Analysis of seismic waves generated by two separate earthquakes, occurring in the same region of the globe but 20 years apart, revealed changes in the composition of the Earth’s outer core.
“The fact that they move faster indicates the presence of a new, lighter material”
Today, most of our knowledge about the bowels of the Earth comes from the study of waves generated by earthquakes. The latter can in particular indicate the nature of the rocks and metals that compose it. Swirling layer of liquid iron and nickel separating the mantle from the inner core, the outer core directly influences the magnetic field of our planet, which protects us from deadly cosmic radiation. This is why understanding it and monitoring its evolution is essential.
In the context of work published in the journal Nature Communications Earth & Environmentresearchers from the University of Virginia studied seismic data relating to two earthquakes, which occurred near the Kermadec Islands (South Pacific) between May 1997 and September 2018. It turned out thata particular type of wave took one second less to propagate through a region of the outer core than two decades ago. Which indicates changes in the composition of the latter.
” SKS waves pass through the mantle as shear (S) waves, then enter the outer core as compression (K) waves, before exiting the other side and traversing the mantle again as shear wave explains Ying Zhou, lead author of the study. ” The fact that they move faster indicates the presence of a new, lighter material. »
Valuable insight into Earth’s outer core convection
Occurring in the liquid iron of the outer core as it crystallizes on contact with the inner core, the convection causes electric currents to be created and flow, controlling the magnetic field around us. However, the relationship between the latter and the outer core remains poorly understood (it is largely based on hypothetical modeling).
” If you look at the geomagnetic north pole, it is currently moving at a rate of about 50 kilometers per year, moving away from Canada and closer to Siberia says Zhou. ” The fact that the magnetic field varies daily suggests that convection also changes with time, but there is no direct evidence. We never saw her. »
Although the variations observed here turn out to be quite small, they offer valuable insight into the evolution of the outer core. The study authors believe that lighter elements such as hydrogen, carbon and oxygen have been released there since 1997, implying a loss of density of approximately 2 to 3% and a convective flow velocity of about 40 kilometers per hour.
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