"Old Buried Mountains" – The massive earthquake of 1994 in Bolivia reveals the secrets of the Earth



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Posted on February 15, 2019

Earthquake in Bolivia in 1994

According to geophysicist Jessica Irving and Wenbo Wu, Princeton University geophysicists, a more spectacular topography than the Rockies was discovered at the border between the 660 and 660 kilometers deep in the interior of the planet.

"It's easy to assume, since we can only detect seismic waves crossing the Earth in their current state, that seismologists can not help to understand how the Earth's interior has changed over the past 4 years. , 5 billion years past, "said Irving. "What's exciting about these results is that they provide us with new information to understand the fate of the old tectonic plates that have descended into the mantle and where the old mantle material might still reside." . "

Most of us have learned that the Earth has three (or four) layers: a crust, a mantle, and a core, sometimes subdivided into an inner core and an outer core. This is not false, but it leaves out many other layers that scientists have identified on Earth, including the transition zone in the mantle.

In a study published this week in Science, Irving and Wu, in collaboration with Sidao Ni of the Institute of Geodesy and Geophysics in China, used data from a huge earthquake in Bolivia to find mountains and another topography at the base of the transition zone, a straight layer of 660 kilometers (410 miles) that separates the upper and lower mantle. (In the absence of an official name for this layer, researchers simply call it "the 660 km limit.")

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To search the Earth in depth, scientists use the most powerful waves of the planet, generated by massive earthquakes. "You want a powerful, deep earthquake that will shake the entire planet," said Irving, assistant professor of geoscience.

Large earthquakes are much more powerful than small ones – the energy increases by 30 times at each level of the Richter scale – and deep earthquakes ", instead of wasting their energy in the crust, can make the whole coat disappear, "said Irving. She's getting her best data from earthquakes of magnitude 7.0 or higher, she said, because the shock waves that they send in all directions can cross the heart from the other side of the planet – and vice versa. For this study, key data come from waves captured after a magnitude 8.2 earthquake, the second-largest-ever deep-sea quake that rocked Bolivia in 1994.

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"Big earthquakes do not happen very often," she said. "We are lucky now to have a lot more seismometers than 20 years ago. Seismology is a different field from that of 20 years ago, between instruments and computer resources. "

Seismologists and data scientists use powerful computers, including the Princeton Tiger supercomputer group, to simulate the complex behavior of diffusion waves in the deep Earth.

Technology is based on a fundamental property of waves: their ability to bend and bounce. Just as light waves can bounce (mirror) on a mirror or bend (refract) when passing through a prism, seismic waves pass through homogeneous rocks but reflect or refract as soon as they meet a boundary or a roughness.

"We know that almost all objects have a surface roughness and therefore a diffuse light," said Wu, the main author of the new paper, who had just completed his PhD in Geoscience. and is now a postdoctoral fellow at the California Institute of Technology. "That's why we can see these objects: scattering waves carry information about the roughness of the surface. In this study, we studied dispersed seismic waves propagating inside the Earth in order to limit the roughness of the 660 km land boundary.

The researchers were surprised at how rough this border is – rougher than the surface layer on which we all live. "In other words, the 660 km limit is stronger than the Rocky Mountains or Appalachians," said Wu.

Their statistical model did not make it possible to determine the height precisely, but it is possible that these mountains are larger than anything on the surface of the Earth. The roughness was not evenly distributed. Just as the surface of the crust has smooth seabed and gigantic mountains, the 660 km limit has rough areas and smooth areas. The researchers also examined a layer located 410 km below, at the top of the "transition zone" in the middle of the mantle, without finding a similar roughness.

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"They find that the deep layers of the Earth are as complicated as what we see on the surface," said Christine Houser, seismologist, assistant professor at the Tokyo Institute of Technology, who did not participate in this research. "Finding elevation changes of 1 to 3 km over a limit of more than 660 km using waves that roam the entire Earth and return is an inspiring feat. … Their findings suggest that as earthquakes occur and seismic instruments become more sophisticated and expand into new regions, we will continue to detect new small-scale signals that reveal the new properties of Earth's layers. . "

The presence of roughness on the 660 km limit has important implications for understanding how our planet has formed and continues to function. This layer divides the mantle, which represents about 84% of the volume of the Earth, in upper and lower parts. For years, geoscientists have debated the importance of this boundary.

In particular, they have studied how heat travels through the mantle – if the warm rocks are transported smoothly from the central-mantle boundary (nearly 2,000 miles) up to the top of the mantle or if this transfer is interrupted at this stage. layer. Some geochemical and mineralogical evidence suggests that the upper and lower layers are chemically different, supporting the idea that the two sections do not mix thermally or physically.

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Other observations suggest no chemical differences between the upper and lower mantle, leading some to argue for what is called a "well-mixed mantle", with the upper and lower mantles both involved in the same transfer cycle. heat.

"Our findings provide insight into this issue," Wu said. Their data suggests that both groups may be partially right. Smoother areas of the 660 km limit could result from deeper vertical mixing, while more rugged, mountainous areas may have formed where the upper mantle and lower mantle do not mix equally well.

In addition, the roughness discovered by researchers, which existed at large, medium and small scale, could theoretically be caused by thermal anomalies or chemical heterogeneities. But because of the way heat is transported in the mantle, Wu explained, any small-scale thermal anomaly would be smoothed in a few million years. This leaves only chemical differences to explain the small-scale roughness found.

What could cause significant chemical differences? The introduction of rocks that belonged to the crust, now resting quietly in the mantle. Scientists have long debated the fate of undercaped ocean floor slabs in subduction zones. The collisions occurred all around the Pacific Ocean and elsewhere in the world. Wu and Irving suggest that the remains of these slabs could now be just above or just below the 660 km limit.

"Seismology is very exciting when it allows us to better understand the inside of our planet in time and in space," Irving added.

The Daily Galaxy via Princeton University

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