Most of our knowledge about what lies at the center of our planet comes from the study of seismic waves coming out of earthquakes. A careful analysis of these waves can reveal the composition of rocks and metal beneath the Earth’s surface.
A new study of seismic waves propagating from two different earthquakes, in similar places but separated by a gap of 20 years, has revealed the changes that are taking place in the outer core of the Earth, the rotating layer of liquid iron and nickel between the mantle (the rock below). the surface) and the inner core (the deepest layer).
The outer core and the iron contained in it directly influence the magnetic field of our planet, which in turn provides protection against space and solar radiation that would otherwise make life on Earth impossible.
This makes understanding the outer core and its evolution over time vital. Recorded data from four seismic wave monitors in both earthquakes showed that the waves from the subsequent event traveled about a second faster as they passed through the same region of the outer core.
“Something has changed along the way of this wave, so it can go faster now,” says Virginia Tech geoscientist Ying Zhou. “The material that was there 20 years ago is no longer there.”
“This is a new material and it’s lighter. These lightweight elements will move up and change density in the region where they are.”
The types of waves analyzed here are SKS waves: they pass through the mantle as shear waves (the S), then to the outer core as compression waves (the K), then exit on the other side and return through of the mantle as more shear. waves (the second S). The timing of this journey can be revealing.
As for the two earthquakes, both were near the Kermadec Islands in the South Pacific Ocean: the first in May 1997 and the second in September 2018, giving researchers a unique opportunity to see how the core of the Earth may have changed over time.
How seismic waves travel through the outer core. (Ying Zhou)
The convection that occurs in the liquid iron of the Earth’s outer core when it crystallizes in the inner core creates flowing electric currents, which is what controls the magnetic field around us. However, the relationship between the outer core and the Earth’s magnetic field is not fully understood: much of it is based on hypothetical modeling.
“If you look at the North Geomagnetic Pole, it is currently moving at a speed of about 50 kilometers. [31 miles] per year, “says Zhou.” It’s moving away from Canada and into Siberia. The magnetic field is not the same every day. It’s changing. “
“Because it’s changing, we’re also speculating that convection in the outer core is changing over time, but there’s no direct evidence. We’ve never seen it.”
This new study, and possibly future studies like this, could provide useful information on how exactly the outer core and its convection are changing. While the changes outlined here aren’t great, the more we know, the better.
In this case, Zhou suggests that lighter elements such as hydrogen, carbon, and oxygen have been released into the outer core since 1997. It corresponds to a 2-3 percent density reduction and a velocity. convection flow of about 40 kilometers (25 miles) per hour, according to the published paper.
There are currently 152 stations on the Global Seismic Network worldwide, which monitor seismic waves in real time. While we cannot control the location or timing of earthquakes, we can make sure that as much data as possible is recorded about them.
“Now we can see it,” Zhou says. “If we are able to see it from seismic waves, in the future, we could install seismic stations and control that flow.”
The research has been published in Nature Communications Earth & Environment.