SBU’s Weisen Shen uses seismic signal to see inside Earth

SBU’s Weisen Shen uses seismic signal to see inside Earth

Weisen Shen in front of a twin-otter airplane in the Antarctic during the 2017-18 season. Photo by Zhengyang Zhou

By Daniel Dunaief

Ever sit alone in a house and hear noises you can’t explain? Was that the wind, the house settling (whatever that means) or the cat swatting at the string hanging from the blinds?

Those sounds, which are sometimes inexplicable and are called ambient noise, are often hard to trace, even if we walk around the house and listen outside every room.

Weisen Shen
Photo by John Griffin

For Weisen Shen, an assistant professor in the Department of Geosciences at Stony Brook University, ambient noises deep below the Antarctic continent and elsewhere can be and often are clues that unlock mysteries hidden miles below the frozen surface.

A geoscientist who uses computer programs in his research, Shen would like to study the temperature well below the surface. He developed an in-house code to understand and interpret seismic data.

The speed at which Earth rumbling passes from one area to another can indicate the relative temperature of an area. Seismic activity moves more slowly through warmer rocks and moves more rapidly through colder crust, which has a higher rigidity. According to Shen, these temperature readings can help provide a clearer understanding of how much heat is traveling through the surface of the solid Earth into the ice sheet.

Shen traveled to the Ross Ice Shelf in the 2015-16 season and ventured to the South Pole in the 2017-18 season. He is currently seeking funding to go back to the Antartica. Earlier this year, he published an article in the journal Geology in which he found evidence that the lithosphere beneath the Transantarctic Mountains is thinner than expected.

Shen pointed out that seismic properties aren’t just related to temperature: They can help determine the density of the material, the composition and the existence of fluid such as water. He looks for surface geology and other types of geophysical data to detect what is the dominant reason for seismic structure anomalies. He also uses properties other than speed, such as seismic attenuation and amplitude ratios, in his analysis.

This kind of information can also provide an idea of the underlying support for mountain ranges, which get built up and collapse through a lithographic cycling.

As for ambient noises, Shen explained that they can come from ocean fluctuations caused by a hurricane, from human activities or, most commonly, from the bottom of the ocean, where the dynamic ocean wave constantly pushes against the bottom of the earth. By processing the noises in a certain way, he can extract information about the materials through which the noise traveled.

Shen published an article in the Journal of Geophysical Research in which he discussed a noise source in Kyushu Island in the Japanese archipelago. “The noise is so subtle that people’s ears will never catch it,” he said. “By deploying these very accurate seismic sensors, we will be able to monitor and study all the sources of those noises, not just the earthquakes.”

Studying these lower volume, less violent noises is especially helpful in places like Antarctica, which is, Shen said, a “quiet continent,” without a lot of strong seismic activity. He also uses the images of earthquakes that occur elsewhere, which travel less violently and dramatically through Antarctica.

Shen decided to study Antarctica after he earned his doctorate at the University of Colorado at Boulder. “I have this ambition to get to all the continents,” he said. In graduate school he told himself, “If you ever want to get that work done, you have to crack this continent.”

During his postdoctoral work, Shen moved to St. Louis, where he worked at Washington University in the laboratory of Doug Wiens, professor of Earth and planetary sciences.

In addition to conducting research in Antarctica, Shen collaborated with Chen Cai, a graduate student in Wiens’ lab. Together with other members of the Washington University team, they used seismic data in the Mariana Trench to show that about three to four times more water than previously estimated traveled beneath the tectonic plates into the Earth’s interior.

That much water rushing further into the Earth, however, is somehow offset by water returning to the oceans, as ocean levels haven’t changed dramatically through this part of the water cycle process.

“People’s estimates for the water coming out is probably out of balance,” Wiens said. “We can’t through millions of years bring lots of water through the interior. The oceans would get lower. There’s no evidence” to support that, which means that “an upward revision of the amount of water coming out of the Earth” is necessary. That water could be coming out through volcanoes or perhaps through the crust or gas funnels beneath the seafloor, he suggested.

Wiens praised all the researchers involved in the study, including Shen, whom he said was “very important” and “wrote a lot of the software we used to produce the final images.”

A resident of Queens, Shen lives with his wife Jiayi Xie, who works as a data scientist at Xaxis, a subcompany of the global media firm GroupM. The couple has an infant son, Luke.

Shen grew up in the southwestern part of China. When he was younger, he was generally interested in science, although his particular passion for geoscience started when he was in college at the University of Science and Technology of China, USTC, in Hefei, Anhui, China.

The assistant professor, who teaches a geophysics class at Stony Brook University, currently has two graduate students in his lab. He said he appreciates the support Stony Brook provides for young faculty.

As for his work, Shen is excited to contribute to the field, where he enjoys the opportunity and camaraderie that comes from exploring parts of Earth that are relatively inaccessible. He feels his detailed studies can help change people’s understanding of the planet.

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