In September 2023, seismic detectors around the world began picking up a mysterious signal. Something – it wasn’t clear what – was causing the entire Earth to shake every 90 seconds. After a period of puzzlement, and a second, similar signal in October, theoretical studies proposed an explanation. The tremors, these studies suggested, were caused by standing waves, or seiches, that formed after landslides triggered huge tsunamis in a narrow waterway off the coast of Greenland.

Engineers at the University of Oxford, UK, have now confirmed this hypothesis. Using satellite altimetry data from the Surface Water Ocean Topography (SWOT) mission, the team constructed the first images of the seiches, demonstrating that they did indeed originate from landslide-triggered mega-tsunamis in Dickson Fjord, Greenland. While events of this magnitude are rare, the team say that climate change is likely to increase their frequency, making continued investments in advanced satellite missions essential for monitoring and responding to them.

An unprecedented view into the fjord

Unlike other altimeters, SWOT provides two-dimensional measurements of sea surface height down to the centimetre across the entire globe, including hard-to-reach areas like fjords, rivers and estuaries. For team co-leader Thomas Monahan, who studied the seiches as part of his PhD research at Oxford, this capability was crucial. “It gave us an unprecedented view into Dickson Fjord during the seiche events in September and October 2023,” he says. “By capturing such high-resolution images of sea-surface height at different time points following the two tsunamis, we could estimate how the water surface tilted during the wave – in other words, gauge the ‘slope’ of the seiche.”

The maps revealed clear cross-channel slopes with height differences of up to two metres. Importantly, these slopes pointed in opposite directions, showing that water was moving backwards as well as forwards across the channel. But that wasn’t the end of the investigation. “Finding the ‘seiche in the fjord’ was exciting but it turned out to be the easy part,” Monahan says. “The real challenge was then proving that what we had observed was indeed a seiche and not something else.”

Enough to shake the Earth for days

To do this, the Oxford engineers approached the problem like a game of Cluedo, ruling out other oceanographic “suspects” one by one. They also connected the slope measurements with ground-based seismic data that captured how the Earth’s crust moved as the wave passed through it. “By combining these two very different kinds of observations, we were able to estimate the size of the seiches and their characteristics even during periods in which the satellite was not overhead,” Monahan says.

Although no-one was present in Dickson Fjord during the seiches, the Oxford team’s estimates suggest that the event would have been terrifying to witness. Based on probabilistic (Bayesian) machine-learning analyses, the team say that the September seiche was initially 7.9 m tall, while the October one measured about 3.9 m.

“That amount of water sloshing back and forth over a 10-km-section of fjord walls creates an enormous force,” Monahan says. The September seiche, he adds, produced a force equivalent to 14 Saturn V rockets launching at once, around 500 GN. “[It] was literally enough to shake the entire Earth for days,” he says.

What made these events so powerful was the geometry of the fjord, Monahan says. “A sharp bend near its outlet effectively trapped the seiches, allowing them to reverberate for days,” he explains. “Indeed, the repeated impacts of water against fjord walls acted like a hammer striking the Earth’s crust, creating long-period seismic waves that propagated around the globe and that were strong enough to be detected worldwide.”

Risk of tsunamigenic landslides will likely grow

As for what caused the seiches, Monahan suggests that climate change may have been a contributing factor. As glaciers thin, they undergo a process called de-buttressing wherein the loss of ice removes support from the surrounding rock, leading it to collapse. It was likely this de-buttressing that caused two enormous landslides in Dickson Fjord within a month, and continued global warming will only increase the frequency. “As these events become more common, especially in steep, ice-covered terrain, the risk of tsunamigenic landslides will likely grow,” Monahan says.

The researchers say they would now like to better understand how the seiches dissipated afterwards. “Although previous work successfully simulated how the megatsunamis stabilized into seiches, how they decayed is not well understood,” says Monahan. “Future research could make use of SWOT satellite observations as a benchmark to better constrain the processes behind disputation.”

