Adapted from a release written by Bianca Scolaro for Louisiana State University.
With unprecedented clarity, scientists have directly observed a subduction zone—the collision point where one tectonic plate dives beneath another—actively breaking apart. The discovery, reported in Science Advances, sheds new light on how Earth’s surface evolves and raises fresh questions about future earthquake risks in the Pacific Northwest.
Subduction zones are the sites of Earth’s most powerful tectonic events. They drive continents across the globe, unleash devastating earthquakes and volcanic eruptions, and recycle the planet’s crust deep into the mantle.
But they don’t last forever. If they did, continents would endlessly collide and stack up, erasing oceans and wiping out the record of Earth’s past. The big question geologists have wrestled with is: how exactly do these mighty systems finally shut down?
“Getting a subduction zone started is like trying to push a train uphill—it takes a huge effort,” said Brandon Shuck, an assistant professor 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.” Shuck conducted the research while he was a postdoctoral research fellow at the Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School.
Off the coast of Vancouver Island, in a region of Cascadia where the Juan de Fuca and Explorer plates slowly move beneath the North American plate, scientists have found the answer. Using a combination of seismic reflection imaging—essentially an ultrasound of the Earth’s subsurface—and detailed earthquake records, the team has captured a subduction zone in the process of tearing itself apart.
The seismic data were collected during the 2021 Cascadia Seismic Imaging Experiment (CASIE21) aboard the Lamont-Doherty Earth Observatory’s research vessel, the Marcus G. Langseth. The experiment was led by Lamont scientist Suzanne Carbotte, who is also co-author on the new paper, along with Lamont colleague Anne Bécel. The researchers sent sound waves from the ship into the seafloor and recorded the echoes using a 15-kilometer-long streamer of underwater listening devices. This produced high-resolution images of faults and fractures deep beneath the ocean floor, revealing places where the plate is snapping.
“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.”
Carbotte adds that scientists have known for decades that subduction can stall as buoyant regions of oceanic plates reach a subduction zone. “But we haven’t previously had such a clear picture of the process in action,” she says. “These new findings help us better understand the life cycle of the tectonic plates that shape the Earth.”
The team observed tears slicing through the Juan de Fuca plate, including a massive break where the plate has dropped by about five kilometers. “There’s a very large fault that’s actively breaking the [subducting] 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 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.
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. The timing for each piece to break away takes several million years, but together these episodes may 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.
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.
CASIE21 is supported by the National Science Foundation under awards OCE 1827452 and OCE 2217465.