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GEOLOGY

Heading northwest

GPS measurements indicate that part of the Earth’s crust in the Brazilian Northeast is moving a few millimeters northwest each year

062-063_GEO_230Fragments making up one-third of the terrestrial crust of the Brazilian Northeast are slowly sliding towards the north and west at a maximum rate of 5.6 millimeters per year, according to a scientific paper by Brazilian researchers, published in the Journal of South American Earth Sciences in March 2015. The movement of sections of the Borborema Province—the name geologists use to refer to the rocky block that comprises about 540,000 square kilometers and encompasses much of the states of Ceará, Rio Grande do Norte, Paraíba, Pernambuco, Alagoas and Sergipe—causes subtle extensions and contractions at different points on the surface and raises the risk of local tremors. “The province is under pressure from all sides,” says geophysicist Giuliano Sant’Anna Marotta of the Seismological Observatory at the University of Brasília (UnB), principal author of the paper. “The situation is similar to what happens when we squeeze a rubber eraser.” Some points contract while others stretch; some parts sink as others rise.

There is no cause for alarm, however. The movement of pieces of the geological province, where much of Brazil’s tectonic activity is concentrated, is an expected phenomenon. The pace of movement is relatively modest, about 12 times slower than the pace observed at the famed San Andreas Fault near the coast of California—the region presenting the greatest risk of earthquakes in the United States, and nine times slower than the pace found in sections of the Andes, another region with strong earth tremors. “We knew that the Borborema Province was moving, and now we’ve been able to quantify the maximum velocity of this type of occurrence,” says geologist Francisco Hilário Bezerra of the Federal University of Rio Grande do Norte (UFRN), a co-author of the scientific paper.

The measurement was obtained from data supplied by a network of Global Positioning System (GPS) receiver stations installed at 12 different points in the province (see map). Nine stations are part of the Brazilian Network for Continuous Monitoring of GPS (RBMC), maintained by the Brazilian Institute of Geography and Statistics (IBGE), and three belong to the Potiguar Basin GPS Network, an initiative of the Department of Geology at UFRN. Each receiver provides a nearly uninterrupted record of its location on a horizontal plane as well as along a vertical axis, with an extremely low margin of error of around one millimeter. In other words, it measures whether the ground on which the GPS unit is fixed has moved upward, downward or to either side over time. A completely immovable point on the planet—which does not sink, rise or move horizontally—always shows the same coordinates over a given time interval.

Within the tectonic plate
In the case of the Borborema Province study, the information on the location of the stations was recorded for at least two consecutive years. Because the analyzed time series is small, it is not possible to say whether the maximum velocity of movement found in the study indicates a continuous movement trend for sections of the province, or whether it reflects a transitory phenomenon. “Ideally, we should have data for at least three consecutive years,” says João Francisco Galera Mônico of São Paulo State University (Unesp) at Presidente Prudente, an expert in geodetic studies using GPS who was also involved in the study.

There are no major earthquakes in Brazil because its terrain, including the Northeast, lies within the South American tectonic plate, one of the enormous blocks of rock that form the Earth’s surface. Large-magnitude tremors occur in areas located near the edges of the plates, where there are large geological faults—fissures in the crust that mark the zone of contact between the end of one plate and the beginning of another—such as those located in the Andes near Chile and Peru (the boundary between the South American plate and the Nazca plate) and on the coast of California (the boundary between the North American plate and the Pacific plate). The movements of the crust cause the edge of a plate to collide with the boundaries of the contiguous block of rock.

The Borborema Province is thousands of kilometers from the area of closest contact between two tectonic plates, the submerged mountain range known as the Mid-Atlantic Ridge, which establishes the boundary between the South American plate and the African plate. Even so, this piece of the Northeast feels the effects of the gradual distancing of the South American plate as it moves westward, away from the rocky block that includes Africa. “The Borborema Province has many shear zones,” Marotta explains. This type of geological structure corresponds to ancient zones of weakness that are subject to instability. Crustal movements can cause tremors in such places. The area adjacent to the Senador Pompeu Shear Zone—a fault that cuts through the interior of Ceará State to the Potiguar Basin—had the largest variations of movement towards the northwest, according to the data from the GPS network. Sections of the Potiguar Basin, halfway between the cities of Natal and Fortaleza, moved 4 mm per year to the west and 4.1 mm to the north within the South American plate. The basin has medium-intensity tremors, with magnitudes as high as 5.2.

Scientific article
MAROTTA G. S. et al. Strain rates estimated by geodetic observations in the Borborema Province, Brazil. Journal of South American Earth Sciences. V. 58, p. 1–8. Mar. 2015.

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