Earth’s inner core

Secrets of Earth’s inner core revealed by large quakes

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Seismic waves travel differently through innermost core than through outer section.

Earth core structure 3D illustration. Cross section of planet with visible layers on space backround
Seismic waves have helped researchers to learn about the layers that comprise Earth’s solid centre.

The reverberations from earthquakes as they bounce back and forth through the centre of Earth have revealed new details about the structure of the planet’s inner core, according to a study published in Nature Communications this week1.

For several decades, evidence has been mounting to suggest that the planet’s solid inner core is made up of distinct layers2,3 but their properties have remained mysterious.

To better understand the inner core’s structure, researchers used multiple seismometers to examine how seismic waves are distorted as they pass through the solid ball of iron nickel at Earth’s heart. “Earth oscillates like a bell after a large earthquake, and not just for hours, but days,” says co-author Hrvoje Tkalčić, a geophysicist at the Australian National University in Canberra, Australia.

To detect these oscillations, researchers recorded the waveforms at close to the original site of the earthquake and at the antipode —the direct opposite position on the surface of Earth. This enabled them to look at the multiple journeys through Earth’s centre. “It’s like a ping-pong ball that’s bouncing back and forth,” says co-author Thanh-Son Pham, a postdoctoral fellow at the Australian National University. Each reverberation takes around twenty minutes to cross from one side of the planet to the other, and the seismometers recorded up to five bounces from a single event.

Stacked measurements

The original earthquakes each reached a magnitude of greater than six, but the waves got progressively weaker as they passed through Earth’s core. The researchers used a technique called ‘stacking’, in which they combined the waveforms from a single event to build a more detailed picture of the distortion from the innermost core.

They found that the waves travelled differently through the innermost inner core — which they estimate to be around 650 kilometres thick — than through the outer part. Waves passing through the innermost part of the core slowed down in one direction, whereas waves passing through the outer layer slowed down in another direction. “It simply means that the iron crystals — iron, which is dominant in the inner core — is probably organized in a different way than in the outer shell of the inner core,” Tkalčić says.

Geophysicist Vernon Cormier at the University of Connecticut in Storrs says that the study is important because it offers a measurement of Earth’s innermost section that was very difficult to achieve. “It requires finding seismic waves recorded at very long distance and that are fairly weak in amplitude, and then enhancing the amplitude so that you could measure the wave speed in the very deep interior of the Earth,” says Cormier.

Although the technique is routinely used for minerals exploration, it is not commonly used in geophysics.

The latest finding will help in understanding how Earth’s solid inner core formed — a process that is thought to have started somewhere between 600 million and 1.5 billion years ago — and what role that might have had in shaping the magnetic field.

Secrets of Earth’s inner core revealed by large quakes

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Seismic waves travel differently through innermost core than through outer section.

Earth core structure 3D illustration. Cross section of planet with visible layers on space backround
Seismic waves have helped researchers to learn about the layers that comprise Earth’s solid centre.

The reverberations from earthquakes as they bounce back and forth through the centre of Earth have revealed new details about the structure of the planet’s inner core, according to a study published in Nature Communications this week1.

For several decades, evidence has been mounting to suggest that the planet’s solid inner core is made up of distinct layers2,3 but their properties have remained mysterious.

To better understand the inner core’s structure, researchers used multiple seismometers to examine how seismic waves are distorted as they pass through the solid ball of iron nickel at Earth’s heart. “Earth oscillates like a bell after a large earthquake, and not just for hours, but days,” says co-author Hrvoje Tkalčić, a geophysicist at the Australian National University in Canberra, Australia.

To detect these oscillations, researchers recorded the waveforms at close to the original site of the earthquake and at the antipode —the direct opposite position on the surface of Earth. This enabled them to look at the multiple journeys through Earth’s centre. “It’s like a ping-pong ball that’s bouncing back and forth,” says co-author Thanh-Son Pham, a postdoctoral fellow at the Australian National University. Each reverberation takes around twenty minutes to cross from one side of the planet to the other, and the seismometers recorded up to five bounces from a single event.

Stacked measurements

The original earthquakes each reached a magnitude of greater than six, but the waves got progressively weaker as they passed through Earth’s core. The researchers used a technique called ‘stacking’, in which they combined the waveforms from a single event to build a more detailed picture of the distortion from the innermost core.

They found that the waves travelled differently through the innermost inner core — which they estimate to be around 650 kilometres thick — than through the outer part. Waves passing through the innermost part of the core slowed down in one direction, whereas waves passing through the outer layer slowed down in another direction. “It simply means that the iron crystals — iron, which is dominant in the inner core — is probably organized in a different way than in the outer shell of the inner core,” Tkalčić says.

