A stunning announcement by geophysicists earlier this month suggests the inside of our planet still has many deep mysteries that remain hidden from us under the surface.
A team of researchers using decades of seismographs and an advanced algorithm say they have discovered a series of vast structures embedded in the boundary between the planet’s liquid outer core and the lower mantle.
Scientists arrived at this conclusion by studying the seismographic data from hundreds of earthquakes captured between 1990 and 2018. To analyze all this information, scientists also tapped a machine learning algorithm called Sequencer, which was originally designed to study distant galaxies.
The team prioritized the secondary shear (S) waves from deep in the Earth’s mantle border, as their signals are easier to read than primary (P) earthquake waves.
According to team leader Doyeon Kim, a seismologist and postdoctoral fellow at the University of Maryland,
“We normally like to use S waves because they are larger in amplitude and the data is more or less clean because there is less P wave traffic,” said Kim, noting that such a wave is also ideal “because it diffracts along that surface, it’s a great phase to look for these tiny structures on top of the core-mantle boundary.”
While scientists already had a hunch that there were large structures yet to be identified, to confirm their existence and study their contours, they needed to chart the journey of the S waves and map the echoing signals created when they intersect with the large unknown masses, called ultra low-velocity zones (ULVZs).
This signal is called a “postcursor” and it is the tell-tale indicator of a ULVZ.
Kim’s team found evidence for two “mega-ULVZs” over 1,000 kilometers in diameter beneath Hawai’i and the Marquesas Islands.
Whatever these structures are, they are located 3,000 kilometers below the surface of the Earth in the hellishly hot zone where the outer core meets the lower mantle.
“By looking at thousands of core-mantle boundary echoes at once, instead of focusing on a few at a time, as is usually done, we have gotten a totally new perspective,” said Kim, who was also the lead author of the published paper describing the team’s findings. “This is showing us that the core-mantle boundary region has lots of structures that can produce these echoes, and that was something we didn’t realize before because we only had a narrow view.”
Unsurprisingly, the announcement immediately triggered a flood of conspiratorial speculation online. However, while scientists don’t know the composition of the structures, it is widely believed to be an extremely dense and hot heterogeneous rock.
Scientists believe the further study of these structures will provide us with a better picture of the complex and powerful forces at work inside the Earth. Such information could assist an enhanced understanding of plate tectonics that shaped the geological evolution of our planet.