Forces deep underground seem to be deforming Earth’s inner core

Seismic waves suggest the planet’s solid inner core is being pulled out of shape – and it has undergone these changes over just a few decades

By James Dinneen

10 February 2025

An illustration showing Earth’s inner structure

Rostislav Zatonskiy/Alamy

Earth’s solid inner core appears to have changed shape in the past 20 years or so, according to seismic wave measurements – but the behaviour of these waves could also be explained by other shifts at the centre of the planet.

Since the 1990s, models and seismic measurements have indicated that Earth’s iron-nickel inner core moves at its own pace. Over decades, the rotation of the inner core speeds up and slows down relative to the rest of the planet, affecting things such as the length of a day.

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Those changes in rotation are mainly due to magnetic forces generated by convection in Earth’s liquid outer core, says John Vidale at the University of Southern California. “That flow is continually torquing the inner core.”

Those magnetic forces, or related processes, could change the shape of the inner core as well as its rotation – in fact, some previous measurements of seismic waves passing through the planet’s centre seemed to indicate just that. But uncertainty about the core’s rotation made it impossible to distinguish between a change in rotation and a change in shape.

Now, Vidale and his colleagues have analysed seismic waves generated by 128 earthquakes off the coast of South America between 1991 and 2023. The waves were all measured by instruments in Alaska after passing through the planet.