Most of the knowledge about what is at the center of our planet comes from studying seismic waves emanating from earthquakes. Careful analysis of these waves can reveal the composition of rock and metal beneath the Earth’s surface.
A new study of seismic waves propagating from two different earthquakes — in similar locations but separated by a 20-year gap — has revealed changes taking place in the Earth’s outer core, the swirling layer of liquid iron and nickel between the mantle (the underlying rock). the surface) and the inner core (the deepest layer).
The outer core and the iron it contains directly affect our planet’s magnetic field, which in turn provides protection from space and solar radiation that would otherwise make life on Earth impossible.
This makes understanding the outer core and its evolution over time crucial. Data recorded by four seismic wave monitors at both earthquakes showed waves from the later event traveling about a second faster as they passed through the same region of the outer core.
“Something has changed in the path of this wave so that it can move faster now,” says Virginia Tech geoscientist Ying Zhou. “The material that was there 20 years ago is no longer there.”
“This is new material and it’s lighter. These light elements move up and change the density in the region they are in.”
The wave types analyzed here are SKS waves: they pass through the mantle as shear waves (the S), then into the outer core as compression waves (the K), then out the other side and back through the mantle as stronger shear waves (the second S). The timing of this trip can be revealing.
The two earthquakes occurred near the Kermadec Islands in the South Pacific – the first in May 1997 and the second in September 2018, giving researchers a unique opportunity to see how the Earth’s core may have changed over time.
The convection that occurs in the liquid iron of the Earth’s outer core as it crystallizes on the inner core creates flowing electric currents that control the magnetic field around us. However, the relationship between the outer core and the Earth’s magnetic field is not fully understood – much of this is based on hypothetical models.
“If you look at the geomagnetic north pole, it’s currently moving at a speed of about 50 kilometers [31 miles] per year,” says Zhou. “It’s moving away from Canada and towards Siberia. The magnetic field is not the same every day. It’s changing.”
“Since it changes, we also suspect that convection in the outer core changes over time, but there is no direct evidence. We never saw it.”
This new study – and possibly future studies like this one – could provide useful insights into the exact changes in the outer core and its convection. While the changes noted here aren’t huge, the more we know the better.
In this case, Zhou suggests that since 1997, lighter elements such as hydrogen, carbon, and oxygen have been released in the outer core. This corresponds to a density reduction of about 2-3 percent and a convective flow rate of about 40 kilometers (25 miles) per hour, according to the published paper.
There are currently 152 Global Seismographic Network stations around the world that monitor seismic waves in real time. Although we cannot control the location or timing of earthquakes, we can ensure that as much data as possible is logged about them.
“We can see it now,” Zhou says. “If we’re able to see it from seismic waves, in the future we could set up seismic stations and monitor that flow.”
The research was published in Nature communication earth & environment.