Analysing thousands of recordings of seismic waves, or sound waves travelling through the Earth, scientists have detected widespread, heterogenous structures – areas of unusually dense, hot rock – at the core-mantle boundary than previously known.
The new research, published in the journal Science, provides the first comprehensive view of the core-mantle boundary over a wide area with such detailed resolution.
The researchers focused on echoes of seismic waves travelling beneath the Pacific Ocean basin.
Their analysis revealed a previously unknown structure beneath the volcanic Marquesas Islands in the South Pacific and showed that the structure beneath the Hawaiian Islands is much larger than previously known.
“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 University of Maryland Department of Geology and the lead author of the paper.
“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 realise before because we only had a narrow view.”
Earthquakes generate seismic waves below Earth’s surface that travel thousands of miles. When the waves encounter changes in rock density, temperature or composition, they change speed, bend or scatter, producing echoes that can be detected.
Echoes from nearby structures arrive more quickly, while those from larger structures are louder. By measuring the travel time and amplitude of these echoes as they arrive at seismometers in different locations, scientists can develop models of the physical properties of rock hidden below the surface. This process is similar to the way bats echo-locate to map their environment.
For this study, Kim and his colleagues looked for echoes generated by a specific type of wave, called a shear wave, as it travels along the core-mantle boundary.
In a recording from a single earthquake, known as a seismogram, echoes from diffracted shear waves can be hard to distinguish from random noise. But looking at many seismograms from many earthquakes at once can reveal similarities and patterns that identify the echoes hidden in the data.
Using a machine learning algorithm called Sequencer, the researchers analysed 7,000 seismograms from hundreds of earthquakes of 6.5 magnitude and greater occurring around the Pacific Ocean basin from 1990 to 2018.
Sequencer was developed by the new study’s co-authors from Johns Hopkins University and Tel Aviv University to find patterns in radiation from distant stars and galaxies. When applied to seismograms from earthquakes, the algorithm discovered a large number of shear wave echoes.
The study revealed a few surprises in the structure of the core-mantle boundary.
“We found echoes on about 40 per cent of all seismic wave paths,” said Vedran Lekic, an associate professor of geology at the University of Maryland and a 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.”
The scientists found that the large patch of very dense, hot material at the core-mantle boundary beneath Hawaii produced uniquely loud echoes, indicating that it is even larger than previous estimates.
This study also found a previously unknown ultralow-velocity zones beneath the Marquesas Islands.
Better understanding of the shape and extent of the structures at the core-mantle boundary can help reveal the geologic processes happening deep inside Earth. This knowledge may provide clues to the workings of plate tectonics and the evolution of our planet.