Far below the Earth’s surface, slow convection currents swirl within the mantle. These flows are closely related to the movement of tectonic plates and do more than just move the plate above. The mantle material itself also stretches and deforms.
New research published in Earthquake record The results show that much of this deformation in the deepest parts of the mantle is occurring in areas where scientists believe ancient tectonic slabs have sunk over millions of years.
Earth map reveals deformation near the core
Scientists have long suspected a link between deep mantle deformation and these buried slabs, but this study provides the first complete picture. The research team investigated almost 75% of the lower mantle. This layer is located about 2,900 kilometers (1,800 miles) below the Earth’s surface, just above the core-mantle boundary.
Jonathan Wolff and his colleagues at the University of California, Berkeley, used a huge dataset to build this world map. They collected and analyzed over 16 million earthquake records from 24 data centers around the world, creating one of the most comprehensive seismic datasets ever collected.
Seismic waves reveal hidden structures
When an earthquake occurs, shear waves are generated that travel through the Earth’s interior. These waves travel at different speeds depending on their direction and the properties of the material they are passing through. This change in direction, known as seismic anisotropy, allows scientists to pinpoint areas where the mantle has deformed.
By studying these patterns, researchers can gain valuable insight into how the mantle flows and circulates over time.
“We know that upper mantle deformation is dominated by the drag of plates moving across the upper mantle, and that’s very similar to what we know about upper mantle deformation from seismic anisotropy,” Wolf explained. “But we don’t have this large-scale understanding of the flow at the bottom of the mantle, and that’s what we really want to achieve.”
Large-scale dataset reveals patterns deep in the Earth
Using what Wolff describes as “the largest collection of earthquake seismic data in history,” the team analyzed the multiple stages of seismic waves as they travel through the mantle, enter the core, and then return to the mantle.
These waves are particularly useful for mapping seismic anisotropy over distances of hundreds of kilometers, providing a clearer picture of how deformation is distributed in the deepest mantle.
The results showed anisotropy over approximately two-thirds of the investigated area. Although the pattern is complex, most of the deformation appears in areas where deeply subducted slabs are thought to exist.
“This is predicted by geodynamic simulations, so in some ways it’s not that surprising,” Wolf said. “But at the scale we’re looking at, it’s not really shown with the methods we’re using.”
Causes of deformation of subducted slabs
Scientists are still working to understand exactly why these slabs exhibit seismic anisotropy. One possibility, Wolf said, is that the slab retains some “fossil” anisotropy from when it was close to the surface.
However, a more likely explanation is that intense deformation occurs as the slab sinks and interacts with the core-mantle boundary. As they descend, the surrounding material is also pushed in and reformed. The extreme heat and pressure at this depth can change the minerals within the slab, creating a new anisotropic “fabric.”
Detection limits and future research
Wolf emphasized that regions lacking detectable anisotropy signal should not be assumed to be deformed. In some areas, the signal may be too weak to be detected by current methods.
The vast dataset behind this research remains a valuable resource. Wolf described it as a “treasure trove” that researchers continue to explore for further insights into the Earth’s deep interior.
“If we can dream, one day we will know the seismic anisotropy of the mantle across different lateral scales, illuminate the mantle from many directions, and have enough information to say more about the direction of global flow in the lowermost mantle,” he said.
