We now have proof that the outer core of the Earth is indeed animated by convection currents

It is a widely accepted theory now and which allows in particular to explain the origin of the Earth’s magnetic field: the outer core, which is mainly composed of iron liquidwould be animated by large currents of convection, thus giving rise to the geodynamo effect. Although these movements of matter been widely modeled and studied, they had never been observed directly. Finally, as directly as a layer located more than 2,900 kilometers below our feet can be.

Two similar earthquakes for two different speeds

It is indeed the seismic waves that have made it possible to image this phenomenon of convection. The core, like all layers of the Earth, is often traversed by seismic waves, generated during major earthquakes that occur regularly on the surface. However, each terrestrial envelope has different physico-chemical characteristics which will influence the speed and mode of propagation of seismic waves. The study of the waves that reach us therefore allows us to find these characteristics. This is how we have been able to construct a fairly accurate image of the Earth’s layers and continue to study them.

In May 1997, a powerful earthquake shook the Kermadec Islands in the South Pacific. Twenty years later, in September 2018, a new earthquake, having the same origin, was also recorded in this region. This redundancy of similar events allowed scientists to compare the paths of seismic waves. By having been generated and recorded in the same places, the waves of the two earthquakes should indeed have similar travel times. However, a team of scientists noted a significant difference between these two earthquakes. A wave train named SKS arrived a second ahead in 2018, compared to the same wave train recorded in 1997.

A local change in the density of the outer core

SKS waves are recognized as waves that have passed through the outer core. This acronym means that they crossed the coat under the S waveform (shearing), that they entered the outer core as P (compression) waves, then exited the core to cross the mantle again as S waves. being liquid, it does not allow the propagation of shear waves, but only of compression waves. The speed of the waves in each medium (mantle and outer core), and therefore the travel time of the waves between their origin and the place of recording, depends on several parameters, including the density of the medium. For a medium with a certain rigidity, the denser this medium, the slower the waves passing through it, and vice versa. The difference in travel time between 1997 and 2018 therefore shows that something in the parameters of the environments crossed has changed.

For Ying Zhou, from the Department of Geosciences at Virginia Tech, author of the study published in type communications, this difference is associated with a change in the density of the region of the outer core crossed by the waves during the two earthquakes. The faster speed recorded in 2018 would be attributed to a local decrease in the density of the outer core, related to the presence of lighter elements (hydrogen, carbon, oxygen). The fact is that in 20 years, the material crossed by the waves is no longer the same: that of 1997 has been replaced by less dense material.

A short-term creep responsible for the variations of the magnetic field

This change in density would be a direct observation of the fluid movements that animate the outer core and are associated with its cooling and the migration of light elements outward from the nucleus. It is these movements of iron liquid which are at the origin of the earth’s magnetic field. This, easily measurable, also shows a very high variability, with daily modifications. It was assumed that the outer core would evolve in a similar fashion, but this had never been demonstrated. It is now done.