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The Earth is shaking under our feet


The inner core of the Earth moves slightly back and forth every six years, which explains the variations in the time it takes for our planet to rotate on its axis and that one day is a few times longer than another.

A new study has found something surprising: Earth’s inner core undergoes oscillations that contradict the theory that it is constantly spinning at an ever-increasing speed, relative to the planet’s surface.

The study was carried out by a team of experts from the University of Southern California, (USA) and has been published in the scientific journal Science Advances.

The core of the Earth is the innermost part of the Earth, located almost 3,000 kilometers deep. It consists of an inner part that is solid, with a liquid outer core around it. Both consist of metals, mainly iron.

The outer core is liquid because the temperature is thousands of degrees, but in the inner core the pressure is so high that the metal solidifies into a sphere. That sphere is about the size of Pluto and grows slowly.

round trip kilometers

That sphere rotates, just like our planet. But in the 1990s, scientists discovered that that core doesn’t rotate completely in sync with the rest of Earth.

This occurs because the mass surrounding the inner core is liquid. The core rotates slightly faster than the mantle and the Earth’s crust, about one degree every million years. Until now, this was thought to be stable behavior.

What the new research has discovered is that the Earth’s core does not rotate in a stable manner as had been theorized, but instead fluctuates slightly relative to the outer core, moving a little faster for six years and then a little slower. In total, he travels several kilometers on his annual round trips.

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Scientists are trying to understand how the inner core formed and how it moves over time, because they are interested in better understanding the oscillation it exhibits. They hope that future research will provide sufficiently precise observations to compare with the results obtained in this study.

Revealing seismographs

The problem is that it is impossible to directly observe the inner core of the Earth, which means that researchers use indirect measurements to explain the pattern, speed, cause of movement and changes in the core.

To achieve this, scientists work with seismographs, which measure the movement of earthquakes or explosions. Nuclear tests are also tracked this way.

When several of these devices are set up close to each other as a network, researchers can follow the impact of nuclear tests accurately, even over different time periods.

They use measurements from underground nuclear bomb experiments that took place decades ago, as the vibrations caused by an atomic bomb experiment travel through the Earth’s crust and mantle: they bounce like a ball against the hard inner core, when they return to the surface.

two observations

That means that the vibrations caused by atomic explosions are affected by the movement of the Earth’s core: if the vibration returns to the surface at different times, it indicates that the Earth’s core moved, as well as how and in which direction, when the atomic explosion occurred.

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This same research team, led by John Vidalhad previously analyzed data from nuclear experiments that the Soviet Union had carried out in the Arctic archipelago Nova Zembla between 1971 and 1974, to track the behavior of the Earth’s core.

In the new research, he applied the same method to analyzing data from American experiments in the Amchitka Island in 1969, off Alaska.

In the first observation he had found that the inner core was spinning slower than expected, about 0.1 degrees per year. But on the second observation the surprise arose: the inner core had reversed direction, turning at least a tenth of a degree per year.

proven relationship

The new study supports speculation that the inner core oscillates based on variations in day length, among other causes.

Every six years, the Earth takes about 0.2 seconds less to rotate on its axis, and then it takes longer again. That fluctuating duration from day to day can be explained by a fluctuating core, the researchers say. Until now, no tangible evidence had been found that this was the cause, and it is the main result provided by the new research.


Seismological observation of Earth’s oscillating inner core. Wei Wang, John E. Vidale. Science Advances, 10 Jun 2022; Vol 8, Issue 23. DOI: 10.1126/sciadv.abm9916

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