Earthquake Echoes Reveal Mysterious 'Donuts' Inside Earth's Core

2,890 kilometers beneath our feet lies a giant ball of liquid metal: the core of our planet. Scientists like me use seismic waves generated by earthquakes as a kind of ultrasound to “see” the shape and structure of the core.

Using a new method for studying these waves, my colleague Xiaolong Ma and I made a surprising discovery: There is a large, doughnut-shaped region in the core around the equator, a few hundred kilometers thick, where seismic waves travel about 2 percent slower than the rest of the core.

We believe this region contains lighter elements like silicon and oxygen, and may play a crucial role in the massive streams of liquid metal that run through the core and generate Earth's magnetic field. Our findings are published today in the journal Science Advances.

'Final Correlation Wave Field'

Most studies of seismic waves caused by earthquakes focus on the large primary wave fronts that propagate around the world within an hour or so after the earthquake.

We realized that we could learn something new by looking at the later, fainter part of these waves, known as the “coda”—the part that concludes the piece of music. In particular, we looked at how similar the “coda” was recorded by different seismographs, several hours after it had started.

In mathematical terms, this similarity is measured by what is called correlation. We call these similarities in the later parts of the earthquake waves the “final correlation wave field.”

By looking at the final correlation wave field, we detected tiny signals emanating from multiple resonance waves that we would not have otherwise seen. By understanding the paths these resonant waves took and matching them to the signals in the final correlation wave field, we were able to determine how long these waves took to travel across the planet.

Then we compared what we saw in the seismographs near the poles with the results we saw near the equator. Overall, the waves we saw near the poles were faster than the waves we saw near the equator.

The Earth's core, shown in red “donuts” that contain more light elements around the equator. Xiaolong Ma and Hrvoje Tkalcic

We ran many computer models and simulations of the conditions that might lead to these results in the core. Ultimately, we found that there must be a circular ring in the outer core around the equator, where waves travel more slowly.

Seismologists have not been able to detect this region before. However, using the correlation wave field allows us to “see” the outer core in greater detail and more evenly.

Previous studies have found that waves move more slowly everywhere around the “roof” of the outer core. However, in this study we have shown that the low-velocity region is only located near the equator.

Outer core and geodynamo

The radius of the Earth's outer core is about 3,480 kilometers, making it slightly larger than Mars. It is composed mainly of iron and nickel, with traces of lighter elements such as silicon, oxygen, sulfur, hydrogen, and carbon.

The bottom of the outer core is hotter than the top, and the temperature difference makes the liquid metal move like water in a pot boiling on the stove. This process is called convection, and we think the constant movement must mean that all the material in the outer core is well mixed and uniform.

But if all parts of the outer core are filled with the same material, then seismic waves should travel at roughly the same speed everywhere too. So why do they slow down in the circular region we found?

We believe that there must be a higher concentration of lighter elements in this region. These elements may be released from the Earth's solid inner core into the outer core, where their buoyancy creates more convection.

Why do lighter elements accumulate more in the equatorial donut region? Scientists think this could be explained if more heat is transferred from the outer core to the rocky mantle above it in this region.

A cross section of the Earth's core, showing the “donut” containing more light elements around the equator. Ma Wtkalcic / Scientific Progress

There's another planetary-level process going on in the planet's outer core. The rotation of the Earth and its small, solid inner core causes the fluid in the outer core to organize itself into long, vertical vortices that run north-south, like giant waterways.

The turbulent motion of liquid metal in these vortices creates the “geological dynamo” responsible for creating and maintaining Earth’s magnetic field. This magnetic field protects the planet from harmful solar winds and radiation, making life possible on the surface.

Gaining a more detailed view of the composition of the outer core—including the new mass of lighter elements—would help us better understand Earth’s magnetic field. In particular, how the field changes in strength and direction over time is crucial to life on Earth and the habitability of planets and exoplanets.

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