Scientists using a machine-learning algorithm that was originally developed to analyze remote galaxies to probe 30 years of earthquake data, have discovered a vast structures composed of exotic materials occupying the boundary between Earth’s liquid outer core and the lower mantle, a zone some 3,000 kilometers (1,864 miles) beneath our feet that date back to a time before Earth had a Moon. These massive anomalous objects could be partially melted material that predate the formation of the Moon from a colossal collision between early Earth and a Mars-sized object more than four billion years ago.
Scientists led by Doyeon Kim, a seismologist and visiting scientist at Cornell University, fed seismograms captured from hundreds of earthquakes that occurred between 1990 to 2018 into an algorithm called “Sequencer” to analyze thousands of recordings of seismic waves to identify echoes from the boundary between Earth’s molten core and the solid mantle layer above it. These seismic waves, each with a magnitude of at least 6.5—that shook the subterranean world under the Pacific Ocean within the past three decades, are warped patterns are captured in seismograms, recordings of wave activity inside Earth, enabling seismologists to capture rare glimpses of Earth’s deep, inaccessible underworld.
The researchers analysis. published in Science, 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.
“Sequencer” Reveals an Alien Underworld
“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, 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 realize before because we only had a narrow view.”
The above image shows how areas of hot, dense rock called ultralow-velocity zones deep inside the earth bend and diffract sound waves produced by earthquakes. In a new analysis of the diffracted waves recorded by seismograms, UMD geologists reveal a new ULVZ under Marquesas and a bigger ULVZ beneath Hawaii than previously known.
Seismograms Unveil Previously Unknown Structures
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 echolocate to map their environment.
Shear Wave Echoes Hidden in the Data
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 analyzed 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.
“Machine learning in earth science is growing rapidly and a method like Sequencer allows us to be able to systematically detect seismic echoes and get new insights into the structures at the base of the mantle, which have remained largely enigmatic,” Kim said.
Earthquakes, seen as yellow stars here, send sound waves through the Earth. Seismograms, seen as blue triangles here, record the echoes as those waves travel along the core-mantle boundary, diffracting and bending around dense rock structures. New research from University of Maryland provides the first broad view of these structures, revealing them to be much more widespread than previously known. Credit: Doyeon Kim/University of Maryland
“We found echoes on about 40% of all seismic wave paths,” said Vedran Lekic, an associate professor of geology at UMD 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. 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.
“We were surprised to find such a big feature beneath the Marquesas Islands that we didn’t even know existed before,” Lekic said. “This is really exciting, because it shows how the Sequencer algorithm can help us to contextualize seismogram data across the globe in a way we couldn’t before.”
Source: “Sequencing seismograms: A panoptic view of scattering in the core-mantle boundary region” Science (2020). science.sciencemag.org/cgi/doi … 1126/science.aba8972