Geophysicist Vernon Cormier at the University of Connecticut in Storrs says that the study is important because it offers a measurement of Earth’s innermost section that was very difficult to achieve. “It requires finding seismic waves recorded at very long distance and that are fairly weak in amplitude, and then enhancing the amplitude so that you could measure the wave speed in the very deep interior of the Earth,” says Cormier.

Although the technique is routinely used for minerals exploration, it is not commonly used in geophysics.

The latest finding will help in understanding how Earth’s solid inner core formed — a process that is thought to have started somewhere between 600 million and 1.5 billion years ago — and what role that might have had in shaping the magnetic field.

Source: Nature

Why Earth’s Inner Core May Be Slowing Down

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The planet’s solid inner core might rotate at a different rate than the rest of the planet, and that rate might be changing

Why Earth's Inner Core May Be Slowing Down

Earth’s core structure. Elements of this image were furnished by NASA.

The spin of Earth’s inner core may have slowed, with the heart of the planet now rotating at a slightly more sluggish clip than the layers above, new research finds. The slowdown could change how rapidly the entire planet spins, as well as influence how the core evolves with time.

For the new study, published in the journal Nature Geoscience, scientists used a database of earthquakes to probe the behavior of Earth’s solid inner core over time. The inner core sits suspended like a ball bearing in the molten-metal ocean of the outer core. Because of this liquid cocoon, the “ball bearing” may not spin at the same rate as the rest of the planet. Over the years, some researchers have found that the core rotates slightly faster than the mantle and crust, a condition called “super rotation.” But studies have not returned consistent numbers, with the first study to observe differential core rotation estimating that the inner core rotates up to one degree faster per year than the rest of the planet; others found an annual speedup of just tiny fractions of a degree.

These differences aren’t dramatic. The variation in rotation time between the inner core and the rest of Earth is very minor. Nor are the differences a threat to life on the surface: In contrast to the 2003 science-fiction movie The Core, there’s no need to call in a crack team of geophysicists and astronauts to drill to the center of our planet and start blowing things up. At most, the inner core rotation might influence Earth’s overall spin and contribute to fluctuations in the planet’s magnetic field. Each year the core expands by about a millimeter, as some of the molten iron in the outer core solidifies, seismic studies have shown. The solidification also drives the circulation of the outer core, which, in turn, creates the planet’s magnetic field. The rotation of the inner core could influence this solidification process in ways that are not yet fully understood, thus impacting the magnetic field, says study author Xiaodong Song, a geophysicist at Peking University in China.

The rotation might also matter for how the inner core grows over billions of years, says John Vidale, a geophysicist at the University of Southern California who was not involved in the study, but who has researched core rotation.

The catch, however, is that no one really knows how fast the inner core spins. In the new study, Song and geophysicist Yi Yang, also at Peking University, found that the core appeared to hold a steady spin, faster than the overall spin of Earth, between the 1970s and the early 2000s. Around 2009, though, that spin rather abruptly slowed to match Earth’s speed and then perhaps slowed so much that the rest of the planet now spins faster, Song says.

Song and Yang measured this spin by using pairs of almost-identical earthquakes that originated at the same spots, separated only by time. Because the quakes are nearly identical, their shock waves should also look identical when they travel through the core and back out, where they are detected by seismometers around the planet—that is, unless the core itself changes and alters the path of one earthquake’s waves relative to the other. If the core is spinning differently than the rest of the planet, identical earthquake waves that happen months or years apart will hit the core at slightly different spots and therefore bounce back with some subtle differences. The researchers compared quake waves going back to 1964 to track the changes in how the core might be moving over time. If they’re right, the spin of the core now lags that of the overall planet by a tiny amount.

“We are hypothesizing that this [slowed rotation] will continue in the coming years and decades, and we should be able to see that in [our] relatively short human time frame,” Song says.

The new findings likely won’t end the debate over the inner core. The work is well done and does an admirable job of combining different data, Vidale says. But there are several competing explanations for what’s going on. For example, Vidale’s research hints that the core may alter its rotation every six years or so, while researchers Guanning Pang and Keith Koper reported a single “lurch” in the early 2000s and little change since in a 2022 study. “I don’t view [the new work] as entirely conclusive,” Vidale says.

Lianxing Wen, a geodynamicist at Stony Brook University, who was not involved in the new study, also researches the core’s spin. He doesn’t believe the inner core spins any differently than the rest of the planet. A better explanation for the changes in seismic waves that travel through the core, Wen says, is that the surface of the inner core isn’t smooth like a ball bearing but rather uneven and constantly changing. “We believe the inner core has a shifting topography that best explains observed temporal changes of seismic waves that reflect off the inner core,” he says. The new research, Wen says, misinterprets these changes as caused by the core’s spin rather than to its fluctuating surface.

