Researchers analyzing data from the 2021 CASIE21 expedition report that the northern Cascadia subduction zone is fragmenting in stages. Using a 15-km hydrophone streamer towed by the RV Marcus G. Langseth, the team imaged large tears in the Juan de Fuca plate, including an offset of about 5 km. The process — described as "episodic" or "piecewise" termination — can produce microplates but unfolds over millions of years and does not lower short-term earthquake risk for the Pacific Northwest.
Scientists Watch a Subduction Zone Slowly Break Apart Off the Pacific Northwest
Researchers analyzing data from the 2021 CASIE21 expedition report that the northern Cascadia subduction zone is fragmenting in stages. Using a 15-km hydrophone streamer towed by the RV Marcus G. Langseth, the team imaged large tears in the Juan de Fuca plate, including an offset of about 5 km. The process — described as "episodic" or "piecewise" termination — can produce microplates but unfolds over millions of years and does not lower short-term earthquake risk for the Pacific Northwest.
18:35, 07.11.2025Science
Earth in Slow Motion: A Subduction Zone Unraveling
From a human perspective, tectonic systems can seem eternal — yet even subduction zones eventually die. New research published in Science Advances reports evidence that the northern end of the Cascadia subduction system is fragmenting in stages.
High-resolution images from CASIE21
During the 2021 Cascadia Seismic Imaging Experiment (CASIE21), researchers towed a 15-kilometer hydrophone streamer behind the research vessel RV Marcus G. Langseth. The seismic data allowed the team to build a high-resolution image of faults and fractures deep beneath the seafloor where the Juan de Fuca and Explorer plates subduct beneath the North American plate.
“Starting a subduction zone is like trying to push a train uphill — it takes enormous work. But once it’s moving, it’s like a train racing downhill; stopping it requires something dramatic — basically, a train wreck.” — Brandon Shuck, lead author (Louisiana State University)
Piecewise or "episodic" termination
Instead of collapsing at once, the images show the subducting plates detaching in segments; the team calls this process episodic or piecewise termination. The seismic profiles reveal extensive tearing along the Juan de Fuca plate, including one striking offset in which a block appears to have dropped by roughly 5 kilometers. While this block is not yet fully detached, the geometry and patterns of earthquakes in the region are consistent with staggered break-off: some sections remain seismically active while others are quiet, likely because previously separated blocks are no longer locked together.
Why this matters
Understanding episodic termination illuminates how isolated fragments called microplates form — for example, fragments off Baja California are thought to be remnants of similar dying subduction systems. Although scientifically important, this slow disintegration unfolds over millions of years and does not materially reduce short-term earthquake risk for people in the Pacific Northwest.
Bottom line: The CASIE21 images provide a rare, high-resolution view of a subduction zone in the process of breaking apart piece by piece. The discovery sheds light on long-term tectonic evolution and the origin of microplates, while reminding us that these processes operate on geological time scales far beyond human lifetimes.