Discovering new faults in the area of ​​the devastating earthquake

Earthquakes are usually sudden and unexpected, but deep within the Earth, the slow and steady movement of tectonic plates forms areas where strong seismic activity is likely to occur.

One of the most complex earthquake zones in the United States is located off the coast of Northern California.

This is where the San Andreas Fault meets the Cascadia Subduction Zone. Scientists now say this region is much more complex than previous maps suggested.

A new study has shown how tracking very small earthquakes can reveal movements hidden deep underground.

The results help explain past earthquakes and may improve estimates of future earthquake hazards.

Tectonic plates and collisions

Mendocino Triple Junction is located offshore near Humboldt County, California. This spot represents the meeting point of three large tectonic plates that form the Earth’s crust.

South of this junction, the Pacific plate slides to the northwest, overtaking the North American plate, creating the San Andreas Fault.

North of this point, the Gorda plate moves northeast and sinks beneath the North American plate in a process called subduction.

For many years, scientists believed that plate boundaries follow clear, simple lines. Real data tells a different story.

A powerful 7.2 magnitude earthquake occurred in 1992 at a depth much lower than expected. This event raised serious questions about what lies beneath the surface.

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

Small earthquakes reveal movement

To look deeper into this problem, scientists focused on very small earthquakes called low-frequency earthquakes.

These events are too weak for people to feel them. Special instruments called seismometers detect these signals deep within the Earth, where tectonic plates are pressing, rubbing, and sliding.

The study’s first author, David Shelley of the US Geological Survey’s Geological Hazards Center in Golden, Colorado, explained how this works.

“You can see a little bit on the surface, but you have to know what the formation is underneath,” he said.

Researchers studied thousands of small earthquakes across the Pacific Northwest to map plate movements hidden from view.

Low-frequency earthquakes track the movement of part of a captured slab. Proposed plate tectonic earthquake geometry at the Mendocino triple junction. Credit: Science. Click on the image to enlarge. The effect of the moon and sun on faults

The team also used an unusual tool to confirm the model. Gravitational forces from the Sun and Moon affect the Earth’s tectonic plates, similar to the effect of tides on ocean water.

When this drag matches the normal direction of plate movement, more small earthquakes occur.

By comparing the timing of the earthquake with tidal forces, the scientists tested whether the plate movements in the model made sense. The results matched expectations, giving strong support for the new structure.

This approach allowed the team to see how deep plate pieces respond to external forces, even when those pieces remain invisible from the surface.

More tectonic plate cuts than expected

The most surprising result came from the new panel model. Instead of three panels meeting at the intersection, the study showed five moving pieces.

Two of these pieces are located deep underground and cannot be seen on the surface.

At the southern end of the Cascadia subduction zone, part of the North American plate has broken off. This piece moves downward with the Gorda plate as it sinks into the mantle.

South of the junction, the Pacific Plate is dragging another hidden piece called the Pioneer Plate beneath North America. The Pioneer piece once belonged to the ancient Farallon plate, which no longer exists as a complete plate.

The boundary between this piece and the North American plate lies almost flat, making it impossible to detect from surface observations.

This hidden structure helps explain why the 1992 earthquake occurred much closer to the surface than expected.

Rethinking earthquake risks

The researchers note that the subducting surface lies much higher than previous models suggested.

“It was assumed that the faults follow the leading edge of the subducting slab, but this example deviates from that,” said Katherine Materna of the University of Colorado Boulder. “It looks like the board boundaries aren’t where we thought they were.”

By revealing pieces of invisible plates, this research is changing the way scientists look at earthquake zones in Northern California.

Better maps of underground movement will help improve risk estimates and guide future safety planning.

The study shows that even the smallest earthquakes can reveal key clues about the Earth’s hidden structure.

The study was published in the journal Science.

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