Researchers measured the speeds at which seismic waves penetrate and pass through Earth’s inner core.
The inner layers of Earth.
In Short
Researchers analysed data from about 200 magnitude-6 earthquakes
The team assessed seismic waves that travel directly through the Earth’s center
The team analysed the variation of travel times of seismic waves
By India Today Web Desk: Researchers trying to uncover the secrets of Earth’s geology have revealed the fifth layer of the planet. Seismic waves generated by earthquakes have revealed new insights about the deepest parts of Earth’s inner core.
The team of researchers from the Australian National University measured the speeds at which these seismic waves penetrate and pass through the Earth’s inner core. The team believes that this has presented evidence of a distinct layer inside Earth known as the innermost inner core.
This layer is a solid ‘metallic ball’ that sits within the center of the inner core. The findings of the study have been published in the journal Nature Communications, which states that probing the Earth’s center is critical for understanding planetary formation and evolution.
So far, four layers of Earth’s structure were found.
“The existence of an internal metallic ball within the inner core, the innermost inner core, was hypothesised about 20 years ago. We now provide another line of evidence to prove the hypothesis,” Dr. Thanh-Son Phạm, from the ANU Research School of Earth Sciences, said.
So far, four layers of Earth’s structure had been identified. This includes – the crust, mantle, outer core, and inner core. The new findings indicate a fifth layer beneath that.
The team assessed the seismic waves that travel directly through the Earth’s center and ‘spit out’ at the opposite side of the globe to where the earthquake was triggered. The waves then travel back to the source of the quake. The team studied the earthquake, which originated in Alaska. The waves bounced off somewhere in the south Atlantic Ocean, before traveling back to Alaska.
“By stacking waveforms recorded by a growing number of global seismic stations, we observe up-to-fivefold reverberating waves from selected earthquakes along the Earth’s diameter,” the paper read.
The researchers studied the anisotropy of the iron-nickel alloy that comprises the inside of the Earth’s inner core. Anisotropy is used to describe how seismic waves speed up or slow down through the material of the Earth’s inner core, depending on the direction in which they travel. They found that bouncing seismic waves repeatedly probed spots near the Earth’s center from different angles.
The team analysed the variation of travel times of seismic waves for different earthquakes. They found that crystallised structure within the inner core’s innermost region is likely different from the outer layer. The team suspects that there could have been a major global event at some point during Earth’s evolutionary timeline that led to a “significant” change in the crystal structure of the inner core.
Seismic waves travel differently through innermost core than through outer section.
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.
The seismic waves began roughly 15 miles off the shores of Mayotte, a French island sandwiched between Africa and the northern tip of Madagascar. The waves buzzed across Africa, ringing sensors in Zambia, Kenya, and Ethiopia. They traversed vast oceans, humming across Chile, New Zealand, Canada, and even Hawaii nearly 11,000 miles away.
These waves didn’t just zip by; they rang for more than 20 minutes. And yet, it seems, no human felt them.
Only one person noticed the odd signal on the U.S. Geological Survey’s real-time seismogram displays. An earthquake enthusiast who uses the handle @matarikipax saw the curious zigzags and posted images of them to Twitter. That small action kicked off another ripple of sorts, as researchers around the world attempted to suss out the source of the waves. Was it a meteor strike? A submarine volcano eruption? An ancient sea monster rising from the deep?
“I don’t think I’ve seen anything like it,” says Göran Ekström, a seismologist at Columbia University who specializes in unusual earthquakes.
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“It doesn’t mean that, in the end, the cause of them is that exotic,” he notes. Yet many features of the waves are remarkably weird—from their surprisingly monotone, low-frequency “ring” to their global spread. And researchers are still chasing down the geologic conundrum.
Why are the low-frequency waves so weird?
In a normal earthquake, the built-up tensions in Earth’s crust release with a jolt in mere seconds. This sends out a series of waves known as a “wave train” that radiates from the point of the rupture, explains Stephen Hicks, a seismologist at the University of Southampton.
The fastest-traveling signals are Primary waves, or P-waves, which are compression waves that move in bunches, like what happens to an extended slinky that gets suddenly pushed at one end. Next come the secondary waves, or S-waves, which have more of a side-to-side motion. Both of these so-called body waves have relatively high frequencies, Hicks says, “a sort of ping rather than a rumbling.”
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Finally, chugging along at the end come slow, long-period surface waves, which are similar to the strange signals that rolled out from Mayotte. For intense earthquakes, these surface waves can zip around the planet multiple times, ringing Earth like a bell, Hicks says.
However, there was no big earthquake kicking off the recent slow waves. Adding to the weirdness, Mayotte’s mystery waves are what scientists call monochromatic. Most earthquakes send out waves with a slew of different frequencies, but Mayotte’s signal was a clean zigzag dominated by one type of wave that took a steady 17 seconds to repeat.
