Scientists have discovered six never-before-seen mystery structures lurking deep within the Earth.
Although the deep layers of our planet are completely inaccessible, scientists can use the seismic waves of powerful earthquakes to explore the hidden depths.
Now Chinese scientists have discovered structures that lie at the boundary between the viscous mantle and the liquid outer core, 2,900 kilometers beneath our feet.
These ‘deep-seated scatterers’ are virtually invisible, except for the way they subtly influence the path of passing seismic waves.
In their paper, published in the journal JGR Solid Earth, the scientists suggest that these structures may have formed when material from the outer layers was dragged deep into the Earth.
They could be chunks of continental crust or even the remains of Theia, a Mars-sized protoplanet that is thought to have formed the moon after impacting Earth 4.5 billion years ago.
Under the enormous pressure and incredible temperatures at the mantle boundary, these chunks transform, partially melt, or undergo mineral transformation.
The researchers suggest that this has created six ‘thermochemical piles’ of material that are very different from the surrounding mantle.
Scientists have discovered six never-before-seen mystery structures lurking deep within the Earth (indicated in dotted lines)
These structures are located right at the boundary between the solid yet malleable mantle and the liquid metal outer core (stock image)
Although Earth looks relatively stable on the surface from our perspective, deep beneath the planet it is violent and dynamic, especially at the boundaries between layers.
A particularly dramatic boundary is between the mantle and the outer core, where solid but viscous rocks meet liquid nickel and iron.
There is a huge temperature jump across the border, with a difference of about 1,000 °C (1,800 °F) between one layer and the next.
The enormous heat escaping from the outer core drives convective currents in the mantle, known as mantle plumes, which play a key role in determine where volcanic activity occurs on the surface.
The huge density differences also cause seismic waves to slow down dramatically at this boundary, allowing geologists to ‘see’ what is going on.
To learn more about this strange part of the planet, the researchers looked at a special type of seismic wave, a PKP precursor.
These are relatively weak waves that arrive shortly before the stronger seismic waves caused by earthquakes.
PKP precursors are extremely useful to scientists because they are dispersed by the subtle differences in the hidden structures, known as heterogeneities, at the mantle boundary.
To learn more about this strange part of the planet, the researchers looked at a special type of seismic wave called a PKP precursor, weak waves that arrive shortly before the stronger seismic waves caused by earthquakes.
These scattered waves then pass through the liquid outer core, but not the solid inner core, and bounce back to arrive at seismic detectors ahead of the main wave.
The problem is that PKP precursors, because they are so faint, are extremely difficult to find in seismic data and must be manually sought out among the thousands of signals collected every year.
In their paper, the researchers explain: ‘Manual identification of these precursors is inefficient, subjective and insufficient for massive global seismic datasets.’
To get around this, the researchers trained an artificial intelligence (AI) model to recognize PKP precursors in seismic data.
After training the model on human-identified waves, the team unleashed their AI on more than two million recordings of 5,000 different earthquakes.
Ultimately, their model identified 174,929 high-quality PKP precursor signals – more than ten times all previous studies combined.
This has provided an unprecedented view of the mantle boundary, revealing vast areas of previously unknown structures.
The researchers write: ‘We also discovered six areas that likely harbor significant heterogeneities never previously documented, providing clear priority targets for future exploration of Earth’s deep interior.’
Using an AI model, the researchers identified 174,929 high-quality PKP precursor signals from more than two million recordings of 500 earthquakes. Pictured: Earthquakes with identified PKP precursors traveling from the source (pink stars) to seismic array detectors (blue triangles).
Previous studies have found fragmented, seemingly random structures in a few locations around the world.
But this new map shows that these fragments were actually connected into much larger, continuous belts.
Currently, researchers don’t know exactly what these structures are made of or how they are formed, only that they are different from the surrounding mantle.
However, with AI models enabling even more powerful analysis of the data collected over the past decades, the picture could soon become clearer.
The researchers add: ‘As the catalog continues to expand, the high-resolution spatiotemporal coverage will advance the refinement of fine-scale structural models of the lower mantle, providing increasingly rich constraints for deepening our understanding of the geodynamic state of Earth’s deep interior.’
