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Geology

Ten tremors a day

Study reveals intense movement of the crust in the central region of the country

The tranquility of the vast plains of Goiás and Tocantins is only apparent. Under the surface, there rules an unrest that is now coming to light. A team of researchers from São Paulo, Brasília and Minas Gerais has found that 30 low intensity tremors happen there every year, with a magnitude of between 2 and 4, with extremely rare episodes above this limit. When the milder shocks are taken into consideration, from zero magnitude upwards, the earth there shakes ten times a day, on average, in episodes of no more than three seconds.

“We knew that there were tremors in the central region of Brazil, but not that there were so many of them,” observes the project’s coordinator, geophysicist Jesús Antonio Berrocal Gomez, from the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG), at the University of São Paulo (USP). Since the 70’s, only one or two tremors a year had been detected in the region – as today, they could not even be called earthquakes, a term reserved for the intense shocks that are capable of bringing down houses and buildings.

The fact is that there were only ten seismographs – the apparatuses that record the movement of the crust, the outermost layer of the Earth’s surface -, and these were concentrated around Brasilia and the Mesa mountain range, in the northern region of Goiás. Four years ago, when this study started, another ten were installed, to cover other strategic points, and the real activity of the crust in this part of Brazil started to emerge. The results earned reinforcement by a technique adopted for the first time in Brazil, the so-called deep seismic refraction: the researchers set off artificial tremors and sounded the crust of the heart of Brazil, at depths of up to 40 kilometers.

The ceaseless underground agitation does not mean a cause for concern for buildings or human activities: in the immense majority, only seismographs record the tremors. It is important, though, for enriching with new details the remote history of this part of the country, and for reflecting phenomena on a planetary scale that are still under way, probably in no other place in Brazilian territory. “The tremors of the central region of Brazil are echoes of the breaking up of Pangaea,” rules Berrocal, referring to the supercontinent formed some 250 million years ago, which, when it broke up, in a process that started 200 millions of years ago, gave rise to today’s Americas, Africa, Europe, Asia and Oceania.

The Goiás ocean
Berrocal and his team interpret the tenuous shocks of the central region of Brazil as a minor local response to the closest movement of the accommodation of the tectonic plates. This is the case of Africa and South America moving apart, two continents that 250 million years ago were united, and that today make up different plates and are moving away from each other by a few centimeters each year. They can have an echo in the heart of Brazil, in such a way that the scientists intend to detail shortly even the movements of accommodation of the blocks of rocks the encounter of which resulted in the Andes, a relatively young mountain range, formed 150 million years ago, and which has plenty of energy to release, as is attested by the earthquakes that still frighten Colombians, Peruvians, Ecuadorians and Chileans.

With this work, it was possible to take a leap to even more distant ages, in a way that made clearer the initial stage of the formation of this part of the Brazilian territory called the Tocantins Tectonic Province, an area measuring 1,000 kilometers in the north-south direction and from 400 to 600 kilometers in width, which runs from the south of Tocantins to the border of Goiás with Minas, on the one side, and the east of Mato Grosso, on the other. Auscultation of the crust – yes, this is the term that is used, as if geologists were listening to a heart beating – confirmed something that had only been cogitated some years ago: between 1 billion and 640 million years ago, an ocean of proportions close to today’s Atlantic occupied the central portion of the now Brazilian territory.

The blocks of rocks that emerged formed an archipelago like the Japanese archipelago today. The landscape, though, was then far different from what it is nowadays, with mountains like the Andes and active volcanoes. “What we see today,” says Reinhardt Fuck, a geologist from the University of Brasilia (UnB) who is taking part in the project, “is just the root of the more ancient terrain, since the mountain range has been almost completely eroded.” At the beginning of the research, it was thought that today’s tremors were merely the final accommodation of two cratons (sub-units of the tectonic plate), those of the Amazon and the São Francisco, the meeting of which originated the Tocantins Province. This hypothesis has not been entirely discarded, which ought to have some weight, albeit limited, but one detail made it wobble. “The accommodation of the cratons ought to have ended at least 500 million years ago,” observes Fuck, who began to study the region in 1969.

Younger arc
The artificial tremors, which help to delimit the range of the movements of the blocks of rocks, were generated using 21 holes with a depth of between 40 and 60 meters, filled with up to 1,000 kilos of gelatinous explosive and spread out along three routes of 300 kilometers each. Accompanied by means of 120 seismographs distributed along the three lines, the explosions revealed a variation of almost 11 kilometers in the thickness of the crust in the region, which grows from west to east. Among the points most distant from the average of 36 kilometers of thickness found there, São Luís dos Montes Belos, in Goiás, is to be picked out, where the inhabitants are settled on a surface layer of rock measuring 33.7 kilometers. At the other extreme, the most strengthened crust was found beneath Brasilia: the inhabitants of the federal capital live on top of 43 kilometers of crust rock.

The tremors are concentrated in a strip almost 700 kilometers long, in which the crust is thinnest, called the Goiás Magmatic Arc. Its age has dropped down some 1.6 billion years, following the study of the crust and the dating of the rocks. According to Fuck, this stretch of Tocantins Province was formed with the material taken out of the Earth’s mantle and transferred to the continental crust between 930 and 640 million years ago, as a consequence of the destruction of the ocean that separated the Amazon and São Francisco cratons. “It used to be thought that this part of the Tocantins Province was one of the most ancient in Brazil, but the data obtained indicates that we are dealing with one of the youngest,” says the researcher from the UnB.

The project
Geophysical Studies and Tectonic Model of the Central and Southeastern Sectors of the Tocantins Province, Central Brazil (nº 96/01566-0); Modality Thematic project; Coordinator Jesús Antonio Berrocal Gomez – IAG/USP; Investment R$ 423,507.56 and US$ 112,073.90

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