The fragile crust of the Northeast

Thickness of surface explains constant earth tremors in the region

FABIO COLOMBINIPaulo Afonso, Bahia state: 35-kilometer thick crustFABIO COLOMBINI

The most detailed geophysical study of the Borborema Province, the name by which experts refer to the huge block of rock making up most the Brazilian northeastern region, revealed that the Earth”s crust there is far thinner than the global average. A hypothesis explaining this is that the thinning of the crust happened during the Cretaceous period (some 126 million to 65 million years ago), when Africa and South America separated due to the movement of the blocks that form these two continents. In this process, the crust presumably stretched in that region, like melted cheese pulled by its edges. “Normally, the crust is some 40 kilometers thick. In the Himalayas, it can reach as much as 70 kilometers,” says Reinhardt Fuck, a researcher from the University of Brasília (UnB). “However, in some parts of the Northeast it”s about 30 to 35 kilometers, reaching less than 30 at some points. This means that this crust was very stretched out and became thinner as a result of tectonic processes.”

Though one of the most striking findings, it is far from being the only one of the studies conducted in the Borborema Province. Fuck is the coordinator of the National Institute of Science and Technology (INCT) for Tectonic Studies, which, since late 2009, has brought together groups from several institutions in Brazil, such as the State University of Campinas (Unicamp), Paulista State University (Unesp), the University of São Paulo (USP), the Federal University of Rio Grande do Norte (UFRN), the Federal University of Ceará (UFC), the National Institute for Space Research (Inpe), and the National Observatory in Rio de Janeiro. With an investment of R$2.6 million, the researchers produced approximately 30 scientific articles in the last two years. These efforts began to fill a major gap in the geological research of Brazil”s territory. “We have today a reasonable geological understanding of Brazil”s surface,” explains Fuck. “But this is not enough for us to understand how its crust was formed and evolved. This is where geophysics comes in, with a series of methods for analyzing the composition of rocks, gravimetric anomalies, rock magnetism in depth, and so on and so forth.”

These studies started to evolve in the 1990s, when Jesus Berrocal, then a researcher at USP, now retired, obtained FAPESP support for a geophysics project in Brazil”s central-west area. Together with Fuck and people from UnB, Berrocal conducted for the first time ever in Brazil a seismic refraction survey. This technique allows one to determine the thickness of the Earth”s crust and the transition between this and the mantle, the layer immediately beneath the crust. This, nevertheless, was still a modest effort. Still, the results led to obtaining more substantial funding when the Ministry of Science and Technology instituted the so-called Millennium Institutes, at the end of the twentieth century. One of these, in turn, was the kernel of the current INCT, which is now producing the most complete results ever obtained for any region of the Brazilian crust.

The technique employed to measure the thickness of the crust was the same as had been used, years before, in the central-west. It consists of creating explosions near the surface and studying how the shock waves propagate inside the Earth. When the waves move from one environment to the next, they are partially reflected and partially refracted, which means that they change speed. As the internal regions of our planet have different densities and consistencies, one can, by measuring the speed of the propagation of waves over time, estimate where the border between the crust, which is solid, and the mantle, which is denser, lies.

In the early hours of the morning
The procedures dictates taking great care with several things. First, a number of wells with a 25-centimeter diameter and 45-meter depth are drilled at 50 kilometers from each other and gel explosives are placed at the bottom. Each well is then sealed completely, so that the explosion cannot dissipate through the air, where it would meet with far less resistance, of course. To measure the effects of the explosion, the researchers install seismographs in the region every two kilometers. The time of the detonations, of which several are conducted in sequence, in intervals that range from 15 minutes to one hour, are meticulously controlled. All of this is done in the early hours of the morning.

046-049_crostas_184“We”re very careful not to damage the properties. Only very little energy is released. At most, it can cause a landslide around the hole,” says Fuck. “At a distance of 100 to 200 meters from the site, one feels a certain vibration caused by tremor. However, at 300 hundred meters one can”t feel a thing,” he tells us. The explosion is muffled because the well is plugged. The main reason for the experiments being conducted in the early hours of the morning is to avoid vibrations unrelated to the experiment, given the sensitivity of the seismographs. “If a truck or a cow go past, [the seismograph] will record that,” the geologist explains.

