Deep within Earth, subtle variations in how seismic waves travel are revealing a hidden pattern of deformation in the planet’s lowest mantle layer.
Deep beneath our feet, Earth’s mantle is in constant motion. Vast currents of slowly circulating rock, driven by heat from the planet’s interior, help move tectonic plates across the surface. But these same currents also stretch, twist, and reshape the mantle itself in ways that are difficult to observe directly.
A new study in The Seismic Record offers one of the clearest views yet of this hidden activity. It shows that much of the deformation in the mantle’s deepest layer is concentrated in regions where long-sunken tectonic slabs have accumulated after plunging thousands of kilometers below the surface.
Researchers had suspected this connection, but the study is the first to examine it on a global scale. It covers nearly 75% of the lowermost mantle just above the core-mantle boundary, about 2,900 kilometers (1,800 miles) beneath Earth’s surface.
Jonathan Wolf of the University of California, Berkeley, and his team built this global map using more than 16 million seismograms collected from 24 data centers worldwide.
Earthquake-generated shear waves travel at different speeds depending on their direction through a material. These variations reflect the material’s internal structure and composition. Scientists call this effect seismic anisotropy, and it helps identify regions where the mantle is being deformed.
By mapping these deformation patterns, researchers can better understand how material circulates within the mantle.
Understanding Mantle Flow
“We know that deformation in the upper mantle is dominated by the drag of the plates that move across it. And that extremely well approximates what we know from seismic anisotropy about the deformation of the upper mantle,” Wolf explained. “But we don’t have any of this kind of large-scale understanding for flow in the lowermost mantle. And that’s really what we want to get at.”
Using what Wolf describes as “the largest-ever assemblage of earthquake seismic data,” the team examined seismic waves that pass through the mantle, enter the core, and then return to the mantle.
These waves allow scientists to map anisotropy across distances of hundreds of kilometers, offering a clearer picture of where deformation occurs in the deepest mantle.
The researchers detected anisotropy in roughly two-thirds of the regions they analyzed. While the pattern is complex, most of these signals appear in areas where deeply subducted slabs are thought to exist.
“This isn’t that surprising in a sense, because that is predicted by geodynamic simulations,” Wolf said. “But at the scale that we’re looking at, it’s not really been shown using those methods that we’re using.”
Scientists are still working to determine exactly what causes anisotropy within these slabs, Wolf noted.
What Causes Anisotropy?
Some slabs may preserve “fossil” anisotropy from their time near Earth’s surface. However, a more likely explanation is that intense deformation occurs as the slabs reach the core-mantle boundary, along with deformation in the surrounding material they push aside.
Extreme pressure and heat at these depths can also alter the minerals within the slabs, creating new forms of anisotropic “fabric.”
Wolf emphasized that areas without a clear anisotropic signal do not necessarily lack deformation. In some cases, the signal may simply be too weak to detect with current methods.
The dataset used in this study remains a “treasure trove” for future discoveries about Earth’s deep interior.
“If I can dream, we will someday have enough information to really say much more about global flow directions of the lowermost mantle, knowing the seismic anisotropy across different lateral scales in the mantle, illuminating it from many directions,” he said.
Reference: “Widespread Deformation at the Base of the Mantle Linked to Subducted Slabs” by Jonathan Wolf, Barbara Romanowicz, Ed Garnero, Weiqiang Zhu and John D. West, 1 April 2026, The Seismic Record.
DOI: 10.1785/0320260001
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