The newly discovered structure, located thousands of kilometers beneath the Earth's surface, is confined to low latitudes and aligns parallel to the equator. Despite its significance, this region had eluded detection until now.
Earth's core consists of two layers: the solid inner core and the liquid outer core, which is encased by the mantle. The doughnut-shaped region is positioned at the upper boundary of the outer core, where it meets the mantle.
Professor Hrvoje Tkalcic, a geophysicist at ANU and co-author of the study, explained that the seismic waves in this region travel more slowly than in other parts of the liquid outer core. "The region sits parallel to the equatorial plane, is confined to the low latitudes and has a doughnut shape," he said. "We don't know the exact thickness of the doughnut, but we inferred that it reaches a few hundred kilometers beneath the core-mantle boundary."
Instead of relying on conventional seismic wave observation methods, which typically analyze signals shortly after earthquakes occur, the ANU team studied the similarities between waveforms several hours post-quake. This approach enabled them to uncover this previously hidden region.
"By understanding the geometry of the paths of the waves and how they traverse the outer core's volume, we reconstructed their travel times through the Earth, demonstrating that the newly discovered region has low seismic speeds," Professor Tkalcic added. The unique discovery was made possible by analyzing waves that reverberated for hours after major earthquakes, providing more comprehensive coverage of the outer core's volume than earlier studies.
Dr. Xiaolong Ma, another co-author of the study, emphasized the importance of this finding in unraveling the complexities of Earth's magnetic field. "There are still mysteries about the Earth's outer core that are yet to be solved, which requires multidisciplinary efforts from seismology, mineral physics, geomagnetism, and geodynamics," Dr. Ma said.
The outer core, composed mainly of liquid iron and nickel, generates Earth's magnetic field through the vigorous movement of electrically conductive liquid. This magnetic field is crucial for life on Earth, as it protects the planet from harmful solar winds and radiation.
The researchers believe that understanding the composition of the outer core, including the presence of light chemical elements, is vital for predicting changes in the magnetic field. "Our findings are interesting because this low velocity within the liquid core implies that we have a high concentration of light chemical elements in these regions that would cause the seismic waves to slow down," Professor Tkalcic explained. These light elements, coupled with temperature variations, play a role in stirring the liquid in the outer core.
"The magnetic field is a fundamental ingredient that we need for life to be sustained on the surface of our planet. The dynamics of Earth's magnetic field is an area of strong interest in the scientific community, so our results could promote more research about the magnetic field on both Earth and other planets."
Research Report:Seismic low-velocity equatorial torus in the Earth's outer core: Evidence from the late - coda correlation wavefield
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