A tear 75 kilometres long runs through the oceanic plate that dives beneath the Pacific Northwest, and one fault along it has dropped the slab by about 5 kilometres. Off Vancouver Island, the Juan de Fuca and Explorer plates are being forced under the North American plate, and a seismic survey has imaged the point where that sinking slab is coming apart. The tear is real, and it is also very slow.
The finding comes from Brandon Shuck, lead author and assistant professor at Louisiana State University, who describes a subduction zone shutting down in pieces. Shuck described the result in plain terms: this is the first time there is a clear picture of a subduction zone caught in the act of dying.
Seismic imaging off Vancouver Island
The evidence comes from the 2021 Cascadia Seismic Imaging Experiment, known as CASIE21, according to LSU’s account of the work. The research vessel Marcus G. Langseth towed a 15-kilometre underwater listening cable, called a streamer, behind it, and sound pulses bounced off buried layers returned a cross-section of the crust and the plate beneath it. The National Science Foundation funded the survey, and about 20 scientists from LSU, the University of Washington, Oregon State University, Auburn University, the University of Texas at Austin, the United States Geological Survey, Woods Hole Oceanographic Institution and Lamont-Doherty Earth Observatory are analysing the data through the Cascadia Region Earthquake Science Center.
The results appear in the journal Science Advances, in a paper titled “Slab tearing and segmented subduction termination driven by transform tectonics”, published in 2025 in volume 11, issue 39. Its subject is how a subduction zone ends: not with a single break but as a sequence of separations, each guided by a transform fault, the kind of boundary where plates slide past one another instead of colliding or pulling apart.
The geometry matters for scale. The Pacific Northwest Seismic Network says the Juan de Fuca plate is pushed beneath the North American plate along the Cascadia subduction zone, a megathrust that ruptured along much of a 1,000-kilometre length in its last great earthquake. A 75-kilometre tear is therefore a feature of roughly one-thirteenth that span, located in the descending slab off Vancouver Island, at the northern end where the Explorer plate sits beside the Juan de Fuca plate.
Transform faults as scissors
The mechanism is what the authors call episodic, or piecewise, termination. According to the LSU summary, transform boundaries act like natural scissors, letting one section of the plate detach from its neighbour while subduction carries on elsewhere. Shuck put it this way: “Rather than shutting down all at once, the plate is ripping apart piece by piece, creating smaller microplates and new boundaries.”
The Lamont-Doherty Earth Observatory release adds that some sections of the plate remain seismically active while others show no earthquake activity, an indication that those portions have already separated and no longer take part in the ongoing descent of the plate beneath the continent. Suzanne Carbotte of Lamont-Doherty said the findings help scientists understand the life cycle of the tectonic plates that shape the Earth.
Each cut leaves a smaller microplate that moves somewhat independently. The process may also open slab windows, where mantle material rises toward the surface and alters volcanic activity.
Earthquake hazard on a geological clock
The timescale is the key qualifier. The researchers emphasise that each piece of the breakup takes several million years, far longer than any human planning horizon, and the LSU release as carried by ScienceDaily says the findings do not significantly alter near-term hazard predictions. The Pacific Northwest remains capable of producing very large earthquakes and tsunamis.
The reference event for that capability is the Cascadia earthquake of January 26, 1700, the region’s reference event. The Pacific Northwest Seismic Network, which tracks the region’s earthquakes and publishes the historical record, describes it as a megathrust rupture of magnitude between 8.7 and 9.2 along much of the fault’s 1,000-kilometre length, the most recent of 19 great Cascadia events identified in the geologic record over the last 10,000 years. A magnitude range that wide, nearly half a unit of magnitude, reflects the modelling needed to reconstruct a quake that left geologic traces and tsunami records instead of seismograms, and it frames how large a rupture the region’s planners must assume regardless of any slab tear.
What the new structure may eventually improve is the modelling. The LSU account says the newly identified features could help refine understanding of rupture behaviour in the region, since a plate cut by a 75-kilometre tear, with a vertical offset of about 5 kilometres in one fault section, is a different structure from an intact slab, and a rupture model built on the real geometry has one more constraint to satisfy.
The three numbers that define the finding all belong to the geological frame: about 75 kilometres of tear, about 5 kilometres of drop in one fault section, and several million years for each piece of the plate to separate. None of them is a forecast, and the LSU and Lamont-Doherty accounts present none.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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