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The massive Bolivian earthquake reveals mountains 660 km below our feet




In a study published this week in Science geologists from Princeton Jessica Irving and Wenbo Wu, in collaboration with Sidao Ni of the Institute of Geodesy and Geophysics in China, used data from a massive earthquake in Bolivia to find mountains and other topographies on a layer located 660 km (410 miles) straight down, separating the top and bottom mantle. (There is no official name for this layer, scientists just call it "the 660km.")

In order to get deeper into the Earth, scientists use the most powerful waves on the planet that are generated by massive earthquakes. "You want a great, deep earthquake to shake the entire planet," Irving, an assistant professor of geology, said.

Big earthquakes are much more powerful than small earthquakes – energy increases 30 times each step to Richter. scale and deep earthquakes, "instead of shattering their energy into the earth's crust, they can make the whole mantle," Irving said. It gets its best data from earthquakes with a magnitude of 7.0 or higher, she said, as the shock waves they send in all directions can travel through the nucleus to the other side of the planet – and vice versa. For this study, key data is derived from waves that have occurred after a magnitude earthquake of 8.2, the second deepest earthquake ever to have been hit, which shook Bolivia in 1994. "This great earthquake does not come very often," I said . "We are happy today that we have so many seismometers as it was 20 years ago." Seismology is a different area than 20 years ago, between tools and computing resources. "

Seismologists and data specialists use powerful computers. , including the Princeton supercomputer printer, to simulate the intricate behavior of distracting waves in the deep Earth.

Technology depends on the fundamental property of the waves: their ability to bend and jump. Just as light waves can mirror or bend as they pass through the prism, the waves of the earthquake travel straight through homogeneous rocks but reflect or break when they encounter a border or roughness. all objects have superficial roughness and therefore distract the light, "says Wu, the lead author of the new document that has just finished his PhD studies in Earth Sciences. now he is a post-graduate student at the California Institute of Technology. "That's why we can see these objects – the distracting waves bring information about the roughness of the surface." In this study we explored scattered seismic waves that travel inside the Earth to limit the roughness of the 660-kilometer boundary on Earth.

the researchers were surprised how rough that border is – rougher than the surface layer we all live on. "In other words, the stronger topography of the Rocky Mountains or the Appalachians is at the 660-kilometer border," Wu said. Their statistical model does not allow precise height determination, but these mountains are likely to be larger than anything on the surface of the Earth. The unevenness was not evenly distributed; just as the surface of the earth's crust has smooth ocean flooring and massive mountains, the 660 km border has rough areas and smooth spots. Researchers have also studied a layer of 410 km (255 miles) down at the top of the middle mantle transition area, and they have not found such roughness.

"They find that deep layers of Earth are as complex as what we are observing on the surface," said Seismologist Christine Hauser, an assistant professor at the Tokyo Institute of Technology, who is not involved in this study. "To find a 2- kilometer (1-3 km) altitude change of a border that is over 660 km deep, using waves that travel across the Earth and vice versa, is inspiring feats. earthquakes occur, and seismic tools become more complex and expand into new areas, we will continue to find new, small-scale signals that reveal new properties on the Earth's layers.

The roughness of the 660 km border has significant implications for understanding how our planet is formed and continues to function. This layer separates the mantle, which accounts for about 84% of the Earth's volume in its upper and lower parts. For years, geologists have discussed how important this limit is. In particular, they studied how the heat moves through the mantle – whether the hot rocks are moving smoothly from the boundary between the core and the mantle (almost 2000 miles down) to the top of the mantle, or whether this interruption is interrupted on that layer. Some geochemical and mineralogical evidence suggests that the upper and lower mantle are chemically different, which supports the idea that the two parts are not thermally or physically mixed. Other observations show that there is no chemical difference between the upper and lower mantle, which causes some to argue about what is called a "well mixed mantle", with the upper and lower mantle involved in the same heat transfer cycle. to give an idea of ​​this, Wu said. Their data show that both groups may be partially correct. Smoother areas at the 660 km limit may result from fuller vertical mixing, while coarser, mountainous areas may have formed where the top and bottom mantle are also not mixed. , which exist on large, moderate and small rocks, theoretically can be caused by thermal anomalies or chemical heterogeneities. But because heat is transported into the mantle, Wu explains, every little thermal anomaly will be smoothed within a million years. This leaves only chemical differences to explain their small roughness

What can cause significant chemical differences? The introduction of rocks that belonged to the earth crust now rests peacefully in the mantle. Scientists have long been discussing the fate of seabed plates that are being pushed into the mantle in the subduction zones, with clashes occurring around the Pacific Ocean and elsewhere in the world. Wu and Irving suggest that the remains of these plates can now be slightly above or slightly below the 660 km limit. "It's easy to assume that we only have the chance to detect seismic waves that travel through the Earth in its current state so seismologists can not understand how the interior of the Earth has changed over the last 4.5 billion years, "Irving said," What is exciting in these results is that they give us new information to understand the fate of ancient tectonic plates that have come down in the mantle, and where the ancient material of the mantle can still reside. "

She added: "Seismology is the most exciting when that allows us to better understand the interior of our planet, both in space and in time. "


See also:
Laboratory experiments grounded the theory that submerged crust at the base of the top mantle of the Earth

More Information:
W. Wu el al., "The Removal of the Broken Chemical Laying of Earth from the 660-Kilometer Topography at the Border," Science (2019). science.sciencemag.org/cgi/doi… 1126 / science.aav0822

"Low Mantle of the Earth" Science (2019). science.sciencemag.org/cgi/doi… 1126 / science.aaw4601

Journal Title:
science

Provided by:
Princeton University


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