Scientists detect unexpected widespread structures near Earth's core


Date:
June 11, 2020
Source:
University of Maryland
Summary:
A new study has produced the first analysis of seismic echoes from
hundreds of earthquakes at once, revealing widespread structures at
the core-mantle boundary. Previous studies were limited to analysis
of single earthquakes, providing only a narrow window into the
structure deep inside the Earth. This study enables a much wider
view than ever before, revealing new, unexpected features and
expanding the size of a previously known feature beneath Hawaii.



FULL STORY ========================================================================== University of Maryland geophysicists 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, heterogenous structures -- areas of unusually dense, hot rock -- at the core-mantle boundary than previously
known.


========================================================================== Scientists are unsure of the composition of these structures, and previous studies have provided only a limited view of them. Better understanding
their shape and extent 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.

The new research provides the first comprehensive view of the core-mantle boundary over a wide area with such detailed resolution. The study was published in the June 12, 2020, issue of the journal Science.

The researchers focused on echoes of seismic waves traveling 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
UMD 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 realize
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 echolocate 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
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.

The study revealed a few surprises in the structure of the core-mantle boundary.

"We found echoes on about 40% of all seismic wave paths," said Vedran Lekić, 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.

This study also found a previously unknown ULVZ 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," Lekić 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."

========================================================================== Story Source: Materials provided by University_of_Maryland. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. D. Kim, V. Lekić, B. Me'nard, D. Baron and M. Taghizadeh-Popp.

Sequencing Seismograms: A Panoptic View of Scattering
in the Core-Mantle Boundary Region. Science, 2020 DOI:
10.1126/science.aba8972 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200611143101.htm

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