Low-velocity seismic structure discovered in Earth’s outer core

Two Australian scientists discovered a ‘seismic abnormality’ in the Earth’s outer core — a “donut” of energy around the equator, where seismic waves move about 2% slower than in other core regions. Their study provides new clues about the dynamics of our planet’s magnetic field.

• The structure was discovered using a novel seismic analysis approach that looks at the later, fainter parts of seismic waves, known as the coda.
• The study calls into question conventional models of the core, implying the presence of lighter elements and potential repercussions on Earth’s magnetic field, which is vital for safeguarding life from solar radiation.

The Earth has two core layers: the inner core — a solid layer, and the outer core — a liquid layer. The newly discovered donut-shaped region is found at the top of Earth’s outer core, where the liquid core meets the mantle.

“We present evidence for a low-velocity volume within the outer core via the global coda correlation wavefield,” researchers Xiaolong Ma and Hrvoje Tkalčić of the Research School of Earth Sciences, The Australian National University (ANU) stated.

The “donut” is a separate zone around the equator that is a few hundred kilometers thick and in which seismic waves travel roughly 2% slower than the rest of the core. This has baffled scientists since it implied an abnormality in the outer core’s composition, which might have higher amounts of lighter elements such as silicon and oxygen.

This seismic feature was found globally, using data collected from seismic stations located all over the world. The “donut” is near the Earth’s outer core, near the Equator. Unlike many seismic phenomena limited to specific regions on the Earth’s surface, this phenomenon is observed globally via seismic waves that travel into the Earth’s interior.

The study included data from earthquakes that happened between 2000 and 2021, with results published in Science Advances, a peer-reviewed magazine, in 2024. The timing of this discovery is remarkable because it coincides with breakthroughs in seismic data processing and computational modeling, which have permitted such thorough investigation.

The discovery of this “donut” structure is essential since it called into question conventional conceptions of Earth’s inner core. The anomaly also revealed previously unknown intricate interactions within the outer core.

Scientists believe this region may be essential in the vast currents of liquid metal that run through the core and generate the Earth’s magnetic field. The magnetic field is necessary for life on Earth because it shields the globe from dangerous solar radiation. Understanding this mechanism and the structure of the core can reveal new information regarding the activities of Earth’s interior, as well as the behavior of the geodynamo that maintains the magnetic field.

The researchers used a technique known as the “coda-correlation wavefield,” which involved cross-correlation between seismic waves. They discovered the structure by examining late coda waves, which arrive long after the initial earthquake signals.

This strategy allowed them to identify minute signals from several echoing waves that would otherwise go undetected. To establish that this seismic feature was an actual physical structure rather than an artifact, the researchers performed synthetic simulations using powerful models of Earth’s innards. These models considered features such as Earth’s ellipticity and probable mantle heterogeneities.

However, they could not fully explain the observed “donut” shape, concluding that it is a novel characteristic that requires additional exploration.

“To summarize, we have revealed a latitudinal pattern of inhomogeneity in the OC using the coda correlation wavefield. Such heterogeneity was mentioned in several previous studies. However, more seismological investigations are required to confirm the existence of such a structure in the OC, considering the limited coverage of body-wave phases and effects from strong heterogeneities in the lowermost mantle. We anticipate that our findings can serve as the starting point for future investigations of lateral heterogeneity in the evolution of OC dynamics and composition.”

References:

¹ Seismic low-velocity equatorial torus in the Earth’s outer core: Evidence from the late–coda correlation wavefield – Xiaolong Ma, Hrvoje Tkalčić – Sci. Adv.10, eadn5562(2024) – August 30, 2024 – 10.1126/sciadv.adn5562 – OPEN ACCESS

² Doughnut-shaped region found inside Earth’s core deepens understanding of planet’s magnetic field – ANU – September 3, 2024

Harsha Borah

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