Why put an earthquake sensor in Minnesota?

When Doug Wiens, a planetary scientist, approached Minnesota farmers to ask permission to install a seismometer on their land, he got a puzzled look.

“You could tell they were thinking ‘Why are you putting a seismometer here?'” says Doug Wiens, a professor at Washington University in St. Louis. “‘We don’t have earthquakes and we don’t have volcanoes. Do you know something we don’t?'”

Actually, he did.

Deep beneath the fertile flat farmland, there is a huge scar in the Earth called the Midcontinent Rift. This ancient and hidden feature bears silent witness to a time when the core of what would become North America nearly ripped apart. If the U-shaped rip had gone to completion, the land between its arms—including at least half of what is now called the Midwest—would have pulled away from North America, leaving a great ocean behind.

Weisen Shen, a postdoctoral research associate with Wiens, will present seismic images of the rift at the annual meeting of the Geological Society of America (GSA). The images were made by analyzing data from Earthscope, a National Science Foundation program that deployed thousands of seismic instruments across America in the past 10 years.

Understanding the rift

The Midcontinent Rift was discovered by geophysicists who noticed that gravity was stronger in some parts of the upper Midwest than in others. In the 1950s and 1960s, they mapped the gravity and magnetic anomalies with airborne sensors. Shen is contributing to a session at the GSA dedicated to Bill Heinze, a geophysicist who helped discover and map the Midcontinent Rift.

“A huge volume of lava erupted here. It was perhaps the largest outflowing of lava in our planet’s history. “

But understanding of the rift then stalled until 2003, when the NSF funded Earthscope, a program whose mission is to use North America as a natural laboratory to gain insight into how the Earth operates.

As part of Earthscope, the Incorporated Research Institutions for Seismology (IRIS) installed a network of 400 seismometers, called the USArray,  that rolled across the United States from west to east, gathering data at each location for two years before moving on. USArray was installed on the West Coast beginning in 2004, and had advanced to the Midwest by 2010.

US map of seismometers
The USArray consisted of 400 seismometers that rolled across the United States from 2004 until 2015. The instruments are currently in Alaska. (Credit: Washington University)

Earthscope also made available a pool of seismometers, called the flexible array, for more focused field experiments. A consortium of universities installed 83 of these stations along and across the rift in 2011, creating a dense array called SPREE.

2 Congo disasters uncover new source of strong earthquakes

Seismologists had never before been able to blanket the landscape with seismometers in this way, and so the USArray has stimulated many innovations in the manipulation of the seismic data to extract information about Earth’s crust and upper mantle.

Seismic interpretation is a thorny version of what is called an inverse problem. If the Earth’s interior were of uniform composition, seismic waves would travel in straight lines. But instead, underground structures or differences in temperature and density refract and reflect them. The problem is to figure out mathematically which obstructions could have produced the wave arrivals that the seismometers recorded.

It’s a bit like trying to figure out the shape of an island in a pond by throwing a pebble into the lake and recording the ripples arriving at the shore. Shen has devised new techniques for combining many types of seismological data to create sharper images of Earth’s interior.

The farmers in Minnesota have a point when they wonder what an “earthquake sensor” could detect in an area where there are no earthquakes. The answer is that the seismometers record distant earthquakes, such as those on the Pacific Ring of Fire on the opposite side of the planet, and ambient noise, caused by activity such as powerful storms slamming into the Jersey Shore.

What have scientists learned?

Shen has seasoned the mix with several other measurements that can be extracted from the seismic record as well. By inverting all of these data functions simultaneously within a Bayesian statistical framework, he is able to obtain much clearer images of Earth’s interior than one type of data alone would produce, together with estimates of the probability that the images are correct.

“When you pull apart a continent, like a piece of taffy, it starts to stretch and to thin,” says Michael Wysession, professor of earth and planetary sciences and a member of the SPREE team. “And as it sags, the dip fills with low-density sediment. So if you go over a rift with a gravity sensor, you expect to find  a negative gravity anomaly. Mass should be missing. But that’s not what happened with the Midcontinent Rift. Instead of being thinner than the surrounding crust, it is thicker.

“We know that lava comes out at rifts,” Wysession says. “The East African rift zone, for example, includes a number of active and dormant volcanoes, such as Mount Kilimanjaro. But the Midcontinent Rift was flooded with lava, and as it sank under the weight of the cooling basaltic rock, even more lava flowed into the depression.

“A huge volume of lava erupted here. It was perhaps the largest outflowing of lava in our planet’s history. And then, after the eruptions ended, the area was compressed by mountain building event to its east, thickening the scar by squeezing it horizontally.”

Earthquake tech can ID different shocks and booms

Shen published images of the rift made with USArray data in the Journal of Geophysical Research 2013. But at that time, he had only sparse coverage in the rift’s vicinity. At this week’s GSA meeting he’ll present images made with both USArray and SPREE data (especially many more “receiver functions,” a type of seismic data that is particularly sensitive to seismic boundaries) that show what lies beneath the rift more clearly.

