has lingered in the background of hazard discussions.

The new model offers a possible explanation, because it places the subducting surface shallower than previously thought in this part of the system. In simple terms, if the sliding boundaries are closer to the surface, they can change where harmful vibration can start.

Katherine Materna, a geophysicist at the University of Colorado Boulder, summed up the surprise: “It was assumed that the faults follow the leading edge of the subducting slab, but this example deviates from that. It appears that the plate boundary is not where we thought it was.” This is a big statement, because wrong limits can mean a misreading of risk.

What changes in risk estimates?

This study does not predict when the next big earthquake will occur. Instead, it sharpens the deep engineering on which hazard models rely, using small earthquakes as markers of underground movement.

It also suggests that an older portion of a plate from the ancient Farallon system may be moving northward beneath North America, widening the deep plate interface fault, the surface where the two plates press together and slide.

This idea challenges some explanations for how the triple junction opened at depth, including the concept of a plate window, a gap left when a subducting plate breaks.

Scientists have grappled with the behavior of triple junctions for decades, starting with a foundational 1969 paper on how these triple boundaries, available in nature, evolve, and a subsequent synthesis of the Mendocino region in 2004 in the Annual Review of Earth and Planetary Sciences.

The project was funded by a grant from the National Science Foundation, and the next step is to test this deep map against future data, the kind that quietly accumulates as daily life goes on.

The main study was published in the journal Science.

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