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Geology

Quaking earth

Recurrent earthquakes helped to shape the northeastern terrain

JOÃO ALEXANDRINOPlace in the sun: native Brazilian pine tree needs a lot of light to grow JOÃO ALEXANDRINO

The Brazilian Northeast is an exciting region, and not only because of Carnival and other festivities. According to researchers from the states of Rio Grande do Norte and São Paulo, the earthquakes that hit the region every once in a while are far from being anything new. They hit the region way before the world was populated by humans and still do. Geologist Francisco Hilario Bezerra, from the Federal University of Rio Grande do Norte (UFRN), resorts to popular culture to emphasize the wrong idea people have of the movement of Brazilian land: “Go, go, go, get on my motorcycle, come, come, come, there are no earthquakes here,” are the lyrics of the song Insolação do coração, by Carlinhos Brown, sung by Claudia Leitte. According to the researcher, what the lyrics say is not entirely true. There are many earthquakes in Brazil, especially in the region the researcher comes from.

“The Brazilian Northeast is where most earthquakes occur,” says Bezerra, “but nobody knows exactly why.” The results of the study conducted by the group from UFRN make it clear that earthquakes have been a recurrent phenomenon in the region in the last 400 thousand years. In addition to explaining the characteristics of the northeastern terrain, this knowledge can be useful, for instance, to provide guidelines in the field of civil engineering. “If we determine that a given area has been prone to magnitude 5 earthquakes for thousands of years, then constructions must be earthquake-resistant,” explains the geologist.

The tectonic aspects of the region are part of a more extensive research project coordinated by geologist Reinhardt Fuck, from the University of Brasília, covered by the National Institute of Science and Technology (INCT) of Tectonic Studies. Part of the work was done by Francisco Cézar Nogueira during his doctoral studies under Bezerra. He studied a 35-kilometer long tectonic fault. The Jundiai River, which runs along the fault, crosses the city of Natal. Nogueira noticed that every 16 thousand years, the movements along this fault had caused earthquakes, according to an article published in the Journal of Geodynamics this year.

Nogueira’s main source of information was the sand that fills the deep cracks in the soil. Sand can be a challenge when it is used as the main material for geological studies. Sandy regions in dry climates are unsuitable to preserve fossils and this is why it is difficult to date them with carbon-14, the standard dating technique. This challenge was solved through a partnership with the laboratory headed by Sonia Tatumi, from the College of Technology of São Paulo (Fatec-SP). Sonia Tatumi is an expert in the analysis of optically stimulated luminescence. This technique measures the position of electrons inside quartz sand grains to evaluate the age of the sand. Sunlight attracts these electrons to the outermost layer, and the electrons go back inside the sand grain when the layer of sand is buried. This method allows one to estimate how long the grain has been lying underground, within up to one million years. By assuming that the sand infiltrated inside the Jundiai fault was buried as a result of the crack, the dating technique could estimate that the fault was formed approximately 100 thousand years ago. The fault has been active ever since, even though this fact was never recorded in historic data. Historic data on earthquakes in the region goes back just 200 years. No seismic activity was recorded in the Jundiai fault during this period, which might lead to the erroneous conclusion that the fault is inactive.

ELISSANDRA MOURA-LIMA / UFRNPebbles form a dome-like structureELISSANDRA MOURA-LIMA / UFRN

Loose soil
Studying faults is not the only way to research the seismology of a given region. Although the group from UFRN had no direct access to the fault that causes quakes in a specific area, they resorted to other alterations in the soil to infer past movements. One of these phenomena is liquefaction, which occurs when a combination of water and sand trapped in the subsoil is submitted to enormous pressure, such as the pressure generated by an earthquake. Bezerra makes an analogy to help one understand what happens in this case, by comparing it to the pressure that is created when a bottle of champagne is shaken. “The champagne bottle stopper, which, in the case of soil, can be compared to a rock, keeps the water and sand mixture from expanding and the pressure increases until everything bursts,” he explains. In the case of champagne, the movement is similar to quartz grains when they are shaken by an earthquake and are ejected together with the water after the rock breaks open. The end result is massive destruction and, nowadays, buildings being demolished.

The signs of such earthquakes solidify later on and are registered: this is what Elissandra Moura-Lima focused on in her doctorate. The witnesses to this activity are the pebbles that lie on top of the sand. Once again, Bezerra resorts to an image to clarify the instability of this arrangement: “Imagine a bowl of gelatin of the kind we eat, with an iron on top of it.” A small shake will destroy the balance and cause the iron to drown in the gelatin. The iron would probably slide sideways into a position that offers less resistance. This is what happens with the pebbles: when they are found under the surface in a vertical position, researchers can infer the path they travelled along. With the help of luminescence, they can also estimate when these movements took place.