Miles beneath the Earth’s surface, there is a seismic boundary called the Mohorovičić discontinuity, or Moho. At the Moho, seismic waves hit higher density material and suddenly accelerate. But beneath the rift, the Moho is blurred rather than sharp. “Its structure has been destroyed,” he says.

He also sees evidence of something called magmatic underplating. “We think magma might have trapped, or stalled out, at the Moho or within the crust during its rise to the surface,” he said. This might explain why the Moho is so disrupted, although Shen can think of alternative explanations. He compares images of the Midcontinent Rift made with the SPREE array to images of the Rio Grande rift made with a similar seismic array called La Ristra. The La Ristra images show that the Rio Grande rift is thinner than the surrounding crust, not thicker. The Moho is clear and rises rather than sinks under the rift.

“I think we’re looking at different stages of rifting,” Shen says. The Rio Grande Rift is still active, still opening, but the Midcontinent Rift is already dead and has been squeezed shut.

The tremendous outpouring of magma at the Midcontinent Rift might also have disrupted its structure, making it look different from other rifts, Wien says.

“My goal,” Shen says, “is to provide basic seismic models of interesting tectonic regions like this one for geologists, geochemists, and scientists from other disciplines to use—to help them interpret their results and also help the public to better understand the story of the land they live on.”

Source: Washington University in St. Louis

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A zap to the right spot might ‘reset’ your brain

Doctors use brain stimulation to treat epilepsy, depression, pain, and other conditions, but it’s not exactly clear how it works or even which areas to target.

New research suggests stimulating a single region of the brain can activate other regions and even alter global brain dynamics.

“The question we asked in this study was how much of the brain is activated by stimulating a single region.”

“We don’t have a good understanding of the effects of brain stimulation,” says Sarah Muldoon, an assistant professor of mathematics at the University at Buffalo. “When a clinician has a patient with a certain disorder, how can they decide which parts of the brain to stimulate? Our study is a step toward better understanding how brain connectivity can better inform these decisions.”

Danielle S. Bassett, associate professor of bioengineering in the University of Pennsylvania, says if you look at the architecture of the brain, “it appears to be a network of interconnected regions that interact with each other in complicated ways. The question we asked in this study was how much of the brain is activated by stimulating a single region.

“We found that some regions have the ability to steer the brain into a variety of states very easily when stimulated, while other regions have less of an effect.”

8 brains and 83 regions

The researchers used a computational model to simulate brain activity in eight individuals whose brain architecture was mapped using data derived from diffusion spectrum imaging, a type of brain image taken by an MRI scanner.

The research, published in in PLOS Computational Biology, examined the impact of stimulating each of 83 regions within each subject’s brain.

While results varied by person, common trends emerged.

Network hubs—areas of the brain that are strongly connected to other parts of the brain via the brain’s white matter—displayed what researchers call a “high functional effect.” Stimulating these regions resulted in the global activation of many brain regions.

This effect was particularly notable in two sub-networks of the brain that are known to contain multiple regional hubs: the subcortical network (which is composed of regions that evolved relatively early on and are critical for emotion processing) and the default mode network (which is composed of regions that evolved later and are critical for self-referential processing when a person is at rest, or not completing any task).

Creativity goes up with ‘alpha wave’ stimulation

Stimulating regions in the subcortical network culminated in global changes, in which a diversity of areas within a subject’s brain lit up. Stimulating regions in the default mode network also led easily to a plethora of new brain states, though the patterns of activation were constrained by the brain’s underlying architecture—by the white matter links between the nodes of the network and other parts of the brain.

Despite this limitation, the network’s agility supports the idea that the brain at “rest” is well suited for shifting quickly into an array of new states geared toward completing specific tasks.

In contrast to regions within the default mode network and subcortical networks, more weakly connected areas, such as in the sensory and association cortex, had a more limited effect on brain activity when activated.

These patterns suggest that doctors could pursue two classes of therapies when it comes to brain stimulation: a “broad reset” that alters global brain dynamics, or a more targeted approach that focuses on the dynamics of just a few regions.

The Army Research Office, the John D. and Catherine T. MacArthur Foundation, the Alfred P. Sloan Foundation, the National Institute of Mental Health, the National Institute of Child Health and Human Development, the Office of Naval Research, and the National Science Foundation funded the work.

Source: University at Buffalo

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Team models liquid ocean under Pluto’s surface

When NASA’s New Horizons probe zoomed past Pluto, it did more than just give us a close-up look at the dwarf planet. It also sparked a debate about a liquid ocean lurking under the planet’s icy surface crust.

Now, a new study published in Geophysical Research Letters has used computer modeling to show there is a liquid ocean on Pluto more than 62 miles deep with salinity similar to the Dead Sea right here on Earth.

The study team was focused on Sputnik Planum, a basin nearly 560 miles across that comprises the western half the Pluto’s now-famous heart-shaped feature. The basin seems to have been produced by an impact, probably by an object 120 miles across or bigger.

Focusing on Sputnik Planum

Sputnik Planum drew researchers’ attention because of its position. Pluto and its moon Charon are tidally locked with each other, which means they always show the same face to each other as they rotate. Sputnik Planum sits on this tidal axis and this indicates the basin has greater mass than the rest of Pluto’s icy crust, a phenomenon known as a positive mass anomaly. As Charon’s gravity affects Pluto, it yanks proportionally more on parts of greater mass, tilting the planet until Sputnik Planum became lined up with the tidal axis.

