Ванкувер арлын орчимд тектоник хавтан задарч буйг эрдэмтэд илрүүлэв

Off the coast of Vancouver Island, a slab of ocean floor has dropped by about five kilometers along a fault that scientists say is close to breaking clean through. The finding comes from a team that used sound waves to image the seafloor in the northern Cascadia subduction zone, the boundary where the Juan de Fuca and Explorer plates slide beneath North America. What they found, published in Science Advances, is the clearest picture yet of a subduction zone in the process of shutting down.

Subduction zones form wherever one tectonic plate dives beneath another. They are responsible for the planet’s largest earthquakes and its most explosive volcanoes, and they slowly recycle old crust back into the mantle.

Geologists have long wrestled with how these systems finally shut down, since a subduction zone that never stopped would eventually erase oceans and stack continents on top of one another. The new images, gathered off Vancouver Island, answer part of that question by showing the Juan de Fuca plate breaking apart piece by piece rather than snapping all at once.

A Ship-Based Scan Revealed the Fault Five Kilometers Down

The images came from the 2021 Cascadia Seismic Imaging Experiment, known as CASIE21, carried out aboard the R/V Marcus G. Langseth, a research vessel operated by Lamont-Doherty Earth Observatory. The expedition was led by Lamont scientist Suzanne Carbotte, who served as chief scientist and later co-authored the new study with Lamont colleague Anne Bécel.

To see beneath the seafloor, the crew sent sound waves into the ocean floor from the ship and recorded the returning echoes using a 15-kilometer-long streamer of underwater listening devices. The technique, known as seismic reflection imaging, works much like an ultrasound of the Earth’s subsurface, producing high-resolution pictures of faults and fractures buried deep beneath the water. About 20 scientists from Lamont-Doherty, Louisiana State University, Auburn University, the University of Texas at Austin, and several other institutions have been analyzing the resulting dataset.

Those images revealed tears cutting through the descending plate, including one massive break where the slab has dropped by roughly five kilometers. Brandon Shuck, an assistant professor at Louisiana State University who led the study while a postdoctoral researcher at Lamont, said the discovery gives researchers something they have never had before: a subduction zone caught mid-collapse rather than reconstructed after the fact.

“This is the first time we have a clear picture of a subduction zone caught in the act of dying,” Shuck said. “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.”

Quiet Stretches of a 75-Kilometer Fault Mark Where the Plate Has Already Split

The earthquake record along the fault gave the researchers a second, independent line of evidence. Along the 75-kilometer-long tear, some sections still generate small earthquakes, while neighboring stretches have gone quiet. That silence is not a gap in the data but a sign the rock has already come apart, since a fully detached piece of plate no longer grinds against its neighbor.

“Once a piece has completely broken off, it no longer produces earthquakes because the rocks aren’t stuck together anymore,” Shuck said. Where the earthquakes have stopped, that section of plate has likely separated fully and is drifting slowly away from the piece still being pulled under the continent.

The main fault, where the slab dropped five kilometers, has not detached completely. “There’s a very large fault that’s actively breaking the [subducting] plate,” Shuck said. “It’s not 100% torn off yet, but it’s close.” Researchers call this overall pattern episodic, or piecewise, termination: instead of the entire plate breaking in a single event, sections separate one at a time.

Each of these episodes plays out over several million years, but taken together they can gradually shut down an entire subduction system. As each piece detaches, the remaining plate loses some of the downward pull that keeps it subducting, similar to how a train loses momentum once cars are cut loose from the back. Over enough episodes, that lost pull can eventually stall the whole system.

The Pattern Also Explains Old, Scattered Pieces of a Long-Dead Plate

Carbotte said geologists have known for decades that subduction can stall when a buoyant section of oceanic plate reaches the boundary, but catching the breakup itself in progress is new. “We haven’t previously had such a clear picture of the process in action,” she said. “These new findings help us better understand the life cycle of the tectonic plates that shape the Earth.”

The piecewise pattern seen off Vancouver Island helps explain features found elsewhere in the geologic record, including abandoned plate fragments and unusual bursts of past volcanic activity. Off Baja California, researchers have long identified fossil microplates, the scattered remains of the Farallon plate, a much larger plate that once subducted beneath North America before disappearing.

For decades, scientists understood those fragments as evidence of a dying subduction zone, but the mechanism that produced them stayed unclear without a live example to study. Cascadia now supplies that missing piece, showing that subduction zones do not collapse in one catastrophic event but unravel step by step, leaving microplates behind as evidence of the process.

Researchers Are Now Studying How the Tears Affect Earthquake Hazards

The discovery does not change how large an earthquake the Pacific Northwest could see on a human timescale. Cascadia remains capable of producing very large earthquakes and tsunamis, a hazard that decades of prior research have already established independent of this study.

What the new images add is structural detail that researchers did not have before: a sharper picture of faults and fractures running through the plate. Scientists are now studying whether a major rupture could jump across one of these tears, or whether the breaks might instead block a rupture from spreading further along the fault.

Refining that picture is expected to improve the models used to assess seismic hazards across the region. The CASIE21 survey was supported by the National Science Foundation, and the dataset it produced is still being analyzed by researchers working to understand how these structural complexities shape earthquake behavior in the Pacific Northwest.

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