How powerful Bolivian earthquakes discovered mountains at a depth of 660 kilometers underground

All schoolchildren know that the planet Earth is divided into three (or four) large layers: the crust, mantle and core. In general, this is true, although this generalization does not take into account several additional layers determined by scientists, one of which, for example, is a transitional layer inside the mantle.


In a study published on February 15, 2019, geophysicist Jessica Irving and undergraduate student Wenbo Wu of Princeton University, in collaboration with Sidao Ni of the Geodetic and Geophysical Institute in China, used data from the massive 1994 earthquake in Bolivia to find mountains and other topographic elements on the surface of the transition zone deep inside the mantle. This layer, located at a depth of 660 kilometers below the ground, separates the upper and lower parts of the mantle (without a formal name for this layer, the researchers simply called it “660 km border”).

In order to “look” so deep underground, scientists used the most powerful waves on the planet, caused by strong earthquakes. “You need a strong and deep earthquake to shake the planet,” said Jessica Irving, assistant professor of geophysical sciences.

Large earthquakes are much more powerful than ordinary ones - the energy of which increases 30-fold with each additional step up the Richter scale. Irving gets its best data from earthquakes with magnitudes 7.0 and above, because the seismic waves sent by such powerful earthquakes diverge in different directions and can pass through the core to the other side of the planet and back. For this study, key data was obtained from seismic waves that were recorded from an earthquake with magnitude 8.3 - the second deepest earthquake ever recorded by geologists - that shocked Bolivia in 1994.

“Earthquakes of this magnitude do not happen often. We are very lucky that now there are much more seismometers installed around the world than 20 years ago. Seismology has also changed dramatically over the past 20 years thanks to new tools and computer capabilities.

Seismologists and data analysts use supercomputers, such as the Princeton Tiger cluster supercomputer, to simulate the complex behavior of scattering seismic waves deep underground.

Technologies are based on the fundamental properties of waves: their ability to reflect and refract. Just like light waves, they can bounce (reflect) from a mirror or bend (refract) when they pass through a prism, seismic waves pass through homogeneous rocks, but are reflected or refracted when they encounter uneven surfaces in the path.

“We know that almost all objects have an uneven surface and therefore can scatter light,” said Wenbo Wu, the main author of this study, who recently received a PhD in geonomy and is currently undergoing postdoctoral studies at the California Institute of Technology. “Thanks to this fact, we can“ see ”these objects - scattering waves carry information about the roughness of the surfaces that they encounter on their way. In this study, we studied scattering seismic waves propagating deep inside the Earth to determine the irregularities of the found 660-km boundary. "

Researchers were surprised how uneven this border is - even more than the surface layer on which we live. “In other words, this underground layer has a topography more complicated than the Rocky Mountains or the Appalachian mountain system,” Wu said. Their statistical model was not able to determine the exact heights of these underground mountains, but there is a high probability that they are much higher than anything on the surface of the Earth. Scientists also noticed that the 660-kilometer border is also unevenly distributed. In the same way that the ground layer has a smooth ocean surface in some parts and massive mountains in others, the 660-km border also has uneven zones and smooth layers on its surface. The researchers also studied the underground layers at a depth of 410 kilometers and on top of the middle layer of the mantle, but could not find a similar roughness of these surfaces.

“They found that the 660-km boundary is as complex as the surface earth layer,” said seismologist Kristina Hauser, an assistant professor at Tokyo Institute of Technology who was not involved in this study. “Using seismic waves created by powerful earthquakes to find the 3 km difference in elevation of the terrain located 660 kilometers deep underground is an unimaginable feat. ... Their discoveries mean that in the future, using more sophisticated seismic tools, we will be able to detect previously unexplored, inconspicuous signals that will reveal to us new properties of the inner layers of our planet. "

Seismologist Jessica Irving, an assistant professor of geophysics, holds two meteorites from the Princeton University collection that contain iron and are supposedly part of planet Earths.
Photo taken by Denis Appelwight.

What does this mean?

The existence of uneven surfaces at the 660-km boundary is important for understanding how our planet is formed and functions. This layer divides the mantle, which makes up about 84 percent of our planet, into upper and lower sections. For years, geologists have debated how important this boundary is. In particular, they studied how heat is transported through the mantle - and whether heated rocks move from the Gutenberg border (the layer separating the mantle from the core at a depth of 2900 kilometers) up to the top of the mantle or is this movement interrupted at the 660-km boundary. Some geochemical and mineralogical data suggest that the upper and lower layers of the mantle have different chemical compositions, which supports the idea that both layers do not mix thermally or physically. Other observations suggest that the upper and lower layers of the mantle do not have any chemical difference, which gives rise to a debate about the so-called “well-mixed mantle”, where both layers of the mantle are involved in an adjacent heat transfer cycle.

“Our study provides a new perspective on this debate,” said Wenbo Wu. Evidence from this study suggests that both sides may be partially correct. Smoother layers of the 660-km boundary can be formed due to careful vertical mixing, where more uneven, mountainous zones could be formed in a place where the mixing of the upper and lower layers of the mantle did not proceed smoothly either.

In addition, layer roughness at the found boundary was discovered on a large, medium, and small scale by research scientists, which in theory could be caused by thermal anomalies or chemical heterogeneity. But due to how heat is carried in the mantle, Wu explains, any small-scale thermal anomaly would have been smoothed over several million years. Thus, only chemical heterogeneity can explain the roughness of this layer.

What could be the reason for such significant chemical heterogeneity? For example, the appearance of rock in the mantle layers that belonged to the earth's crust and moved there for many millions of years. Scientists have been arguing for a long time about the fate of plates on the seabed, which are pushed into the mantle in subduction zones, which collide around the Pacific Ocean and in other parts of the globe. Weibo Wu and Jessica Irving suggest that the remains of these plates may now be above or below the 660-kilometer border.

“Many people believe that it is quite difficult to study the internal structure of the planet and its changes over the past 4.5 billion years, only using data from seismic waves. But this is far from it! ”Said Irving.“ This study has given us new information about the fate of ancient tectonic plates that descended into the mantle for many billions of years. ”

In the end, Irving added: “I believe that seismology is most interesting when it helps us understand the internal structure of our planet in space and time.”

From the author of the translation: I always wanted to try my hand at translating a popular science article from English into Russian, but I did not expect how difficult it was. Great respect to those who regularly and qualitatively translate articles on Habré. To professionally translate a text, one must not only know English, but also understand the topic itself by studying third-party sources. Add a little "gag" to sound more natural, but also not to overdo it, so as not to spoil the article. Thanks so much for reading :)


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