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They confirm that the rotation of the Earth’s inner core has slowed down.

The motion of Earth’s inner core, a solid sphere of iron and nickel the size of the moon located more than 4,800 km below the surface, has been the subject of debate in the scientific community for two decades. Some … The researchers say the core’s speed slowed, although it was still spinning faster than the surface. A new study from the University of Southern California (USC) has found a change in this dynamic. The core is slowing down, yes, but so much so that it is moving slower than the surface of the Earth. This phenomenon, as they write in the journal Nature, began around 2010 and occurred for the first time in forty years. The changes can change the length of the day by fractions of a second.

“When I first saw the seismograms hinting at this change, I was puzzled,” admits John Vidale, a senior professor of geosciences at the University of Southern California. “But when we found two dozen more observations pointing to the same pattern, the result became inevitable. The inner core slowed for the first time in decades. “Other scientists have recently advocated similar and different models, but our latest study provides the most convincing solution yet,” says the researcher.

The authors believe the inner core is flipping and moving away relative to the planet’s surface because it is moving slightly slower, rather than faster, than Earth’s mantle for the first time in about 40 years. Compared to the speed of previous decades, the inner core is slowing down.

The inner core, surrounded by a liquid outer core of iron and nickel, poses a challenge to researchers: it cannot be visited or seen. Scientists must use seismic waves from earthquakes to create an idea of their movement.

Earthquakes and nuclear tests

Vidale and Wei Wang of the Chinese Academy of Sciences used data from repeated earthquakes, seismic events occurring in the same location, to create identical seismograms. So they collected and analyzed seismic data recorded around the South Sandwich Islands from 121 repeat earthquakes that occurred between 1991 and 2023. They also used data from twin Soviet nuclear tests between 1971 and 1974, as well as French and American nuclear tests.

According to Vidale, the slowing of the inner core was caused by the churning of the surrounding liquid iron outer core, which generates the Earth’s magnetic field, as well as the gravitational pull of dense regions of the overlying rocky mantle.

The consequences of this change in the internal motion of the core for the Earth’s surface can only be speculated. Vidale believes that the recoil of the inner core can change the length of the day by a fraction of a second. Of course, “it’s very difficult to notice, about a thousandth of a second,” he reassures.

Future research by USC scientists aims to map out the trajectory of the inner core in more detail to find out exactly why it is changing. “The dance of the inner core may be even more animated than we know so far,” warns Vidale.

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