An asteroid or meteor impact crater typically has a negative mass anomaly, and the fact that Sputnik Planum is positive immediately drew investigators attention. The study team said the high amount of mass in Sputnik Planum may be due to liquid hanging out beneath the icy surface.

A large impact first digs out a planet’s exterior, and then a bounce back draws material up from deep within the planet. If that rising material is denser than what was there originally, the crater ultimately ends up having the same mass as it had before the impact. This is a situation geologists call “isostatic compensation”.

Because water is denser than ice, liquid water rising up from beneath Pluto’s icy surface would even out the mass of the Sputnik Planum crater, the study team said. Therefore, if the basin began with neutral mass, then nitrogen ice deposited later to form an icy crust would produce a positive mass anomaly.

“We wanted to run computer models of the impact to see if this is something that would actually happen,” study Brandon Johnson, a Brown University geologist, said in a news release. What we found is that the production of a positive mass anomaly is actually quite sensitive to how thick the ocean layer is. It’s also sensitive to how salty the ocean is, because the salt content affects the density of the water.”

The simulation that best modeled Sputnik Planum’s depth, while also generating a crater with compensated mass, was one involving Pluto having an ocean layer greater than 62 miles thick, with a salinity of around 30 percent.

“What this tells us is that if Sputnik Planum is indeed a positive mass anomaly — and it appears as though it is — this ocean layer of at least 100 kilometers has to be there,” Johnson said. “It’s pretty amazing to me that you have this body so far out in the solar system that still may have liquid water.”


Image credit: NASA/JPL

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Did a massive collision form the moon? Here’s proof

Scientists have new evidence that our moon formed when a planet-sized object struck the infant Earth some 4.5 billion years ago.

Lab simulations show that a giant impact of the right size would not only send a huge mass of debris hurtling into space to form what would become the moon. It would also leave behind a stratified layer of iron and other elements far below Earth’s surface, just like the layer that seismic imaging shows is actually there.

“We’re saying this stratified layer might be the smoking gun.”

Johns Hopkins University geoscientist Peter Olson says a giant impact is the most prevalent scientific hypothesis on how the moon came to be, but has been considered unproven because there has been no “smoking gun” evidence.

“We’re saying this stratified layer might be the smoking gun,” says Olson, a research professor in earth and planetary sciences. “Its properties are consistent with it being a vestige of that impact.”

A summary of the study has been published by the journal Nature Geoscience.

“Our experiments bring additional evidence in favor of the giant impact hypothesis,” says Maylis Landeau, lead author of the paper and a postdoctoral fellow at Johns Hopkins when the simulations were done. “They demonstrate that the giant impact scenario also explains the stratification inferred by seismology at the top of the present-day Earth’s core. This result ties the present-day structure of Earth’s core to its formation.”

1,800 miles below Earth

The argument compares evidence on the stratified layer—believed to be some 200 miles (322 kilometers) thick and 1,800 miles (2,897 kilometers) below the Earth’s surface—with lab simulations of the turbulence of the impact. The turbulence in particular is believed to account for the stratification—meaning there are materials in layers rather than a homogeneous composition—at the top of the planet’s core.

The stratified layer is believed to contain iron and lighter elements, including oxygen, sulfur, and silicon. The existence of the layer is understood from seismic imaging; it is far too deep to be sampled directly.

Up to now, most simulations of the hypothetical big impact have been done in computer models and have not accounted for impact turbulence, Olson says. Turbulence is difficult to simulate mathematically, he adds.

Asteroid as long as New Jersey likely gave the moon a ‘right eye’

The researchers simulated the impact using liquids meant to approximate the turbulent mixing of materials that would have occurred when a planetary object struck when Earth was just about fully formed—a “proto-Earth,” as scientists call it.

Olson says the experiments depended on the principle of “dynamic similarity.” In this case, that means scientists can make reliable comparisons of fluid flows without doing an experiment as big and powerful as the original Earth impact, which—of course—is impossible. The study in Olson’s lab was meant to simulate the key ratios of forces acting on each other to produce the turbulence of the impact that could leave behind a layered mixture of material.

The researchers conducted more than 60 trials in which about 3.5 ounces of saline or ethanol solutions representing the planetary projectile that hit the Earth were dropped into a rectangular tank holding about 6 gallons of fluid representing the early Earth. In the tank was a combination of fluids in layers that do not mix: oil floating on the top to represent the Earth’s mantle and water below representing the Earth’s core.

Analysis showed that a mix of materials was left behind in varying amounts and that the distribution of the mixture depended on the size and density of the projectile hitting the Earth. The authors argue for a moon-forming projectile smaller or equal to the size of Mars, a bit more than half the size of Earth.

Landeau, now a fellow at the University of Cambridge, co-wrote the paper with Olson, Johns Hopkins undergraduate Benjamin H. Hirsh, and Renaud Deguen of Claude Bernard University in Lyon, France. The National Science Foundation supported the work.

Source: Johns Hopkins University

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