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Scientists use seismic noise to image first hundred meters of Mars

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Enlarge / InSight places a wind shield over its seismometer.

NASA’s InSight lander installed a seismograph on Mars, and the marsquakes it detected have helped us map the planet’s interior. This data provides the big picture of Mars’ internals—how big the core is, whether anything is molten, and so on. But it doesn’t capture the small details, like what the ground immediately below InSight looks like.

This week, researchers described how they’ve managed to find quiet periods on Mars that lets them image closer to the surface. The results, combined with some nearby surface features, reveal that InSight is likely above two large lava flows, separated by layers of sediment.

Be very quiet

Marsquakes aren’t useful for sorting out local features. If their seismic waves arrive from far enough away, then their behavior is mostly influenced by the materials they spent most of their time traveling through. If the marsquake happens nearby, then things are too energetic to make out the fine details caused by local features. So, in order to look at the local geology, you need to look at the background seismic noise that’s constantly being picked up by InSight.

On Earth, most of the seismic noise is generated by either human activities or the oceans. But Mars lacks both of these noise sources, and its background is dominated by the wind interacting with features on Mars.

But when the data was examined at times of day when winds were generally high, the noise turned out to be dominated by frequencies that were produced by the wind interacting with the lander itself. So the researchers focused on what was early evening, Mars time, when the winds tended to die down. At that point, most of the seismic noise is generated by weak winds interacting with nearby geology rather than with the lander itself.

Geologists have used seismic noise to reconstruct features on Earth by comparing the horizontal and vertical components of the noise. This is a process that can be consistent with a large collection of potential structures near the surface of Mars. To constrain the list of possibilities, the researchers focused on features that showed up in the majority of potential solutions. They also looked at the rocks exposed in nearby craters to search for visible features that correlated to the things their models were suggesting might exist.

What’s underneath

Closest to the surface, the regolith of Mars is formed by dust and rock fragments produced by impacts. It appears to be only 1.5 meters thick, although the researchers caution that the data on the uppermost 20 meters of material is very uncertain. By three meters below the surface, there appears to be a layer of volcanic rock, formed by major eruptions in Mars’ distant past.

Below that, from roughly 30 meters to 80 meters (these figures are pretty inexact), is another layer of material where seismic signals move slowly. The researchers conclude this is likely to be a layer of sedimentary rock. Below that are further volcanic deposits.

The researchers conclude that the deepest volcanic deposits date back to the Hesperian, a period of widespread volcanic activity that ended over 3 billion years ago. The overlying sediment deposit formed while Mars experienced cold, dry conditions similar to its present state. After it consolidated, and sometime during Mars’ Amazonian period, additional eruptions covered the sediments. Since then, impacts and Mars’ winds have deposited a layer of loose material on top of the volcanic layers.

Obviously, all of this is consistent with what can be observed in nearby craters. Still, it’s impressive how much information the researchers were able to extract from just a bit of noise.

Nature Communications, 2021. DOI: 10.1038/s41467-021-26957-7  (About DOIs).

Listing image by NASA/JPL-Caltech

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Here’s Why Earthquakes’ ‘Four-Leaf Clover’ Shockwaves Are Dangerous Instead of Lucky

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Geologists have measured a devastating ‘four-leaf clover’ pattern of earthquake shockwaves in greater detail than ever before – and the resulting findings could be crucial in making our buildings and cities more resistant to large quakes in the future.

 

This four-pronged pattern has been analyzed before, but never in as much depth as this. The team behind the new study is hoping that it might remove some of the mystery surrounding how earthquake shockwaves spread out across different frequencies.

Crucially, the cloverleaf shockwaves spread at low frequencies of under 10 hertz, a level of vibration that many buildings and structures are particularly vulnerable to.

The four-leaf clover pattern is visible at lower frequencies. (Trugman et al., Geophysical Research Letters, 2021)

“We find that at low frequencies, a simplified and widely used four-lobed model of earthquake ground motions does a good job describing the observed seismic wavefield,” write the researchers in their published paper.

