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UM researchers using sound to peer beneath Earth's crust

UM researchers using sound to peer beneath Earth's crust
Assistant professor Vedran Lekic highlights a map of seismometers spread across the country. (Scott Dance, Baltimore Sun, Baltimore Sun)

A network of hundreds of seismometers across the eastern United States hears everything — earthquakes on the other side of the world, construction work down the street, the swaying of trees in the wind.

And like bats or ultrasound machines use sound to "see," the instruments can translate those sounds into images. The more instruments listening, the finer the image.

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An assistant professor of seismology at the University of Maryland, College Park and his colleagues are using that principle to peer into the Earth's core and create images with unprecedented resolution. Last month, Vedran Lekic won an $875,000 fellowship to fund a project using the National Science Foundation-funded network of seismometers.

Their work includes learning how to best translate seismographic data into images, getting better pictures of where in the Earth's mantle rock is melting and better understanding of how that molten material travels through continental plates. In recent years, seismologists have been studying two massive and mysterious structures just outside the planet's core, and the research could help explain them, Lekic said.

"We have no idea what they are," Lekic said.

But understanding them could help answer some nagging Earth science questions, such as why volcanic islands form where they do or why continents break apart, he said.

The research data comes from what Popular Science called the universe's "most epic project" in 2011, the NSF's EarthScope. To allow for those finer scans beneath the Earth's crust, it uses not only the network of seismometers, but 1,100 devices that use GPS to track land changes that occur because of shifts in tectonic plates.

The seismometer network, dubbed USArray, has over the past seven years leapfrogged from the West Coast to the East to gather detailed observations across the continent. The results are graphs like one Lekic pored over in a spare office in College Park on a recent afternoon.

The data stacks dozens of seismometer readings on top of one another, showing the waves associated with earthquakes as they travel over long distances. But each one is slightly different, Lekic said, as the waves travel through differing geologic formations.

Looking at only one, or just a few, can hide some features because the waves just travel around them. Without a fuller picture from multiple echoes, the resulting data isn't very useful for creating an image, he said.

"It would be like the bat could only hear the first echo and not the later ones," Lekic said.

The seismometers are built inside large barrel-like tubes that are placed in large depressions in the ground, said Ramon Arrowsmith, a geology professor at Arizona State University and director of EarthScope's national office. They are placed evenly across the continent, instead of more haphazardly as in the past, to allow a denser, deeper look beneath the Earth's crust.

"It's like getting a new set of glasses," Arrowsmith said. "Before, things were more fuzzy and now things are more in focus."

EarthScope's data is provided to scientists freely, enabling wider interpretation. Other projects using the data have included efforts to understand why active faults such as the San Andreas in California are more "slippery" than others, how and why earthquakes occur in cycles, and how vibrations of things like meteors are felt in bodies of water and the atmosphere.

Lekic is working with postdoctoral researchers, doctoral and master's degree students and even some undergraduates on various ways to interpret the data. One project seeks to establish how to most conservatively estimate the size of geological features causing anomalies in the data, while another looks to create a systematic map of the Earth's crust across North America.

An award from the David and Lucile Packard Foundation announced last month will help pay for those efforts over the next five years, at a rate of $175,000 each year. Ninety percent of that funding must be spent directly on science, instead of being kicked back to the university to pay for what are known as "indirect" costs, such as lighting and heating of buildings.

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Lynn Orr, a spokeswoman for the foundation, said in an email that Lekic's proposal — along with 17 others in science and engineering — was chosen for its "creative approach to research" and for pushing the boundaries of its field "in ways that drive innovation and discovery."

Unlike other types of grants Lekic could use to fund the research, the Packard Fellowship is not tied to any specific project, but more broadly to the team's efforts. That gives them the ability to take risks in exploring new methods for interpreting the EarthScope data, for example, even if uncertain of their effectiveness, Lekic said.

"It gives us the freedom to try new things," he said.

The seismology research in itself is somewhat new to the university. While the program has a long history of leading research on geochemistry, including studies of the composition of the Earth's crust, Lekic arrived in 2012 as the geology department's lone seismology professor. Since then, five others have joined the faculty, he said.

Scientists hope the data will help drive a leap in progress for Earth science. Arrowsmith said scientists like Lekic have been able to build their careers on the data, much as space scientists have with other tools.

"EarthScope is like the Hubble Telescope for earth science, but instead of looking out, we look in," Arrowsmith said.

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