Earth’s outer core is changing, as evidenced by seismic wave data

(ORDO NEWS) — Much of our knowledge of what is at the center of our planet comes from studying the seismic waves generated by earthquakes. Careful analysis of these waves reveals the composition of rocks and metal below the Earth‘s surface.

A new study of seismic waves propagating from two different earthquakes – at similar locations but 20 years apart – has revealed changes occurring in the Earth’s outer core, a vortex 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 it contains directly affect our planet’s magnetic field, which in turn provides protection from cosmic and solar radiation that would otherwise make life on Earth impossible.

This makes understanding the outer core and its evolution over time vital. Data recorded from four seismic wave monitors during both earthquakes showed that the waves from the later event traveled about one second faster, passing through the same region of the outer core.

“Something has changed in the path of this wave, so now it can go faster,” says geophysicist Ying Zhou of the Virginia Institute of Technology. “The material that was there 20 years ago is no longer there.”

“It’s a new material and it’s lighter. These lighter 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 (S), then into the outer core as compression waves (K), then exit the other side and again pass through the mantle as even more shear waves (second S ). The timing of these movements can be indicative.

As for the two earthquakes, they both 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.

Earths outer core is changing as evidenced by seismic wave data 2

The convection that occurs in the liquid iron of the Earth’s outer core as it crystallizes on the inner core creates fluid electrical currents that drive the magnetic field around us. However, the relationship between the outer core and the Earth’s magnetic field is not fully understood – much is based on hypothetical modeling.

“If you look at the north geomagnetic pole, it’s currently moving at about 50 kilometers [31 miles] a year,” Zhou says. It’s moving away from Canada and moving towards Siberia. “The magnetic field is not the same every day. It changes.”

“Because it changes, we also assume that the convection in the outer core changes with time, but there is no direct evidence. We have never seen this.”

This new study – and potentially future ones like it – could provide useful insights into exactly how the outer core and its convection are changing. While the changes noted here are not 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 decrease in density of about 2-3 percent and a convective flow velocity of about 40 kilometers (25 miles) per hour, according to the published paper.

Currently, there are 152 stations of the Global Seismographic Network in the world that track 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.

“We can see it now,” Zhou says. “If we can see this from seismic waves, then in the future we can set up seismic stations and track this flow.”


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