Why the Northeast is the region of Brazil with the highest risk of earthquakes

Under Ceará, Rio Grande do Norte, Paraíba, Pernambuco and Alagoas, the earth's crust is 30 to 35 kilometers thick — in some places, even less than that. It is thinner than the world average, which exceeds 40 kilometers, reaching 70 kilometers in the Himalayan region. In the ocean region, it is about 10 kilometers. This is believed to have originated in the Cretaceous period, between 136 million and 65 million years ago. As the blocks that form the continents are in motion over the mantle (layer of the Earth located between the crust and the core), what we know today as Africa and South America separated at that time. Exactly in this region where the Northeast is today, the crust would have stretched a little more than the rest, in a process of accommodation of the plates. The result was this atypical thinning. "It's the so-called stretching effect", explains structural engineer Marcelo Bianco, professor at the University of São Paulo (USP) — he even studied the region's crust in research he carried out at the University of Weimar, in Germany, where he completed his master's and doctorate. "It is a region that suffered the stretching process with the opening of the Atlantic [Ocean] [in the formation of the continents]", says geophysicist Aderson Farias do Nascimento, professor at the Federal University of Rio Grande do Norte (UFRN). "In regions like this, there is often an easy accumulation of forces that can trigger earthquakes." Accommodation of the plates Researchers often make an analogy with a cracked eggshell, a turtle shell or even a football and its segments when explaining that the planet is full of tectonic plates glued together. But perhaps it's better to imagine these signs as a group of people crammed into a public transport carriage — when one moves, the other needs to change position too, the tension is constant and there are common touches that, even involuntary, are uncomfortable. These tensions between materials, this friction resulting from an ancient and constant process of accommodation, is what often causes earthquakes, due to the energy that accumulates and, eventually, disperses. Stresses are greatest at the edges of large tectonic plates, where these immense volumes of rock meet and seek to accommodate their boundaries. The Earth has more than 50 of these plates — the largest being 14. Large earthquake that hit Venezuela originated from the meeting between the South American and Caribbean tectonic plates Getty Images The large earthquake that hit Venezuela a few days ago originated from the meeting between the South American and Caribbean plates, but the Andean region is also susceptible to tremors due to the encounter with the Nazca plate. In this sense, Brazil is privileged. Its territory is relatively protected, located entirely within the South American plate. However, Nascimento explains that the formation of the terrain in the Northeast, with very old rocks, favors the perception of tremors. “It works as an excellent way for seismic waves to travel,” he says. "There is very good efficiency in transmitting seismic energy." Author of the book Planeta Hostil, geologist Marco Moraes explains that the tensions that reach the center of the South American plate come from all sides. To the east, due to the force caused by the mid-ocean ridge, the submarine mountain range formed by the separation of tectonic plates that is in the Atlantic. “It’s a compression component,” he says. On the other side, strength comes from the Andes. "In the Pacific region, the [South American] plate is pushed by the Nazca plate", says the geologist. "The stress is dispersed over large areas, but from time to time fault accommodations occur." BBC Infographic The earthquake that shook Rio Grande do Norte According to the map made by the international scientific organization Global Seismic Hazard Assessment Program (GSHAP), an entity created in 1992, while most of the Brazilian territory presents no risk of earthquakes, the northeast "corner" of the country is classified as moderate to high risk. The entity considers a risk if there is a probability of at least 10% of tremors every 50 years, considering a history of occurrences over the last 475 years. What happens is that if most of Brazil is on stable points in the crust, there in the Borborema Province, the number of geological faults is large. The largest of these faults is Samambaia, in Rio Grande do Norte. 40 years ago, in November 1986, the municipality of João Câmara, 82 kilometers from Natal, recorded a frightening earthquake. With a magnitude of 5.1 points, the tremor caused panic in the city, according to reports published at the time. "It was a weak earthquake, but enough to knock down small houses", points out Bianco. Cover of the newspaper O Poti, in Rio Grande do Norte, the day after the earthquake recorded in João Câmara Reproduction Around 4 thousand houses were destroyed or partially damaged, 10 thousand people were left homeless and many people simply decided to flee the city, according to press reports. The thickness of the crust in the region is one of the factors that favors the formation of geological faults, even if it is not an area on the edge of a tectonic plate. "Due to the stretching, we have some regions that are more fragile than others here in the Northeast. There are faults and fractures, favorable conditions for displacement", points out Nascimento. And it's not just that. This seismic activity in the region occurs because the crustal material there has, in geological history, an accumulation of faults already in its formation — the push-and-pull game between the plates, which has occurred since the formation of the planet, leaves some points less settled. In the case of the João Câmara earthquake, it is believed that the responsible fault existed millions of years ago. But she was "quiet". With the tensions absorbed by the South American Plate, at a certain point it broke out there. Technically, the flaw has been reactivated. In the image, the red line represents the Fern Fault LabSis UFRN "The thinner plate is one of the factors for more tremors. But there are also old faults there that are, from time to time, reactivated due to intraplate tension", explains Moraes. Geological faults are like fractures in the crust, caused by the displacement of immense rock blocks. These failures can occur when one block "goes down" in relation to the other, when it "rises" in relation to the other or when both blocks slide, rubbing against each other. The phenomenon is much more common at the edges of plates. But, inside them, as is the case in Borborema, they may be the result of already existing weaknesses — as if they were unfilled gaps that are rearranged by erosion, sedimentation and even magmatic activity. Comprehensive study A study carried out by the National Institute of Science and Technology for Tectonic Studies between the end of 2009 and the middle of the last decade, financed by the National Council for Scientific and Technological Development (CNPq), scrutinized the geology of Borborema. Geophysicist Aderson Farias do Nascimento was one of those who participated in the research. It was this work that concluded that the thickness of the crust in the Northeast is thinner. To do so, the researchers triggered a series of explosions close to the surface and, using seismographs, measured how the shock waves propagated inside the Earth. This is done because the waves are reflected and refracted, changing speed, depending on the physical differences between the materials that form the crust — after all, different elements have different densities and consistencies and this is especially relevant between the crust and the mantle. 50 kilometers apart, dozens of wells 45 meters deep and 25 centimeters in diameter were made in the region. Inside them, gel explosives were installed. Once the explosives were detonated, the scientists measured the propagation of the waves with seismographs distributed every 2 kilometers. Then, with the numbers in hand, they carried out the calculations. The researchers confirmed not only the thin thickness of the region's crust, but also the diversity of rock materials in its composition, another component that could explain the large number of faults, as there is no homogeneity that facilitates the accommodation of materials. "The Province of Borborema is very heterogeneous from a geological point of view. There are different terrains, a lot of metamorphic rock, many old faults… It is different from other regions that are more stable", says geologist Marco Moraes.