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Geology

Landscape sculptors

Recent studies attest to the importance of the earthquakes that have occurred over the last ten thousand years for the formation of the Brazilian geographical relief

At the beginning of May, three earth tremors shook Caruaru, in Pernambuco. They were low level quakes – the most intense reached a magnitude of 3.1 on the Richter scale that goes as high as 9.0 – and they were not as frightening as much as those that occurred at the back end of the 80s in João Câmara, in Rio Grande do Norte. Over a four-year period, from 1986 until 1989, the thirty thousand inhabitants of this town felt the earth tremble, as a consequence of a succession of close to 40,000 earth tremors. During this sequence, the most spectacular as yet registered in the country, two tremors reached a magnitude of 5, liberating energy equivalent to the Hiroshima atomic bomb, and damaging around four thousand homes.

This series of tremors buried the myth that Brazil is free of earthquakes – there are in the country on average 80 to 90 tremors per year, almost all of them at a magnitude inferior to 4 – and originated studies that have proven that the most recent tremors – or as the geologists say, the seisms – that have occurred over the last ten thousand years, have interfered in the definition of the forms of geographic relief in a manner as intense as climatic phenomena, mainly rain and wind, which cause erosion. These seisms may well have been as decisive for the sculpturing of the sierras, plateaus and plains of the country, as intensive climatic variations of the era of the glaciers, during the geological period known as Quaternary, which began some 1.8 million years ago.

The most recent study about the consequences of earthquakes that have occurred in Brazil during the recent past, was published in March in the magazine Geomorphology. In the article three São Paulo researchers – Claudio Riccomini, from the Geosciences Institute of the University of São Paulo (USP), and May Modenesi-Gauttieri and Silvio Hiruma, both from the Geology Institute of São Paulo – demonstrated that the tremors that had occurred between 10,000 and 3,000 years ago, have contributed to producing the current forms of the most elevated region of the State of São Paulo: the Mantiqueira range, a mountainous chain some 320 kilometers in length that extends from the south of the State of Minas Gerais to that of Rio de Janeiro.

During the same era, the earthquakes, caused by the movements of blocks of rocks that form the crust of the earth – a fine covering of between 5 and 40 kilometers of thickness that makes up the surface of the planet -, were also responsible for the grouping of some 136 lakes in the mid – Doce river, in the region of the State Park named Rio Doce, in Minas Gerais. And, along the coastline, in a band between Rio de Janeiro and the mouth of the Amazon river, it raised or lowered, by up to three meters, the crags – large rocky walls that tower in front of the sea.

The research into the causes and the consequences of earthquakes is still provoking a conceptual trembling: it has led the specialists to re-evaluate the concept about the real activity of areas considered to be geologically stable, such as the eastern part of South America, where Brazil is located. It had been believed that these portions of the tectonic plates – the blocks of rock which go to make up the earth’s crust – had been free of earthquakes since they were submitted to only slight compression pressure. They are not. The geologist Francisco Hilário Bezerra, from the Federal University of Rio Grande do Norte (UFRN), has gathered evidence that tremors of a magnitude of 7 have hit the Northeast, exactly on the passive margins of the continent, from 10,000 years ago up until now, during the final stage of the Quaternary, called Holocene.

It was Bezerra and an English geologist, Claudio Vita-Finzi, from London University College, who consolidated the idea that tremors could occur on these areas of the Brazilian coastline in an article published in July of 2000 in another magazine highly regarded in this area, the magazine Geology. The dating of the sediments on the coast line of the Northeast indicate that in that location there had been an intense movement of the crust during a much more remote era, some five million years ago, at the end of the Tertiary period, and during another more recent time, between 6,700 and 2,900 years ago, then in the Holocene era.

Risks
Once that there is no more the myth of the calmness of the passive margins, the risk of the occurrence of intense earthquakes becomes more sustainable, although Brazil does not even get close to the situation in Japan or of the west coast of the United States, areas of risk much, much greater since they find themselves over active margins (collision areas) of the tectonic plates. “The danger of more intensive tremors occurring in Brazil is statistically low”, tranquilizes the seismologist Jesus Berrocal, from the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG) of USP, one of the centers for accompanying seisms in the country. The two most intense tremors that have been verified in Brazil occurred in 1955 in the state of Mato Grosso, with magnitude 6.2, and the other on the coast of Espírito Santo, some 300 kms from the capital Vitória, with magnitude 6.1 – both would have been catastrophic if they had occurred in populated regions.

But not even the researchers had imagined the earth could shake so much around here. “The risk of seisms is much greater that we had believed”, recognized Bezerra, who for more than five years has been studying the phenomenon on the Northeast coastline, the region most susceptible to seisms: It is estimated that in the Northeast an earthquake of magnitude 4 might occur over the next three to ten years, while in the Southeast the probability is less over a longer period, between ten to fifteen years. “There is no reason to alarm the population, but large engineering works should take this fact into consideration”, alerts the researcher. Depending on the seismic activity of a region, it may well be necessary to plan highways and hydroelectric schemes with reinforced structures or even to avoid human occupation.

Allaoua Saadi, from the Geosciences Institute of the Federal University of Minas Gerais (UFMG), found that the critical regions in Brazil where generally earthquakes occur are not just a few. These make up the so called fault lines or geological fissures – the tremors are caused by the movement of blocks of rock (tectonic plates) exactly in the regions of the faults. During 2000, in the fist national survey of these fault lines, part of a mapping of the active tectonic structures of the planet, Saadi identified 48 fissures, some with hundreds of kilometers of extension and concentrated along the Amazon river, in the Northeast and the Southeast.