The findings, which are detailed in Nature Communications, show how top-of-the-line satellites like SWOT can fill these observational gaps, he adds. To fully leverage these capabilities, however, researchers need better processing algorithms tailored to complex fjord environments and new techniques for detecting and interpreting anomalous signals within these vast datasets. “We think scientific machine learning will be extremely useful here,” he says.

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Scientists in Switzerland and Japan have uncovered what they say is the first direct evidence that materials at the bottom of the Earth’s mantle flow like a massive river. This literally “ground-breaking” finding, made by comparing seismic data with laboratory studies of materials at high pressures and temperatures, could reshape our understanding of the dynamics at play deep within our planet’s interior.

For over half a century, one of the greatest unresolved mysteries in geosciences has been a phenomenon that occurs just above the boundary where the Earth’s solid mantle meets its liquid core, says Motohiko Murakami, a geophysicist at ETH Zurich who led the new research effort. Within this so-called D” layer, the velocity of seismic waves passing through the mantle abruptly increases, and no-one is entirely sure why.

This increase is known as the D” discontinuity, and one possible explanation for it is a change in the material the waves are travelling through. Indeed, in 2004, Murakami and colleagues at the Tokyo Institute of Technology’s department of earth and planetary sciences suspected they’d uncovered an explanation along just these lines.

In this earlier study, the researchers showed that perovskite – the main mineral present in the Earth’s lower mantle – transforms into a different substance known as post-perovskite under the extreme pressures and temperatures characteristic of the D” layer. Accordingly, they hypothesized that this phase change could explain the jump in the speed of seismic waves.

Nature, however, had other ideas. “In an experimental study on seismic wave speeds across the post-perovskite phase transition we conducted three years later, such a sharp increase in velocity was not observed, bringing the problem back to square one,” Murakami says.

Post-perovskite crystals line up

Subsequent computer modelling revealed a subtler effect at play. According to these models, the hardness of post-perovskite materials is not fixed. Instead, it depends on the direction of the material’s crystals – and seismic waves through the material will only speed up when all the crystals point in the same direction.

In the new work, which they detail in Communications Earth & Environment, Murakami and colleagues at Tohoku University and the Japan Synchrotron Radiation Research Institute confirmed this in a laboratory experiment for the first time. They obtained their results by placing crystals of a post-perovskite with the chemical formula MgGeO₃ in a special apparatus designed to replicate the extreme pressures (around 1 million atmospheres) and temperatures (around 2500 K) found at the D” depth nearly 3000 km below the Earth’s surface. They then measured the velocity of lab-produced seismic waves sent through this material.

These measurements show that while randomly-oriented crystal samples do not reproduce the shear wave velocity jump at the D” discontinuity, crystals oriented along the (001) slip plane of the material’s lattice do. But what could make these crystals line up?

Evidence of a moving mantle

The answer, Murakami says, lies in slow, convective motions that cause the lower mantle to move at a rate of several centimetres per year. “This convection drives plate tectonics, volcanic activity and earthquakes but its effects have primarily been studied in the shallower region of the mantle,” he explains. “And until now, direct evidence of material movement in the deep mantle, nearly 3000 km beneath the surface, has remained elusive.”

Murakami explains that the post-perovskite mineral is rigid in one direction while being softer in others. “Since it naturally aligns its harder axis with the mantle flow, it effectively creates a structured arrangement at the base of the mantle,” he says.

According to Murakami, the discovery that solid (and not liquid) rock flows at this depth does more than just solve the D” layer mystery. It could also become a critical tool for identifying the locations at which large-scale mantle upwellings, or superplumes, originate. This, in turn, could provide new insights into Earth’s internal dynamics.

Building on these findings, the researchers say they now plan to further investigate the causes of superplume formation. “Superplumes are believed to trigger massive volcanic eruptions at the Earth’s surface, and their activity has shown a striking correlation — occurring just before two major mass extinction events in Earth’s history,” Murakami says.

Being able to understand – and perhaps even predict – future superplume activity could therefore “provide critical insights into the long-term survival of humanity”, he tells Physics World. “Such deep mantle processes may have profound implications for global environmental stability,” he says. “By advancing this research, we aim to uncover the mechanisms driving these extraordinary mantle events and assess their potential impact on Earth’s future.”

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