“It’s like you have colored glasses and [are] just seeing red or something,” says Anthony Lomax, an independent seismology consultant.
Mayotte’s volcanic roots
Based on the scientific sleuthing done so far, the tremors seem to be related to a seismic swarm that’s gripped Mayotte since last May. Hundreds of quakes have rattled the small nation during that time, most radiating from around 31 miles offshore, just east of the odd ringing. The majority were minor trembles, but the largest clocked in at magnitude 5.8 on May 15, the mightiest in the island’s recorded history. Yet the frequency of these shakes has declined in recent months—and no traditional quakes rumbled there when the mystery waves began on November 11.
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The French Geological Survey (BRGM) is closely monitoring the recent shaking, and it suggests that a new center of volcanic activity may be developing off the coast. Mayotte was formed from volcanism, but its geologic beasts haven’t erupted in over 4,000 years. Instead, BRGM’s analysis suggests that this new activity may point to magmatic movement offshore—miles from the coast under thousands of feet of water. Though this is good news for the island inhabitants, it’s irksome for geologists, since it’s an area that hasn’t been studied in detail.
“The location of the swarm is on the edge of the [geological] maps we have,” says Nicolas Taillefer, head of the seismic and volcanic risk unit at BRGM. “There are a lot things we don’t know.” And as for the November 11 mystery wave, he says, “it’s something quite new in the signals on our stations.”
The early period of rumbling was also overprinted with what seemed to be the P- and S- waves of tiny tremors, explains Lomax, who spotted the faint pings by filtering out the low-frequency signals. Such pings are commonly associated with magma moving and fracturing rock as it squirts through the crust. But even those signals were a little strange, says Helen Robinson, a Ph.D. candidate in applied volcanology at the University of Glasgow.
“They’re too nice; they’re too perfect to be nature,” she jokes, although she quickly adds that an industrial source is impossible, since no wind farms or drilling are taking place in the deep waters off Mayotte’s shores.
Ekström thinks that the events on the morning of November 11 actually did begin with an earthquake of sorts equivalent to a magnitude 5 temblor. It passed by largely unnoticed, he suggests, because it was what’s known as a slow earthquake. These quakes are quieter than their speedy cousins since they come from a gradual release of stress that can stretch over minutes, hours, or even days.
“The same deformation happens, but it doesn’t happen as a jolt,” Ekström says.
So what is actually causing the super-slow vibrations at Mayotte? A submarine eruption could produce these low rumblings, but evidence for such an event has yet to materialize.
Most current guesses revolve around resonance in a magma chamber, triggered by some type of subsurface shift or chamber collapse. The resonance itself can be any type of rhythmic motion, like sloshing of the molten rock, or a pressure wave ricocheting through the magma body, Ekström explains. Studying the intricate features of the seismic waves could yield clues to the size and shape of the molten material lurking below.
It is very difficult, really, to say what the cause is and whether anyone’s theories are correct.
Helen Robinson, University of Glasgow
“It’s like a music instrument,” says Jean-Paul Ampuero, a seismologist at the Université Côte d’Azur in France. “The notes of a music instrument—whether it’s grave or very pitchy—depends on the size of the instrument.”
The signal’s odd uniformity could be due, in part, to the surrounding rocks and sediments, Lomax adds. Perhaps the local geology is filtering the sounds and only letting this single 17-second wave period escape.
Robinson agrees with this idea, noting that the geology here is extremely complex. Mayotte sits in a region crisscrossed by ancient faults—including fracture zones from the final breakup of the southern supercontinent Gondwana. What’s more, the underlying crust is somewhat transitional, shifting between the thick continental crusts and the thinner oceanic crusts. Perhaps this complexity drives the simplicity of the escaping waves, Robinson says.
Secrets of the sea
For now, though, the lack of data makes it tough to say more about the wiggly forms. Hicks’ preliminary models hinted that the waves emanated from subsurface inflation, rather than a magma chamber draining or collapsing. But with a little additional data, the model flipped and pointed to chamber deflation instead.
It also could be a bit of both, notes Robinson: “Some collapse mechanisms, you can get inflation and deflation occurring at the same time,” she says. Or sometimes they can alternate, pumping up and down like Earth’s fiery lungs.
“It is very difficult, really, to say what the cause is and whether anyone’s theories are correct—whether even what I’m saying has any relevance to the outcome of what’s going on,” Robinson says.
BRGM plans to do ocean bottom surveys to get more detailed information about the region and investigate the possibility of a submarine eruption. In the meantime, the seismic sleuthing continues with the data that’s available. Whether the cause is ordinary or extraordinary remains to be seen, Lomax says, but the science—and the fun—is in the chase.
“Depending on what field and what time in history, 99.9 percent of the time, it’s ordinary, or noise, or a mistake, and 0.1 percent, it’s something” he says. “But that’s just the way it goes. That’s the way it should go. That’s scientific advance.”
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