“At higher frequencies, however, this four-lobed radiation pattern becomes less clear, deteriorating due to complexity in earthquake source processes and fault zone structure.”

The researchers looked at data from one of the densest seismic arrays on the planet: the LArge-n Seismic Survey in Oklahoma (LASSO), which is made up of 1,829 seismic sensors within an area of just 15 by 20 miles (25 by 32 kilometers).

LASSO was used to measure P-wave data from 24 small earthquakes across a period of 28 days in 2016, and it’s this data that the new study digs into. Having sensors so close to the epicenter of the quakes meant that patterns could be spotted before they smoothed out and evened off over greater distances.

 

By using algorithms to filter shockwaves by frequency, the four-leaf clover pattern emerged, but only at the lower frequencies. That might be because lower frequency seismic waves can bypass the jumble of broken rock found at earthquake faults, rather than being reflected and scattered in many different directions.

“What happens when you have an earthquake is that pieces of broken rock inside the fault zone start to move around like pinballs,” says geophysicist Victor Tsai, from Brown University in Rhode Island.

The earthquakes recorded by the LASSO array were relatively small – barely perceptible to the sensors – but the same patterns should be repeated across stronger quakes, the researchers predict. The next step is to put that to the test.

Ultimately, new data like this can make earthquake assessments and modeling more accurate. It shows that while people on the ground might experience a consistent level of shockwaves (the higher frequency ones), the buildings around them might be under a greater or lesser level of stress (the lower frequency shockwaves), depending on where they are in the four-leaf clover pattern.

While earthquake faults vary in terms of their age, their geological composition, and other factors, the underlying physics should be the same. The scientists are hoping to put together a catalog of earthquake zones, showing the faults with the most potential for dangerous seismic waves and resulting damage.

“What’s important in these results is that close to the source we’re seeing a variation in ground motion, and that’s not accounted for in any sort of hazard model,” says the study’s first author, earthquake geophysicist Daniel Trugman from the University of Texas at Austin.

The research has been published in Geophysical Research Letters.

 

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Largest Underwater Eruption Ever Recorded Gives Birth to Massive New Volcano

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A huge seismic event that started in May of 2018 and was felt across the entire globe has officially given birth to a new underwater volcano.

Off the eastern coast of the island of Mayotte, a gigantic new feature rises 820 meters (2,690 feet) from the seafloor, a prominence that hadn’t been there prior to an earthquake that rocked the island in May 2018.

 

“This is the largest active submarine eruption ever documented,” the researchers wrote in their paper.

The new feature, thought to be part of a tectonic structure between the East African and Madagascar rifts, is helping scientists understand deep Earth processes about which we know relatively little.

The seismic rumbles of the ongoing event started on 10 May 2018. Just a few days later, on 15 May, a magnitude 5.8 quake struck, rocking the nearby island. Initially, scientists were perplexed; but it didn’t take long to figure out that a volcanic event had occurred, the likes of which had never been seen before.

The signals pointed to a location around 50 kilometers from the Eastern coast of Mayotte, a French territory and part of the volcanic Comoros archipelago sandwiched between the Eastern coast of Africa and the Northern tip of Madagascar.

So a number of French governmental institutions sent a research team to check it out; there, sure enough, was an undersea mountain that hadn’t been there before.

Led by geophysicist Nathalie Feuillet of the University of Paris in France, the scientists have now described their findings in a new paper.

 

The team began monitoring the region in February of 2019. They used a multibeam sonar to map an 8,600-square-kilometer area of seafloor. They also placed a network of seismometers on the seafloor, up to 3.5 kilometers deep, and combined this with seismic data from Mayotte.

Between 25 February and 6 May 2019, this network detected 17,000 seismic events, from a depth of around 20 to 50 kilometers below the ocean floor – a highly unusual finding, since most earthquakes are much shallower. An additional 84 events were also highly unusual, detected at very low frequencies.

Armed with this data, the researchers were able to reconstruct how the formation of the new volcano may have occurred. It started, according to their findings, with a magma reservoir deep in the asthenosphere, the molten mantle layer located directly below Earth’s lithosphere.