But why should these faults, which are found in a region of passive margin, be so active? Riccomini attributes the reactivation of the much older geological faults, formed more than 540 million of years ago, in a recent era, of some 10,000 years until the present, with the movement of the tectonic plates above which is Brazil. The phenomenon is due, in his opinion, to the pushing of the South American plates – which contain the Continent – to the west, brought on by the opening of the floor of the Atlantic Ocean, which began some 130 million years ago with the separation of the African continent, which is still going on.

During these 10,000 years, the eastern edge of the South American continent, which extends kilometers into the ocean, also became heavier because of the accumulation of sediment brought down to the sea by rivers and wind. At the point where the edge of the continent meets the bottom of the ocean, there occurs a continuous cooling of the seabed of the Atlantic, which increase the density of the region. Together, these two factors provoked a type of seesaw, which make the stretches of the continental margin under the ocean sink, while blocks of the crust in the immersed part of the continent are rising.

Summed up, these factors make the plateau of Campos do Jordão, between the cities of São Paulo and Rio de Janeiro, tremble strongly three times during the Holocene. According to Riccomini, the first of the movements of the crust in the region was some 10,000 years ago. The blocks of the rocks that form the floor of the Earth have also moved at some time between 8,000 and 5,000 years ago, while the third agitation must have occurred a mere 3,000 years ago. These tremors have caused displacements of soil of up to one meter.

Combined action
The work of the São Paulo researchers consolidates the idea that this movement of the plates was as essential in defining the geographical relief of the region as much as the large climatic variations of the Quaternary, during the last 1.8 million years.Though the conclusion seems obvious, it was not always that way. “There was a period in which lots of emphasis was given to the climatic changes of the Quaternary to explain the modeling of the relief”, explains the geographer May Modenesi-Gauttieri, from the Geology Institute, who has been studying the region of Campos do Jordão since the 80s. “The geomorphologists did not ignore the tectonic plates, but, in this phase, forgot about them.”

Geologists and geomorphologists, when they decided to work together, concluded that the current forms of geographical relief are the results of the combined action of climate and of the movements of the crust over the last few millenniums. On the plateau of Campos do Jordão, tectonic plates and climate helped to sculpture, for example, the escarpments, inclinations from the foot of the hills, aligned along the geological faults, and the rivers in a hook formation, whose course suffered an abrupt change of direction because of an interruption in the relief, and acquired a trajectory that reminds one of the letter “u”.

May observed, more than twenty years ago, the first indications of the occurrence of recent movement of the blocks in the region. The relief exhibits its own marks, such as semicircular structures that form on the surroundings of hills and remind one of the Roman amphitheaters – for this reason called suspended amphitheaters. Covered by Parana Pines forests (Araucaria angustifolia) and by Pines (Podocarpus lambertii), these areas distinguish themselves from the open country vegetation, which predominates in the flat portions of the lands and of the top of the elevations.

May also found hills with surroundings in the form of a triangle, known as triangular facets. These signs, nevertheless, were insufficient to prove the suspicion that the tectonic plates had recently helped to mold the forms of the relief, which could result both from the action of climatic agents and from the movement of the crust. A strong indicator, the presence of rivers in the form of a hook, such as the Ribeirão Galvão river, was one more element to sustain the hypothesis, but the smoking gun was missing.

The proof only came about in the middle of the 80s, when May and the geologist Silvio Hiruma came upon abundant morphological data, but coming from a deepening in theneo-tectonics studies. This was when they went looking for Riccomini, who would found neo-tectonics movement in the sedimentary basin of the Paraíba do Sul valley at the end of the 80s. On analyzing an area of 220 km2, in São José dos Alpes, to the west of the town of Campos do Jordão, the discoveries fell into place rapidly.

By way of the measurements of geological faults, through the determination of the directions of the forces that reacted over them – the so called force regimes – and by the dating of the sediments, the researchers verified that this phenomenon, finally confirmed, could not have been restricted to the plateau: in the opinion of Riccomini, the recent tectonic movements affected an area much larger, which includes the middle of the Doce River Valley, in Minas Gerais, in Rio de Janeiro and a strip that goes until the south of the State of São Paulo.

This also changed the explanation of the creation of the lakes of the State Park of Doce River, located twenty meters above the level of the river and without connections to a fluvial system, seen, until recently, as the result of the influence of climatic alterations during the Quaternary, which began some 1.8 million years ago. The geologist Cláudio Mello, from the Federal University of Rio de Janeiro (UFRJ), carried out a survey of the seismic movements and of the faults in the region, together with the geomorphological data and concluded: the movements of the crust that happened around some 9,500 years ago were the main actor that influenced the trapping of the lakes in Minas Gerias.

“It was the recent movement of the fault lines of the region that gave origin to the lakes”, says Mello. During this time, there occurred on the northeastern coastline one or two sequences of intense tremors which lifted or dropped by up to some three meters the walls of the rocks that make up the sea cliffs. Bezerra, who characterized the phenomenon, does not see a reason to believe that other areas of the coast and even of the interior have been saved from the restlessness of the Earth.

The project
Neo-tectonics on the Plateau of Campos do Jordão (nº 96/08239-5); Modality Regular line of research assistance; Coordinator Claudio Riccomini – Geosciences Institute/USP; Investment R$ 26,298.74

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