Elissandra also resorted to a kind of CAT scan known as ground penetrating radar/GPR. This allowed her to characterize the dome-like structures found in the valley of the Açu River, which is part of the Potiguar basin. These domes are formed when the pebbles burrow into the soil and push the sand upwards. Mapping these soil deformations within the context of a network of faults that runs throughout the region has allowed researchers to estimate the time and the magnitude of the earthquakes that occurred thousands of years ago. A magnitude 5 or 6 earthquake, for example, causes alterations within a two-kilometer radius. In the Açu River valley, the group showed that earthquakes had already been recurrent 400 thousand years ago. This is why it is very likely that the faults that run under this valley were responsible for most of the seismic activity that occurred in the past in the Potiguar basin.

More than just a landscape
The advantage of being a geologist and an expert on this region is that the geologist can work in a more attractive environment than quarries or deserts. The cliffs that lie along most of the northeastern coast of Brazil are not only breathtakingly beautiful but also a great source of information. The cliffs, some of which are 30 meters high, come in shades of red, yellow, purple and white. These colorful walls display a seismic and geological history that goes back many thousands of years. All an expert needs to do is look at these cliffs to notice the horizontal stripes that outline sediments from different times and recognize the characteristics of the influence of seismic activity in their formation.

This is the landscape where Dilce Rossetti, from the National Institute of Space Research (Inpe) does most of her work. Wherever possible, she analyses the sides of rocks, such as those that were cut for highway construction. “The northeastern coast is perfect for this kind of study,” explains the researcher, “because of the abundance of cliffs that run for many kilometers.” This allows her to compare deformations in the terrain caused by liquefaction in different contexts, such as near a fault and far from a fault. In addition, by studying a single site on a heavenly beach, she has access to thousands of years of history. In an article to be published in January 2011 in a special issue on paleo-earthquakes in Sedimentary Geology, Dilce uses these deformations to show how the tip of the state of Paraíba, the farthermost point of the South American continent to be detached from Africa, is not as passive as had been believed. Seismic activity at that point is disseminated.

She has used carbon-14 to date these events whenever she can find organic material. Otherwise, she resorts to luminescence, the results of which are in the final stages of preparation prior to publication. She noticed various layers with signs of seismic activity on top of the geological formation known as Barreiras, formed approximately 20 million years ago. She has found rocks on a cliff in the state of Paraiba that are 178 thousand years old; the most common signs that she had found before go back 67 thousand years. “There has long been seismic activity in several places in Paraiba and in other northeastern states as well,” she says. According to Dilce, these land movements were responsible for shaping part of the region’s terrain, such as the cliffs and the location of some riverbeds.
It is impossible to extrapolate the results obtained in the Northeast to other parts of Brazil. “Each fault behaves in a specific way,” explains Bezerra. This is why the Taubaté and Santos faults, in the state of São Paulo, are from a different period and have a distinct way of acting – all of which is yet to be studied. In his opinion, the importance of this joint research work is to show how current natural phenomena are insufficient for one to understand what happens at present. “It isn’t enough to have historical and instrumental knowledge. We need to examine the layers of a distant past.”

The projects
1.
Geophysical and geological studies in the province of Borborema (nº 00/13626-5); Type National Institute of Science and Technology (CNPq); Coordinator Reinhardt Fuck – IG/UnB; Investment R$ 3,400,000.00
2. Integration of sedimentology, remote sensing and geochemistry applied to the mapping of cretaceous-tertiary succession in the central part of the Paraíba basin (nº 2006/04687-7); Type Regular Research Awards; Coordinator Dilce de Fátima Rossetti – Inpe; Investment R$ 125,659.43

Scientific articles
NOGUEIRA, F. C. et al. Quaternary fault kinematics and chronology in intraplate northeastern Brazil. Journal of Geodynamics. v. 49, . 2, p. 79-91. Mar. 2010.
MOURA-LIMA, E. N. et al. 3-D geometry and luminescence chronology of Quaternary soft-sediment deformation structures in gravels, northeastern Brazil. Sedimentary Geology. Forthcoming.
ROSSETTI, D. F. et al. Sediment deformation in Miocene and post-Miocene strata, Northeastern Brazil: Evidence for paleoseismicity in a passive margin. Sedimentary Geology. Forthcoming.

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