In a new study, a team of geophysicists from the University of Maryland, Johns Hopkins University and Tel Aviv University has analyzed echoes of seismic waves traveling beneath the Pacific Ocean basin. They’ve detected unusually dense, hot rock structures at the core-mantle boundary beneath the volcanic Marquesas Islands in the South Pacific and found that the structure beneath the Hawaiian Islands is much larger than previously known.
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. Kim et al provide the first broad view of these structures, revealing them to be much more widespread than previously known. Image credit: Doyeon Kim, University of Maryland.
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.
“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 lead author Dr. Doyeon Kim, a postdoctoral researcher in the Department of Geology at the University of Maryland.
“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.”
Dr. Kim and 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 scientists analyzed 7,000 seismograms from hundreds of earthquakes of 6.5 magnitude and greater occurring around the Pacific Ocean basin from 1990 to 2018.
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,” Dr. Kim said.
The authors revealed a few surprises in the structure of the core-mantle boundary.
“We found echoes on about 40% of all seismic wave paths,” said co-author Dr. Vedran Lekic, a researcher in the Department of Geology at the University of Maryland.
“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 team 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, 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 ultralow-velocity zone beneath Hawaii is the largest known.
The researchers also found a previously unknown ultralow-velocity zone beneath the Marquesas Islands.
“We were surprised to find such a big feature beneath the Marquesas Islands that we didn’t even know existed before,” Dr. 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.”
The findings were published in the journal Science.
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D. Kim et al. 2020. Sequencing seismograms: A panoptic view of scattering in the core-mantle boundary region. Science 368 (6496): 1223-1228; doi: 10.1126/science.aba8972
