A surprising new discovery has revealed a previously unknown structure located deep beneath the Pacific Ocean between the Earth’s core and mantle. It is not currently known what this structure – and others found near it – are composed of, but researchers note that their finding provides an opportunity to better understand the intricate processes that have allowed our planet to evolve and change over time.
When an earthquake occurs, it produces seismic waves below the Earth's surface capable of traveling thousands of miles. As these waves encounter changes in temperature, composition, or density of rocks, their speed may change, bend, or scatter in ways that create echoes that can be detected by seismographs strategically placed in locations around the world. Measuring the time and amplitude of these echoes can reveal subsurface physical properties and reveal structures, but research has largely been limited and only able to illustrate this landscape in a “piecemeal way.”
To overcome these limitations, geophysicists at the University of Maryland (UMD) analyzed thousands of recordings of seismic waves as they traveled beneath the Pacific Ocean basin looking at a specific type of wave called a shear wave named for its “s” shape. These waves can be difficult to differentiate from random noise, but analyzing multiple recorders allowed the team to pinpoint similarities and patterns in echoes from the boundary between the molten core and the solid mantle layer above it. A machine learning algorithm called Sequencer analyzed 7,000 seismograms from hundreds of earthquakes with a magnitude of 6.5 or higher occurring around the Pacific Ocean basin between 1990 and 2018 in order to “systematically detect seismic echoes”.
"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 Doyeon Kim, a postdoctoral fellow in the UMD Department of Geology and lead author of the paper published in Science. "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."
In nearly half of the waves that were diffracted, the researchers found three-dimensional structures near the core-mantle boundary. In particular, a large feature was located below the volcanic Marquesas Islands in the Pacific Ocean, which is likely the result of a plume root, a volcanic feature responsible for creating many volcanic islands.
"We found echoes on about 40 percent of all seismic wave paths," said Vedran Lekik, an associate professor of geology at UMD and co-author of the study. "That was surprising because we were expecting them to be more rare, and what that means is the anomalous structures at the core-mantle boundary are much more widespread than previously thought."
This high-detailed comprehensive view of the core-mantle boundary located far more subsurface structures than were previously thought to exist and provide insight into how plate tectonics have shaped our planet over millions of years.