For the first time, scientists have clearly seen a subduction zone in the process of breaking up. These zones form where one plate sinks beneath another and are responsible for some of the most powerful geological phenomena on Earth. The new discovery is scientific progressprovides an unusual perspective on how these large-scale systems evolve and raises new questions about seismic risk in the Pacific Northwest.
Subduction zones dramatically shape the Earth. They shift continents, cause massive earthquakes and volcanic eruptions, and pull old crust deep into the Earth’s mantle. But despite their great power, they don’t last forever.
Why do subduction zones eventually collapse?
If subduction zones continued indefinitely, continents would continue to pile up, oceans would disappear, and much of Earth’s geological history would be erased. Scientists have long wondered why these systems shut down.
“Starting a subduction zone to form is like trying to push a train up the mountain; it takes a lot of effort,” said Brandon Schack, an assistant professor at Louisiana State University and lead author of the study. “But once it starts moving, it’s like a train hurtling downhill and it’s impossible to stop. It takes something dramatic to end the train: a train wreck.” Schack conducted the study while a postdoctoral fellow at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School.
Cascadia reveals torn tectonic plates
The answer appears to lie in the Cascadia region off the coast of Vancouver Island. Here, the Juan de Fuca and Explorer plates are slowly sliding beneath the North American plate. Scientists used advanced imaging techniques and seismic data to confirm that this subduction zone was beginning to collapse.
The research team used seismic reflection imaging, which works similarly to ultrasound inside the Earth, and detailed records of earthquakes. Combining these tools revealed that the plates were not simply sinking, but were being actively torn apart.
Earthquake image experiment in 2021
This data comes from the 2021 Cascadia Seismic Imaging Experiment (CASIE21) conducted aboard a research vessel. Marcus G. Lances. The team, led by Lamont scientist Suzanne Carbot and co-author Anne Bethel, sent sound waves to the ocean floor and captured the echoes using a 15-kilometre-long array of underwater sensors.
This method produced highly detailed images of faults and fractures deep under the seafloor. These images clearly show how parts of the plate are falling apart.
“This is the first time we have a clear picture of a dying subduction zone,” Schack said. “Rather than everything stopping at once, the plates are torn apart bit by bit, creating smaller microplates and new boundaries. So instead of a big train wreck, it’s like watching a train slowly derail one car at a time.”
Carbot noted that scientists have known for years that subduction can slow or stall when the lighter part of a plate reaches a boundary. “But we’ve never had such a clear picture of the actual process,” she says. “These new discoveries help us better understand the life cycles of the plates that shape our planet.”
Massive faults and silent gaps
Researchers have identified several large faults that cut through the Juan de Fuca plate, including one that caused the plate to drop about 5 kilometers. “There’s a very large fault, and it’s actively fracturing the (subducting) plate,” Schack explained. “It’s not 100% peeled off yet, but it’s close to it.”
Seismic data supports this situation. Along the 75-kilometre-long rift, some areas are still experiencing earthquakes, while others are unusually quiet. “Once the pieces are completely broken up, the rocks are no longer stuck together, so there’s no earthquake,” he says. These quiet gaps suggest that parts of the plates have already separated, and the cracks are gradually widening.
slow individual decomposition
This study shows that subduction zones do not collapse all at once. Instead, it shuts down through a process called “episodic” or “piecewise” termination. The plates are torn apart in stages, and different parts break off over time.
When the small part separates, the large plate loses the force pulling it down. If this momentum is gradually lost over millions of years, the entire subduction system can come to a halt.
Clues to Earth’s geological past
This gradual breakdown helps explain puzzling features seen elsewhere on Earth. In some areas, scientists have discovered fragments of old plates and outbursts of volcanic activity whose meaning was not fully understood until now.
One example is off the coast of Baja California, where the remains of the ancient Farallon Plate remain as fossilized microplates. Researchers have long suspected that these debris were associated with a dying subduction zone, but the exact process was unclear. New observations from Cascadia suggest that these ancient plates likely broke apart gradually in the same way.
What this means for the Cascadia earthquake
Scientists are now investigating how these newly discovered fissures will affect future earthquakes. One key question is whether large-scale fractures propagate across these cracks, or whether the cracks can change the way seismic energy spreads.
For now, the findings do not significantly change the overall risk for the Cascadia region. This area is still capable of producing very large earthquakes and tsunamis. However, incorporating these new details into the model will improve the way researchers understand and simulate earthquake hazards in the Pacific Northwest.
The CASIE21 project is supported by the National Science Foundation under awards OCE 1827452 and OCE 2217465.
