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Geophysics

The bones of the Earth

Satellite outlines the densest regions on the Earth's surface

The Andes, one of the difficult-to-access regions investigated by Goce...

Eduardo CesarThe Andes, one of the difficult-to-access regions investigated by Goce…Eduardo Cesar

Before falling into the ocean in November 2013, ending its four-year mission orbiting the Earth, the satellite Goce (Gravity Field and Steady-State Ocean Circulation Explorer) precisely recorded the Earth’s gravitational field, determined by the variations in density. The greater the mass inside the Earth, the greater the gravitational field and gravitational acceleration. Now the information is helping to identify Earth’s great structures, especially in hard-to-reach areas like the Amazon, the Andes and Siberia, where surface data is scarce. Designed, launched and managed by the European Space Agency (ESA), Goce has helped to reconstruct the history of our planet.

Using the information obtained by Goce, Carla Braitenberg, of the University of Trieste, Italy, determined the regions of higher density or with a larger gravitational field, highlighting the densest areas, as if they were the bones of the earth, inaccessible through direct geological observations. She identified the oldest rock structures, called cratons, in Africa and South America and detected the continuity in the structures of greater or lesser density on the two continents, such as Borborema Province, in Northeastern Brazil, which was geologically connected with Central West Africa. The conclusion is that these rock structures must have been continuous before the continents separated, and now reappear united.

Using Goce, the physicist Everton Bomfim, in his doctorate at the Institute of Astronomy, Geosciences and Atmospheric Sciences (IAG), of the University of São Paulo (USP), detected errors in the ground measurements of the variation of gravity in limited areas in the Amazon in the 1970s. He then determined that what is known as the Amazon craton could actually consist of two cratons—one to the north, the Guiana Shield, and another south of the Amazon River, the Brazilian Shield—even though they have geological ages similar to each other, up to 3.2 billion years.

This possibility “could change the geological history of the region a little,” said Bomfim, with the caveat: “We cannot draw a final conclusion from gravitational measurements alone. We also need other sources of data, such as paleomagnetism.” Paleomagnetism is an analysis technique for measuring the variations in the magnetic pole of the Earth and determining the magnetic poles of rocks thousands or millions of years ago (see Pesquisa FAPESP Issue No. 85). Recent paleomagnetic studies in the southern and northern regions of the Amazon craton, coordinated by Manoel D’Agrella-Filho and Franklin Bispo-Santos, also at IAG-USP, detected a possible difference in sense between the two parts of the craton, indicating that they could have different origins and that, at some point in time, they were already separated. “We are only suggesting this, which contradicts widely accepted conclusions about the formation of the Amazon sedimentary basin”, Bomfim said. This is enough, however, to evoke the possibility of mineral deposits and oil not yet identified in the region.

... whose images of variations in the gravitational field resemble a squashed Earth

ESA… whose images of variations in the gravitational field resemble a squashed EarthESA

“Goce does not see details, but rather the whole earth,” summarized Eder Molina, an IAG-USP professor specializing in measurement of the variations in the gravitational field, and who advised Bomfim during his master’s and PhD studies. “Or in other words, it sees large things well, things which other gravity measurement models do not.” For this reason, he said, even with a resolution of 80 km, less than other satellites, data from Goce has helped supplement or correct less-comprehensive surface measurements, and it was the only device to measure the variation of the components of gravity relative to the three spatial axes x, y and z—previously only the vertical variation (along the z axis) of the acceleration of gravity was measured, determined by the force of gravity. Goce measured variations in the gravity field in nine directions (up, down, forward, backward and to the sides), indicating the influence of mountains or denser rocks near the point analyzed, whose format could thus be more accurately delineated.

In 2011, with information from gravitational satellites simpler than Goce, Molina and his team developed a map of variations in sea level, recording a difference of 70 meters between the height of the waterline in South Africa and that in Belém, in the state of Pará, due to fluctuations of the Earth’s gravitational field (see Pesquisa FAPESP Issue No. 181).

Africa and the Andes
His following work, not yet published, portrays the possible gravimetric fit between South America and the west coast of Africa before the separation of the continents. The map, he noted, is very similar to that published in February 2014 by Carla Braitenberg, in Trieste. She herself says in the article that his map is another way to see the unified continents as a single block and certainly will be thoroughly analyzed by geologists who examined only specific regions on both continents that might be linked.

“Some results question the validity of established concepts,” said Orlando Álvarez, a researcher at the University of San Juan, Argentina, who worked with Carla Braitenberg in Trieste for a month in 2010.  After his return to Argentina, using Goce data and other gravimetric models, he mapped the fracture zones in the Andes, the geographic limits of the cratons in Argentina, and the horizontal or inclined advance of the Nazca plate over the South American continent. “The rupture areas caused by strong earthquakes, such as the Valdivia quake in 1960, coincided with our results,” he said. “We can now map the most fragile regions and possible rupture zones before earthquakes, although it is not possible to predict when and where an earthquake might occur.”

Though maybe sometimes it might be possible. On March 27, 2014, the Chilean Hans Agurto Detzel, in a presentation at IAG-USP, where he is a researcher, said that he had observed a sequence of small earthquakes along the northern Chilean coast, based on a network of seismographs, one of the most common devices used for geophysical research. He indicated a still empty region—a seismic gap—and the imminent possibility of an earthquake of magnitude eight or nine in that area; the last earthquake that large in the region was in 1877. On April 1, 2014, an 8.2 magnitude earthquake occurred in the region he had indicated, rupturing only 200 of the 500 km of the seismic gap.

In the following days, monitoring the tremors in northern Chile, he noticed that the tremors began to migrate towards the southern region of the seismic gap. It was precisely to the south, between Iquique and the Mejillones Peninsula, that there seemed to be a lot of accumulated energy, “enough to generate another earthquake of similar or greater magnitude,” he said. A IAG website page contains updated information on tremors in Brazil and neighboring countries.

Scientific articles
BOMFIM, E. P. et alMutual evaluation of global gravity models (EGM2008 and Goce) and terrestrial data in Amazon Basin, BrazilGeophysical Journal International, v. 2, p. 870-82. 2013.
BRAITENBERG, C. Exploration of tectonic structures with Goce in Africa and across-continentsInternational Journal of Applied Earth Observation and Geoinformation. 2014 (no prelo).

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