Seismology becomes exciting when it allows you to better understand the inner structure of our planet, both in space and in time.
We know from school textbooks that the Earth has three (or four) layers: crust, mantle and core, sometimes subdivided into inner and outer. This is not entirely true, since it excludes some of the other layers that scientists distinguish in the structure of our planet. In a study published in the journal Science, geophysicists from Princeton University (USA) and the Institute of Geodesy and Geophysics in China report mountains and other topography on a layer located at a depth of 660 kilometers and separating the upper and lower mantle.
"Finding elevation changes of up to three kilometers on a border of more than 660 kilometers, using waves that travel through the earth and back, is an inspiring feat," said seismologist Christina Hauser, an assistant professor at Tokyo Institute of Technology, Japan, who was not involved in the study.
The structure of the Earth. The roughness at the boundary layer at a depth of 660 kilometers shows the supposed underground mountains. Credit: Image by Kyle McKernan, Princeton University Office of Communications.
To look deep into the Earth, scientists use the most powerful waves on the planet, which are generated by earthquakes. Deep, violent earthquakes can set the entire mantle in motion, and earthquakes with a magnitude of 7.0 propagate shock waves through the core to the other side of the planet and back.
For this study, scientists turned to key data on the waves detected after the 8.2 magnitude earthquake - the second most powerful on record, which shook Bolivia in 1994.
To simulate the complex behavior of wave scattering in the depths of the Earth, seismologists used Princeton University's Tiger supercomputer cluster. The simulation technology depends on a fundamental property of waves: their ability to change direction and bounce. Just as light waves can be reflected or refracted when passing through a prism, seismic waves travel directly through homogeneous rocks, but are reflected or refracted at the boundary of the media. Thus, their scattering carries information about surface irregularities and deep layers.
“We were greatly surprised by the results obtained. The 660-kilometer border has stronger topography than the Rocky Mountains or Appalachians and is as complex as what we see on the surface,”the study authors write.
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Rocky Mountain view from Rocky Mountain National Park USA. Credit: Stanislav Savin.
The statistical model did not allow to accurately determine the height of the mountains found in the depths of the mountains, but it became clear that the irregularities are unevenly distributed, just as the surface of the earth's crust has smooth areas of the ocean floor and high mountains. The researchers also studied a layer at a depth of 410 kilometers, in the upper part of the "transition zone" of the mantle, and did not find such a topographic spread.
The results achieved show how advanced seismic instruments have been to discover new and unexpected properties of the earth's layers.
What does this mean
The presence of irregularities at the 660-kilometer border is essential for understanding how our planet was formed. The explored layer divides the mantle, which makes up about 84 percent of the Earth's volume, into its upper and lower parts. For years, geological scientists have debated how important this boundary is. In particular, they investigated how heat travels through the mantle.
Some geochemical and mineralogical evidence suggests a chemical difference between the upper and lower mantle, which supports the idea that the two sections do not mix thermally or physically. However, the data show that smoother regions at the 660 km boundary may be the result of careful vertical mixing, while mountainous regions may have formed where mixing does not occur.
In addition, the detected irregularities can theoretically be caused by thermal anomalies or chemical irregularities. But due to the redistribution of heat in the mantle, any small thermal anomaly will be smoothed out over a million years, leaving only chemical differences.
So what could have caused the significant difference in layer chemistry? Scientists say the reason for this is the sinking of rocks that used to belong to the earth's crust. Geophysicists have long debated the fate of the seabed plates that cut into the mantle in subduction zones around the world. Researchers speculate that the remains of these ancient plates may now be just above or just below the 660-kilometer border.
“Seismology becomes exciting when it allows us to better understand the inner structure of our planet both in space and in time,” the study authors conclude.
