A subduction zone near Cascadia is unraveling piece by piece. The process offers a rare glimpse into how tectonic plates die and form new geological boundaries.

With unprecedented clarity, researchers have captured a rare geological event: a subduction zone -- the point where one tectonic plate sinks beneath another, actively fracturing. The finding, published in Science Advances, offers new insight into the dynamic processes shaping Earth's crust and raises important questions about long-term earthquake risks in the Pacific Northwest.

Subduction zones are among Earth's most powerful and influential geological systems. They propel continents across the globe, generate catastrophic earthquakes and volcanic eruptions, and recycle the planet's crust back into the mantle.

However, these colossal systems don't last forever. If subduction zones never ended, continents would continually collide and merge, eliminating oceans and erasing the planet's geologic history. For decades, scientists have asked one key question: what exactly causes these immense systems to come to an end?

"Getting a subduction zone started is like trying to push a train uphill -- it takes a huge effort," said Brandon Shuck, a geologist at Louisiana State University and lead author of the study. "But once it's moving, it's like the train is racing downhill, impossible to stop. Ending it requires something dramatic -- basically, a train wreck."

A natural laboratory off Vancouver Island

Just off Vancouver Island, in the Cascadia region where the Juan de Fuca and Explorer plates slowly descend beneath the North American plate, scientists believe they have found the answer. By combining seismic reflection imaging -- essentially an ultrasound of Earth's interior -- with extensive earthquake data, the team observed a subduction zone in the process of breaking apart.

The seismic readings were obtained during the NSF-supported 2021 Cascadia Seismic Imaging Experiment (CASIE21). During this expedition, researchers aboard a ship sent sound waves into the seafloor and tracked their reflections using a 15-kilometer streamer of underwater sensors. The data produced detailed images of the region's subsurface, exposing deep fractures and fault lines where the tectonic plate is beginning to split.

Watching a tectonic plate die in real time

"This is the first time we have a clear picture of a subduction zone caught in the act of dying," said Shuck. "Rather than shutting down all at once, the plate is ripping apart piece by piece, 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."

The team observed tears slicing through the oceanic plate, including a massive offset where the slab has dropped by about five kilometers. "There's a very large fault that's actively breaking the plate," Shuck explained. "It's not 100% torn off yet, but it's close." Earthquake records confirm the pattern: along the 75-kilometer-long tear, some sections are still seismically active, while others are eerily quiet. "Once a piece has completely broken off, it no longer produces earthquakes because the rocks aren't stuck together anymore," he said. That missing gap of seismicity is a telltale sign that part of the plate has already detached and the gap is growing slowly over time.

The step-by-step collapse of tectonic plates

The study found that this breakup happens in stages, through what researchers call "episodic" or "piecewise" termination. Rather than a sudden break across the entire tectonic plate, the plate gradually tears apart one section at a time. Transform boundaries -- faults where plates slide past each other -- play a key role in this process. Acting like natural scissors, these faults cut across the plate, perpendicularly to the tear, allowing one piece to detach and form a microplate while subduction continues in neighboring sections.

By tearing off in smaller chunks, the larger plate loses momentum -- like cutting the cars off a runaway train -- and eventually stops being pulled downward. Each piece that breaks away is a process that takes several million years, but together these episodes gradually shut down an entire subduction system.

This episodic breakup helps explain puzzling features in Earth's history preserved elsewhere, such as abandoned fragments of tectonic plates and unusual bursts of volcanic activity. A striking example lies off Baja California, where scientists have long observed fossil microplates -- the shattered remains of the once-massive Farallon plate. For decades, researchers knew these fragments must be evidence of dying subduction zones, but the mechanism that created them was unclear. Cascadia is now providing that missing piece: subduction zones don't collapse in a single catastrophic event but unravel step by step, leaving behind microplates as geological evidence.

A planet in motion and future implications

And the process isn't over. As each fragment detaches, it reshapes Earth's surface. The torn edges may create "slab windows" where hot mantle rises, fueling bursts of volcanic activity. Over time, the plate boundaries migrate, new microplates form, and the cycle repeats. "It's a progressive breakdown, one episode at a time," said Shuck. "And it matches really well with what we see in the geologic record, where volcanic rocks get younger or older in a sequence that reflects this step-by-step tearing."

Looking ahead, researchers are exploring whether a major earthquake could rupture across one of these newly discovered tears or whether the breaks might influence how ruptures propagate. While these findings help refine models of how structural complexities affect earthquake behavior, they do not significantly change the hazard outlook for Cascadia on a human timescale. The region remains capable of producing very large earthquakes and tsunamis, and understanding how these new breaks influence rupture patterns will improve models used to study seismic hazards in the Pacific Northwest.

Reference: "Slab tearing and segmented subduction termination driven by transform tectonics" by Brandon Shuck, Brian Boston, Suzanne M. Carbotte, Shuoshuo Han, Anne Bécel, Nathaniel C. Miller, J. Pablo Canales, Jesse Hutchinson, Reid Merrill, Jeffrey Beeson, Pinar Gurun, Geena Littel, Mladen R. Nedimović, Genevieve Savard and Harold Tobin, 24 September 2025, Science Advances.

DOI: 10.1126/sciadv.ady8347

CASIE21 is supported by the National Science Foundation under awards OCE 1827452 and OCE 2217465.

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