Other studies conducted by the INCT group led to complementary results, which helped to provide the geological history of the Borborema region with a clearer outline. The group of Aderson Farias do Nascimento, from UFRN, showed that not only the lithosphere of the Northeast is thinner, as the seismic refraction studies had already suggested, but that there is a step there toward a far thicker area in the so-called São Francisco Craton, the block of rock on which part of the states of Bahia and of Minas Gerais lie. The transition between these two crust structures is marked by a clear and visible separation in the local geology: a line of extinct volcanoes that extends from Macau, in the state of Rio Grande do Norte, to Queimadas, in the state of Paraiba.

“We discovered that the characteristics of the rocks that make up the lithosphere to the west of this line of volcanoes are very different from the deep rock that we find to the east of them,” says Nascimento. The group hypothesizes that, as the coastal region was stretched, a step formed, when the mantle of the Earth, which is hotter, was raised to a level close to the surface. With the accumulation of heat, volcanic activity became inevitable. However, there is nothing to fear: the last eruption probably occurred around 7 million years ago. “It”s difficult to tell, but something might happen again one day in the region,” he says.

To obtain these data from within the Earth, the UFRN group used seismographs. However, rather than measuring controlled explosions only a few meters deep, as happens with seismic refraction, they recorded real tremors that took place at least 3 thousand kilometers away from the measuring stations. They then applied a technique called receptor function to the unprocessed information in order to reach their conclusions. The technique consists of processing the seismic measurements. For every event recorded, the unprocessed data reflected four different effects: the seismic source, propagation, the interaction of propagation with the surface structures and finally, the fourth, the effect of the equipment itself upon the measurements taken – this is the best known and most easily subtracted one.

Processing is precisely the path to extract from such confusion the effects of the source and of propagation. “The result is that one is left with only the geology of the site of the station,” explains Nascimento. “To give you an idea of the difficulty involved, it”s as if you had some cake and were able to say it contained this much flour, that much sugar, that much chocolate powder.” The advantage of this technique is that it is cheaper and less laborious than seismic refraction, since it is enough to install the equipment and wait for the earthquakes on the other side of the world. Though less precise, it can cover a far larger area. When explosions are produced along a line, one only gets data for that line. “Seismic refraction is like an X-ray, you get an image on a single plane. In our case, we managed to observe very well the contrast of the composition of the Borborema Province relative to the São Francisco Craton,” says the geophysicist from UFRN.

Patos, Paraíba state: 33-kilometer thick crust, thicker in the south

ROBSON SILVA REIS/ UNBPatos, Paraíba state: 33-kilometer thick crust, thicker in the southROBSON SILVA REIS/ UNB

The geological faults that cause earthquakes are another interesting aspect of the Borborema Province. Brazil is in a stable region in the middle of a tectonic plate, devoid of major seismic events. There are some faults, however, mainly in the Northeast. The tremors in the area are constant, but they rarely exceed 4 degrees on the Richter scale. In general, they cause little damage, but sometimes they do frighten people. “In the town of Sobral, in Ceará state, in 2008 and 2009, people refused to stay inside their homes, because they feared the roof might cave in,” Fuck tells us.

Despite the knowledge that these studies generate, forecasting earthquakes continues to be as difficult as ever. Though no progress has been made in this area, the Brazilian researchers identified the source of the Northeastern tremors and mapped the geological faults that cause them. This is information that henceforth will guide the construction of major engineering projects, such as bridges and dams, or even the settlement of people.

The INCT for Tectonic Studies group spread seismic stations throughout the Northeast precisely in order to monitor earthquakes and identify their epicenter. Several relevant faults had already been identified in the past, such as the Transbrazilian Lineament, that extends from Argentina to the coast of Ceará. Nevertheless, the results achieved by Fuck”s team showed that the earthquakes that often affect Sobral have nothing to do with this lineament, but seem to be connected with another, previously unknown fault. “Our studies indicate that this fault is not shown on geological maps and that it runs in a different direction.”

The researchers are unsure as to why some of the areas that lie on the fault are more liable to undergo tremors than others. Still, it is clear that there are some preferential sites for the release of energy and that Sobral is one of them. “Apparently, some regions of the crust are weaker,” says Fuck. The studies at the Borborema Province continue. “We chose this region as a sort of pilot program. Through these experiments, we want to form teams that are able to take this type of study all over Brazil.”