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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.

Those changes in rotation are mainly due to magnetic forces generated by convection in Earth’s liquid outer core, says 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.

From these, the researchers identified 168 pairs of seismic waves that passed through or near the same area of the inner core – but years apart. Identifying these matches was only possible due to better constraining the changes in rotation of the inner core, says Vidale.

Both waves in each pair that didn’t pass through the inner core shared a similar pattern, suggesting nothing had changed in those areas within our planet between the first and second quake. But the waves in pairs that did intersect with the inner core didn’t match, indicating something about the core had changed beyond what could be explained by differences in rotation.

The researchers say this suggests the inner core not only slows down or speeds up its rotation over decades, but it also changes shape. They say these changes would most likely be caused by convection in the outer core pulling magnetically at the less viscous edge of the solid inner core, or by interactions between the inner core and structures in the lower mantle, the layer between our planet’s core and its crust.

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at Australian National University, who wasn’t involved with the research, says this is a “step forward” towards resolving changes in the inner core beyond rotation. But he says a change in shape isn’t the only explanation for the mismatched seismic waves.

As Vidale and his colleagues acknowledge, those differences could also be caused by unrelated changes in the outer core, convection within the inner core itself or even eruptions of melted material from the inner core. “It’s really hard to tell,” says Vidale. He suggests that studying more repeat earthquakes in the future will help identify changes in more detail.

Tkalčić says more seismological measurements in remote places, like the ocean floor, would also help. “This is critical to understanding the evolution of the Earth’s deepest interior, from the time of the planetary formation to the present day,” he says.