Mapping Earth’s deepest secrets: Scientists detect unusually hot and dense structures near planet's core

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


                            Mapping Earth’s deepest secrets: Scientists detect unusually hot and dense structures near planet's core
(Getty Images)

Scientists have found unexpected widespread structures deep inside the Earth, paving the way towards a new map revealing what the planet’s interior looks like. A research team analyzed thousands of recordings of seismic waves, sound waves traveling through the Earth, to identify echoes from the boundary between Earth's molten core and the solid mantle layer above it. The echoes revealed more widespread and diverse structures -- areas of unusually dense, hot rock -- at the core-mantle boundary than previously known.

The research team is unsure of the composition of these structures, and previous studies have provided only a limited view of them. Better understanding of their shape and extent can help reveal the geologic processes happening deep inside Earth, say experts from the University of Maryland, US; Johns Hopkins University, US; and Tel Aviv University, Israel. This knowledge may provide clues to the workings of plate tectonics and the evolution of our planet, they add.

The researchers focused on the echoes of seismic waves traveling beneath the Pacific Ocean basin. The map shows a large area under the Pacific and reveals hot and dense regions below Hawaii and the Marquesas Islands in French Polynesia. It also reveals that a structure beneath the Hawaiian Islands is much larger than previously known. 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. Known as ultralow-velocity zones (ULVZs), such patches are found at the roots of volcanic plumes, where hot rock rises from the core-mantle boundary region to produce volcanic islands. The ULVZ beneath Hawaii is the largest known. The study also found a previously unknown ultralow-velocity zone beneath the volcanic Marquesas Islands in the South Pacific. “These observations illustrate how approaches flexible enough to detect robust patterns with little to no user supervision can reveal distinctive insights into the deep Earth,” says the team in their analysis published in Science.

According to the research team, the study provides the first comprehensive view of the core-mantle boundary over a wide area with such detailed resolution. “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. 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,” says lead author Doyeon Kim, a postdoctoral fellow in University of Maryland’s Department of Geology, in the study.

The researchers focused on the echoes of seismic waves traveling beneath the Pacific Ocean basin (Getty Images)

Earthquakes generate seismic waves below the 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, say experts. 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 the rock hidden below the surface. “This process is similar to the way bats echo-locate to map their environment,” says the study.

However, while scientists use these seismic waves to probe the Earth's interior, the task is much harder. They need to wait for an earthquake to record data, and when this happens, it only provides information in a piecemeal manner. The data is restricted to a tiny region and most of the time it's impossible to distinguish weaker echoes from noise. For the current study, the team 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,” they explain.

Earthquakes send sound waves through the Earth. Seismograms record the echoes as those waves travel along the core-mantle boundary, diffracting and bending around dense rock structures. The current study provides a broad view of these structures (Doyeon Kim/University of Maryland)

The team of space and earth scientists used a machine learning algorithm called the Sequencer to analyze 7,000 seismograms from hundreds of earthquakes of 6.5 magnitudes and greater occurring around the Pacific Ocean basin for over 30 years, from 1990 to 2018. The Sequencer was developed by the 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. “With this new way to look at the data globally, we were able to see weak signals much more clearly. We were finally able to identify the seismic echoes and use them to create a map,” says Brice Ménard, an astrophysicist at Johns Hopkins University and one of the team members, in the analysis.

The scientists parsed through the thousands of seismograms for echoes to create a new map showing details of the Earth's mantle, just above the liquid iron core, at a depth of 3,000 kilometers. “We found echoes on about 40% of all seismic wave paths. 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,” explains co-author of the study Vedran Leki, an associate professor of geology at UMD. 

In a related article, an expert says that this type of analysis could be applied to various seismic phases and a range of others that are of higher frequency, which would provide a new, higher-resolution, and more comprehensive mapping of the structural heterogeneity of deep Earth. “Knowledge of these physical properties and of inferred chemical and thermal structures is essential to determining whether partial melt of the rocks exists at the core-mantle boundary, whether distinct materials accumulate or stabilize in particular regions, whether some volcanoes have origins in deep Earth, and, last, what the compositional variations are in the lowermost mantle,” says Meghan S Miller from the Research School of Earth Sciences, Australian National University, Canberra. 

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