Chronology of the eruption. (Feuillet et al., Nature Geoscience, 2021)

Below the new volcano, tectonic processes may have caused damage to the lithosphere, resulting in dykes that drained magma from a reservoir up through the crust, producing swarms of earthquakes in the process. Eventually, this material made its way to the seafloor, where it erupted, producing 5 cubic kilometers of lava and building the new volcano.

The low-frequency events were likely generated by a shallower, fluid-filled cavity in the crust that could have been repeatedly excited by seismic strain on faults close to the cavity.

 

As of May 2019, the extruded volume of the new volcanic edifice is between 30 and 1,000 times larger than estimated for other deep-sea eruptions, making it the most significant undersea volcanic eruption ever recorded.

“The volumes and flux of emitted lava during the Mayotte magmatic event are comparable to those observed during eruptions at Earth’s largest hotspots,” the researchers wrote.

“Future scenarios could include a new caldera collapse, submarine eruptions on the upper slope or onshore eruptions. Large lava flows and cones on the upper slope and onshore Mayotte indicate that this has occurred in the past.

“Since the discovery of the new volcanic edifice, an observatory has been established to monitor activity in real time, and return cruises continue to follow the evolution of the eruption and edifices.”

The research has been published in Nature Geoscience.

 

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Spectacular Ice Age Landscapes Beneath the North Sea Revealed by 3D Seismic “MRI” Scans

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Close up image of an esker (a sedimentary cast of a meltwater channel formed beneath an ice sheet), discovered within a tunnel valley using the new 3D seismic reflection data. Credit: James Kirkham

Spectacular ice age landscapes beneath the North Sea have been discovered using 3D seismic reflection technology.  Similar to MRI (magnetic resonance imaging) the images reveal in unprecedented detail huge seafloor channels – each one 10 times wider than the River Thames.

For the first time an international team of scientists can show previously undetectable landscapes that formed beneath the vast ice sheets that covered much of the UK and Western Europe thousands to millions of years ago. These ancient structures provide clues to how ice sheets react to a warming climate. The findings are published this week (September 9, 2021) in the journal Geology.

So called tunnel valleys, buried hundreds of meters beneath the seafloor in the North Sea are remnants of huge rivers that were the ‘plumbing system’ of the ancient ice sheets as they melted in response to rising air temperatures.

Lead author James Kirkham, from British Antarctic Survey (BAS) and the University of Cambridge, says:

“The origin of these channels was unresolved for over a century. This discovery will help us better understand the ongoing retreat of present-day glaciers in Antarctica and Greenland.

“In the way that we can leave footprints in the sand, glaciers leave an imprint on the land upon which they flow. Our new cutting-edge data gives us important markers of deglaciation.”

Comparing the resolution of the new high-resolution 3D seismic reflection data to previous 3D seismic data from this region. The new data revolutionises our ability to image these buried channels and their internal structures, as demonstrated by the contrast between the left and right of the image. Credit: James Kirkham, BAS

Dr. Kelly Hogan, co-author and a geophysicist at BAS, says:

“Although we have known about the huge glacial channels in the North Sea for some time, this is the first time we have imaged fine-scale landforms within them. These delicate features tell us about how water moved through the channels (beneath the ice) and even how ice simply stagnated and melted away. It is very difficult to observe what goes on underneath our large ice sheets today, particularly how moving water and sediment is affecting ice flow and we know that these are important controls on ice behavior. As a result, using these ancient channels to understand how ice will respond to changing conditions in a warming climate is extremely relevant and timely.”

A map of the North Sea showing the distribution of buried channels (tunnel valleys) that have been previously mapped using 3D seismic reflection technology. The limit of the last ice sheet to cover the UK (around 21,000 years ago) is overlain. Credit: James Kirkham

3D seismic reflection technology, which was provided by industry partners, uses sound waves to generate detailed three-dimensional representations of ancient landscapes buried deep beneath the surface of the Earth, in a similar manner to how magnetic resonance imaging (MRI) scans can image structures within the human body. The method can image features as small as a few meters beneath the surface of the Earth, even if they are buried under hundreds of meters of sediment. The exceptional detail provided by this new data reveals the imprint of how the ice interacted with the channels as they formed. By comparing these ancient ‘ice fingerprints’ to those left beneath modern glaciers, the scientists were able to reconstruct how these ancient ice sheets behaved as they receded.

