Earthquakes suggest Earth’s core has started spinning more slowly

Measurements of seismic waves passing through the Earth’s inner core show that the Earth’s rotation is slowing and may be switching its direction relative to the rest of the planet’s rotation.

To Leah Crane

Our planet’s solid inner core may be preparing to slow its rotation and switch spin directions relative to the rest of the planet. appears to be part of a cycle lasting about 60 years, slowing down again.

Beneath the Earth’s mantle is a stirred layer of molten iron and nickel, with a dense inner core of iron held firm by intense pressure at the planet’s core. The movement of the inner core relative to the mantle and surface has been debated for decades, but earthquake measurements are now helping researchers understand it better.

Yi Yang, Xiaodong Song, and their colleagues at Peking University in China analyzed seismic waves from nearly identical earthquakes that have passed through the Earth’s core over the past 60 years or so. If the solid core of the Earth were perfectly spherical and had the same structure throughout, we would expect each set of waves to look exactly the same, regardless of when they passed. But that’s not the case, so we can use wave differences to measure changes deep underground.

Researchers found that by around 2009, the planet’s core appeared to be spinning slightly faster than its mantle and surface. However, around 2009, this rotation began to slow down. If you can look down at the core here, those measurements show that the core isn’t spinning at all, since it’s spinning at about the same speed as the surface.

“While this does mean it’s not the steady rotation that was first reported 20 years ago, it’s actually more complicated,” says Bruce Buffett of the University of California, Berkeley. Yang and Song’s measurements show that the last turning point in the inner core’s rotation was in his early 1970s, so the spin rate appears to oscillate regularly.

“There are several different ideas about how the inner core works. This idea of ​​steady motion with slowdowns at the beginning and end of about 50 years is probably the main idea, but it doesn’t explain everything. No,” says John. Vidale of the University of Southern California. Notably, he says, he doesn’t explain why the core spin rate of change appears to have been much higher in the period from 2001 to 2003 than at other times. “But my guess is that there’s something else going on as well, so it’s not such a bad flaw that all the data isn’t accounted for by his one model.”

Oscillations are most likely caused by interactions between the solid mantle and the inner core. Neither are perfect spheres, so the gravitational pull of each blob or ridge pulls them toward each other. The mantle is much heavier than the inner core, but its effect on the planet’s outer layers is less noticeable.

This is supported by measurements of small changes in day length on the Earth’s surface that fluctuate slightly. Changes in the rotation of the inner core are also expected to affect the planet’s magnetic field, but only on a relatively small scale.

“People are unnerved by the idea of ​​the Earth’s magnetic field reversing, but it’s nothing like that, it’s a small impact,” Buffett says. “The flow in the core changes the magnetic field a little bit, changing the length of a day by about 1 millisecond to 10 minutes in a year.”

But measuring these tiny changes in seismic waves, magnetic fields, and day length is so difficult that we can never know exactly what is happening at the center of the Earth.

“I wish I could say this is final, but I think we still have work to do to arrive at a final explanation,” says Vidale. “Since these waves have very high frequencies across the globe, it is difficult to simulate these waves, and some measurements are rather uncertain and contradictory.” More observations over the years will help researchers sort it out.