Fortunately, Song says, the seismic monitoring of Earth is better than ever, yielding far richer data about the planet’s interior than in earlier decades. By continuing to watch earthquake waves, the researchers should be able to show whether they’re right about the inner core’s spin.

“The exciting news,” Song says, “is that we don’t have to wait too long.”

Has Earth’s inner core stopped its strange spin?

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Earthquake data hint that the inner core stopped rotating faster than the rest of the planet in 2009, but not all researchers agree.

Earth's core, illustration.
Earth’s inner core is made mostly of solid iron, and can rotate separately from the outer parts of the planet.

Thousands of kilometres beneath your feet, Earth’s interior might be doing something very weird. Many scientists think that the inner core spins faster than the rest of the planet — but sometime in the past decade, according to a study, it apparently stopped doing so.Mars’s core has been measured — and it’s surprisingly large

“We were quite surprised,” say Yi Yang and Xiaodong Song, seismologists at Peking University in Beijing who reported the findings today in Nature Geoscience1.

The results could help to shine light on the many mysteries of the deep Earth, including what part the inner core plays in maintaining the planet’s magnetic field and in affecting the speed of the whole planet’s rotation — and thus the length of a day. But they are just the latest instalment in a long-running effort to explain the inner core’s unusual rotation, and might not be the final word on the matter.

“I keep thinking we’re on the verge of figuring this out,” says John Vidale, a seismologist at the University of Southern California in Los Angeles. “But I’m not sure.”

Mysteries of the deep

Researchers discovered the inner core in 1936, after studying how seismic waves from earthquakes travel through the planet. Changes in the speed of the waves revealed that the planet’s core, which is about 7,000 kilometres wide, consists of a solid centre, made mostly of iron, inside a shell of liquid iron and other elements. As iron from the outer core crystallizes on the surface of the inner core, it changes the density of the outer liquid, driving churning motions that maintain Earth’s magnetic field.

Ebeko Volcano, Paramushir Island, Kuril Islands, Russia.
Researchers have learnt about the inner core’s rotation by studying earthquakes that originated in the same region, such as the Kuril Islands (shown here), over decades.

The liquid outer core essentially decouples the 2,400-kilometre-wide inner core from the rest of the planet, so the inner core can spin at its own pace. In 1996, Song and another researcher reported2 studying earthquakes that originated in the same region over three decades, and whose energy was detected by the same monitoring station thousands of kilometres away. Since the 1960s, the scientists said, the travel time of seismic waves emanating from those earthquakes had changed, indicating that the inner core rotates faster than the planet’s mantle, the layer just beyond the outer core.

Later studies refined estimates of the rate of that ‘super-rotation’, to conclude that the inner core rotates faster than the mantle by about one-tenth of a degree per year. But not everyone agrees. Other work has suggested that super-rotation happens mostly in distinct periods, such as in the early 2000s, rather than being a continuous, steady phenomenon3. Some scientists even argue that super-rotation does not exist, and that the differences in earthquake travel times are instead caused by physical changes on the surface of the inner core4.

Last June, Vidale and Wei Wang, an Earth scientist also at the University of Southern California, threw another spanner into the works. Using data on seismic waves generated by US nuclear test blasts in 1969 and 1971, they reported that between those years, Earth’s inner core had ‘subrotated’, or rotated more slowly than the mantle5. Only after 1971, they say, did it speed up and begin to super-rotate.

A rotational shift

Now, Yang and Song say that the inner core has halted its spin relative to the mantle. They studied earthquakes mostly from between 1995 and 2021, and found that the inner core’s super-rotation had stopped around 2009. They observed the change at various points around the globe, which the researchers say confirms it is a true planet-wide phenomenon related to core rotation, and not just a local change on the inner core’s surface.Earth’s magnetic field is acting up and geologists don’t know why

The data hint that the inner core might even be in the process of shifting back towards subrotation. If so, something is probably happening to the magnetic and gravitational forces that drive the inner core’s rotation. Such changes might link the inner core to broader geophysical phenomena such as increases or decreases in the length of a day on Earth.

Still, many questions remain, such as how to reconcile the slow pace of the changes that Yang and Song report with some of the faster changes reported by others. The only way out of the morass is to wait for more earthquakes to happen. A “long history of continuous recording of seismic data is critical for monitoring the motion of the heart of the planet”, say Yang and Song.

“We just have to wait,” Vidale adds.