Close up image of an esker (a sedimentary cast of a meltwater channel formed beneath an ice sheet), discovered within a tunnel valley using the new 3D seismic reflection data. In this image, the esker is shown in context of the high-resolution 3D seismic data which can be ‘sliced’ both vertically and horizontally to reveal ancient glacial landscapes buried beneath the seafloor of the North Sea. The reflections of the curved tunnel valley sides can be seen in the top of the image. Credit: James Kirkham

By diving into the past, this work provides a window into a future warmer world where new processes may begin to alter the plumbing system and flow behavior of the Antarctic and Greenland ice sheets.

Reference: “Tunnel valley infill and genesis revealed by high-resolution 3-D seismic data” by James D. Kirkham, Kelly A. Hogan, Robert D. Larter, Ed Self, Ken Games, Mads Huuse, Margaret A. Stewart, Dag Ottesen, Neil S. Arnold and Julian A. Dowdeswell, 8 September 2021, Geology.
DOI: 10.1130/G49048.1



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Scientists Detect Signs of a Hidden Structure Inside Earth’s Core

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While most of us take the ground beneath our feet for granted, written within its complex layers, like pages of a book, is Earth’s history. Our history.

Now researchers have found more evidence for a whole new chapter deep within Earth’s past – Earth’s inner core appears to have another even more inner core within it.

 

“Traditionally we’ve been taught the Earth has four main layers: the crust, the mantle, the outer core and the inner core,” explained Australian National University geophysicist Joanne Stephenson. 

Our knowledge of what lies beneath Earth’s crust has been inferred mostly from what volcanoes have divulged and seismic waves have whispered. From these indirect observations scientists have calculated that the scorchingly hot inner core, with temperatures surpassing 5,000 degrees Celsius (9,000 Fahrenheit), makes up only one percent of Earth’s total volume.

Now Stephenson and colleagues have found more evidence Earth’s inner core may have two distinct layers.

“It’s very exciting – and might mean we have to re-write the textbooks!” she added.

The team used a search algorithm to trawl through and match thousands of models of the inner core with observed data across many decades about how long seismic waves take to travel through Earth, gathered by the International Seismological Centre.

Differences in seismic wave paths through layers of Earth. (Stephenson et al., Journal of Geophysical Research: Solid Earth, 2021)

So what’s down there? The team looked at some models of the inner core’s anisotropy – how differences in the make-up of its material alters the properties of seismic waves – and found some were more likely than others.

While some models think the material of the inner core channels seismic waves faster parallel to the equator, others argue the mix of materials allows for faster waves more parallel to Earth’s rotational axis. Even then, there’s arguments about the exact degree of difference at certain angles.

 

This study failed to show much variation with depth in the inner core, but did find there was a change in the slow direction to a 54 degree angle, with the faster direction of waves running parallel to the axis.

“We found evidence that may indicate a change in the structure of iron, which suggests perhaps two separate cooling events in Earth’s history,” Stephenson said.  

“The details of this big event are still a bit of a mystery, but we’ve added another piece of the puzzle when it comes to our knowledge of the Earths’ inner core.”

These new findings may explain why some experimental evidence has been inconsistent with our current models of Earth’s structure.

The presence of an innermost layer has been suspected for some time now, with hints that iron crystals which compose the inner core have different structural alignments. 

“We are limited by the distribution of global earthquakes and receivers, especially at polar antipodes,” the team wrote in their paper, explaining the missing data decreases the certainty of their conclusions. But their conclusions align with other recent studies on the anisotropy of the innermost inner core.

A new method currently under development may soon fill in some of these data gaps and allow scientists to corroborate or contradict their findings and hopefully translate more stories written within this early layer of Earth’s history.

This research was published in the Journal of Geophysical Research.

 

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