The echo of large earthquakes has allowed us to explore a border that is 660 kilometers underground and is one of the most unknown regions of the planet.
In textbooks, the interior of the Earth is divided into three layers -Bark, mantle and core- whose limits are smooth and perfect as the surface of a sphere. Now, seismologists from the US and China have analyzed data from 13 major earthquakes and reveal that the reality is quite different from the one learned at school.
The seismic waves of the most powerful tremors cross the interior of the Earth, bounce off the earth's crust on the other side of the globe and return again to the epicenter zone, where they are recorded by dozens of seismometers. If the boundaries between the terrestrial layers were smooth, the bounced signals would arrive more or less at the same time. But until now the seismometers have shown significant irregularities exactly 660 kilometers deep, on the border between the upper and lower mantle. Until now, that border had only been studied at scales of hundreds or thousands of kilometers. Now, for the first time, this border has been surveyed at scales of about 10 kilometers.
The mantle concentrates 80% of all the volume of the Earth. There are still many questions to answer about its composition and behavior since it is impossible to reach it from the surface. "The deepest well that has ever been excavated [el pozo superprofundo de Kola, en Rusia] it reached about 12 kilometers deep, "explains Jessica Irving, seismologist at Princeton University (USA) and co-author of the study, published today in Science. "If you try to go further, the pressure ends up collapsing the walls of the well. We try to study an area that is 50 times deeper, where the temperatures are 1,600 degrees and the pressure thousands of times higher than on the surface, "explains Irving. Previous studies have found similar topographic irregularities even deeper, just at the border between the mantle and the core of the Earth.
Thanks to the study of seismic waves the study has revealed that the boundary between the two layers of the mantle has about 10 times more relief than the earth's surface. "Some of the mountains that we see are as tall as The Everest", Irving highlights.
Evidently these are not mountains to use. The upper and lower mantles are made of solid rock, but with different pressures and densities, something comparable to what happens between the air and the rock of the surface. Seismic waves travel faster through the lower mantle, deeper, than the upper one.
For the time being, the team has been able to map a small part of this border, specifically the opposite of the epicenters of large earthquakes such as that registered in Bolivia in 1994, of magnitude 8.4, or that of the Sea of Okhotsk in 2008, with a magnitude of 7.2, explains Irving. The most interesting earthquakes for these studies are those of magnitude 7 or greater.
The study points out that the two layers of the mantle have different chemical compositions, a key question to know if the mantle moves as a whole or if its two layers work separately and are more or less isolated to the passage of molten material from the core .
"Our work suggests that the exchange of material between the deep and the shallow Earth is blocked", explains Sidao Ni, researcher of the Institute of Geophysics of China and co-author of the study. The great mountains of the mantle would be the cause, although the team has also detected flat areas where it is possible that there is connection between the layers. "In this way, both models have something right, the Earth seems to be an intermediate point between them" , Add.
Geophysics Christine Houser, from the Japan Institute of Technology, highlights the importance of the study. If really the deeper layers of the mantle never reach the surface, they have a huge interest. "The exchange of rock and heat between the two layers of the mantle is crucial for the evolution of the planet, but very little is known of this border at small scales," he writes. "The lower mantle is a relic of when the Earth turned into a planet proper from a disk of dust," he says.
This type of studies are only possible thanks to higher intensity earthquakes. The team now wants to analyze more signals from other earthquakes captured around the planet to begin to compose a complete map of this unreachable Earth frontier.