The Cascadia plate off Vancouver Island is ripping apart piece by piece, seismic imaging shows

Off the west coast of Vancouver Island, the ocean floor that has been sliding under North America for millions of years is coming apart in sections rather than all at once. That is the finding of a Louisiana State University-led team, whose seismic images of the Juan de Fuca and Explorer plates were published in September 2025 in the journal Science Advances. The study is a year old, but it keeps circulating, and ScienceDaily ran it again on September 24, 2026, because it answers an old question about how a subduction zone dies. The work is described here as an explainer of that 2025 paper, not as a new result.

Lead author Brandon Shuck of LSU put the result plainly in the coverage of the study: “Rather than shutting down all at once, the plate is ripping apart piece by piece, creating smaller microplates and new boundaries.” The researchers also stress that the tearing does not meaningfully change the earthquake picture for people living in the Pacific Northwest today.

Inside the 2021 seismic survey

The data come from the 2021 Cascadia Seismic Imaging Experiment, known as CASIE21, in which the research vessel Marcus G. Langseth towed sound sources and hydrophone streamers across the margin. According to the paper on PubMed Central, the team combined multichannel seismic reflection profiles with regional earthquake catalogs and focal mechanisms to see the shape of the sinking slab. The paper, listed on Science Advances under DOI 10.1126/sciadv.ady8347, has the full title “Slab tearing and segmented subduction termination driven by transform tectonics.” It lists Shuck, Brian Boston, Suzanne Carbotte and 11 other authors and carries the publication date September 24, 2025.

The Columbia Climate School account adds that Lamont-Doherty Earth Observatory scientists Suzanne Carbotte and Anne Bécel were co-authors and that the National Science Foundation funded the work.

Tears, offsets and a growing microplate

The paper’s central object is the Nootka Fault Zone, an active transform boundary about 20 kilometers wide that began roughly 4 million years ago as a broad shear zone and has since narrowed. In its abstract the team says that transform boundaries “drive laterally diachronous slab fragmentation and subduction termination,” meaning the plate fails first in one place and later in the next, and that the fault zone splits off a microplate that is subducting less than the plate segments around it.

The numbers differ depending on which description is read. The paper itself reports two slab tears of about 35 kilometers each, separated by roughly 20 kilometers of left-lateral offset and reaching from the top of the slab to about 40 kilometers depth. The Columbia account and the ScienceDaily write-up from October 2025 instead describe a 75-kilometer tear along which part of the slab has dropped by about five kilometers, with some sections still producing earthquakes and others gone quiet. Earthquake patterns support the idea that the silent sections are fragments that detached earlier.

Why a plate would break this way

A subduction zone ends when the spreading ridge that makes the incoming plate approaches the trench. The simplest picture would have the system switching off in a single event. The Vancouver Island images suggest a slower sequence in which the slab is cut into segments, each one detaching on its own schedule.

The researchers say the same mechanism could explain fossil fragments elsewhere, such as the leftovers of the ancient Farallon plate off Baja California, which puzzled geologists because they look like abandoned pieces rather than the products of one clean break. As pieces separate, gaps called slab windows open and hot mantle rock rises through them, which the team links to bursts of volcanic activity in unexpected places.

What it does not change about earthquake hazard

None of this makes the Cascadia margin less dangerous. The Columbia account quotes the researchers: “The region remains capable of producing very large earthquakes and tsunamis.” The version of the story reposted by ScienceDaily on September 24, 2026 repeats that the discovery does not significantly change the hazard on a human timescale, because the fragmentation unfolds over millions of years.

What the finding does do is refine the models. Understanding where the slab is cut, and which pieces are still locked against North America, is expected to improve Pacific Northwest seismic hazard models, according to the Columbia account, and the ScienceDaily write-up says investigation continues into how the newly identified fractures could influence future rupture patterns. Neither source claims that the tears make a large earthquake more or less likely.

The gap between the paper’s two 35-kilometer tears and the widely quoted 75-kilometer figure means any single number from this study should be checked against its source. The larger open question is how the newly identified fractures will shape the way future ruptures begin and end along the northern margin.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview