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GEOPHYSICS

Tension beneath the Earth

Characterization of the movements of geologic faults in the Earth’s crust explains seismic tremors in Brazil

Josenildo Tenório / Estadão Content Residents of João Câmara, in the state of Rio Grande do Norte in early December 1986, after a magnitude 5.3 quakeJosenildo Tenório / Estadão Content

On the morning of May 2, 2017, a red dot began to blink on one of the wall monitors in the Seismology Center at the University of São Paulo (USP), which includes the university’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) and the Institute of Energy and Environment (IEE). A magnitude 4.0 earthquake had struck about 3,000 kilometers (km) from the Center, at the Peru-Bolivia border, and it was detected by the Brazilian Seismographic Network (RSBR), of which the Center is a member. The RSBR’s 80 seismological stations, which have been continuously recording episodes such as this since 2010, enable scientists to carry out a detailed study of the probable causes of earthquakes in Brazil. The stations, each equipped with a seismograph and a data transmitter, are managed by universities, research institutes and companies.

Based on data from the RSBR, previous studies and analysis of waves generated by earthquakes, researchers from USP, the State University of Western Pará and the University of California, Davis, identified the types and direction of the stresses that cause the movement of geologic faults—cracks in the large surface blocks of rock—in South America. The fracturing of these rocky blocks of crust—the upper surface layer of the Earth—releases energy that is expressed in the form of earthquakes.

The researchers studied the horizontal and vertical movements of the geologic faults—known as focal mechanisms—that are associated with nearly 400 earthquakes in South America. Most of these have occurred along the Andes range, with 76 occurring in Brazil. In a paper published in the Journal of South American Earth Sciences in November 2016, they argued that identification of the crustal stress pattern could provide new information on the movements of the lithospheric plates formed by the Earth’s outermost layers, and thus complement the mathematical models used to describe these phenomena.

The characterization of the direction and type of the movements of the geologic faults helped them to understand the stresses that generated the three tremors recorded in recent decades in the state of Amazonas, as well as the largest one ever recorded in Brazil. In 1690, a magnitude 7.0 earthquake churned up the terrain, uprooted trees and generated waves along the Amazon River that inundated villages 45 km from what is now the city of Manaus, according to an account written by Jesuits at the time (see Pesquisa FAPESP Issue No. 224).

“Focal mechanisms reveal the direction of the forces that caused the geologic faults to move, but not the causes of the faults,” explains geophysicist Fabio Luiz Dias, a USP researcher currently working at the National Observatory in Rio de Janeiro and co-author of the study. “Prior to our study, it was not possible to determine the focal mechanism of these tremors because the existing methods were limited.” Improvements in the methodology made it possible to identify the focal mechanism of 12 tremors near the seismographic stations, ranging in magnitude from 3.0 to 5.3, that have occurred in Brazil since 1992 and were of uncertain origin. Now, using this approach, the 4.0 magnitude tremor that occurred in Montes Claros in northern Minas Gerais State in 2012 has been linked to the movement of a geologic fault just 1 km deep, located under one of that city’s neighborhoods.

Fracturing of the rocks beneath the surface is caused by compression or stretching of the crust. These two effects account for the pressure that is applied mainly as a result of the expansion of the Meso-Oceanic Range that occupies the central part of the Atlantic Ocean, and by the sinking of the Nazca tectonic plate beneath the South American plate, on which Brazil rests. “We found that most of the earthquakes in the Southeast and the Pantanal wetlands are generated by stresses that coincide with this east-west compression,” Dias says.

Horizontal compression of crustal rocks also explains the 3.6 magnitude tremor on the morning of January 6, 2006 in the municipality of Telêmaco Borba, in Paraná State. “Identification of the movement of the geologic fault linked to this tremor was one of the most surprising data points from this study, because records of tremors in the South are scarce,” Dias points out. In search of more information, a team led by geophysicist Marcelo Assumpção, head of the Seismology Center and a professor at IAG-USP, is setting up about 40 seismological stations in southern Brazil, in cooperation with institutions in Argentina, Uruguay, Paraguay and Bolivia.

According to the study in South American Earth Sciences, the entire equatorial region of Brazil—from the state of Rio Grande do Norte to the mouth of the Amazon River—is subject to the same type of geological stress: “the overlapping of a compression parallel

to the coast, running east-west with an extension due to the contrasting density of the continental and oceanic crusts,” as Dias describes. This type of stress caused a 4.3 magnitude tremor in Vargem Grande, Maranhão State—the largest-magnitude quake in Brazil in the past four years, in January 2017. “The closest seismological station is 40 km from the epicenter of that quake, which helped us determine its focal mechanism with a good deal of precision,” Assumpção notes.

Crustal compressions are also prevalent in the North and Central-West, but in a northwest-southeast direction. The causes of the different orientation are uncertain, but the specialists think they could be related to convective movements in the Earth’s mantle in the Amazon Region. Conversely, in the states of Ceará and Rio Grande do Norte, the crust is being stretched due to the proximity of the coast and the effect of gravity, according to Assumpção. He says that the continental crust tends to stretch out in the direction of the ocean, “causing tensile stress.” In the Northeast, in addition to the fact that the crust is thinner, the lithosphere—the layer consisting of the crust and the upper layer of the mantle—is also thinner than it is farther inland in Brazil, which facilitates earthquakes. “In places where the lithosphere is thinner, the pressure exerted when two adjacent lithospheric plates meet is distributed across a smaller area, concentrating the stresses and facilitating cracks in the geologic faults.”

Like the Northeast, where quakes occur more frequently, the Central-West has areas where the Earth’s crust is thinner. The difference is that, in the central region, the mantle—the denser layer beneath the crust—is shallower than it is in other regions. As a result, the lithosphere—made up of the crust and the outer mantle—bends and cracks, generating earthquakes (see Pesquisa FAPESP Issue No. 207).

João Miranda /O Tempo /Folhapress A 4.0 magnitude tremor destroyed the balcony of this house in Montes Claros, Minas Gerais, in December 2012João Miranda /O Tempo /Folhapress

Earthquake risk
The data from the RSBR indicated the regions at greatest risk of being hit by earthquakes: the states of Ceará and Rio Grande do Norte, southern Minas Gerais and the Pantanal wetlands of Mato Grosso State, according to the seismic risk map presented in December 2016 in the Bulletin of the Brazilian Geophysical Society. In the article containing the map, the researchers indicate that in Brazil, only critical facilities like nuclear power plants and hydroelectric dams conduct seismological analyses. “Here, almost no one plans to build houses or other buildings because of the potential for earthquakes, which can occur anywhere,” says geophysicist Lucas Vieira Barros, a professor at the University of Brasília (UnB). The destructive power of an earthquake comes not just from the magnitude of the tremor. Building quality and the population’s capacity to respond can increase or reduce the impact.

In Brazil, according to historical records an average of two 6.0 magnitude tremors occur every century, while in the Andes, events of that intensity happen monthly. “An earthquake results from the sudden release of energy that has built up over the course of many years, but soon after a quake, energy starts to build up again,” Barros says. For this reason, a tremor of the same magnitude could occur in the same place many years later.

That prospect could be worrisome for regions like the municipality of Porto dos Gaúchos, in northern Mato Grosso. The biggest earthquake ever recorded in Brazil, a magnitude 6.2, occurred there in 1955, two years before the first settlers arrived in the area. Today, some 300,000 people live within a radius of 100 km around the epicenter of that quake. Using seismographs, Barros and his team have identified a five-kilometer-long geologic fault, highlighting the risk of another devastating quake. In April 2009, an earthquake of similar magnitude—6.3—leveled the Italian city of L’Aquila and killed nearly 300 people.

The idea that earthquakes in Brazil are benign is beginning to change in the wake of tragic episodes. In 2007, a magnitude 4.9 tremor in the rural community of Caraíbas, located in the municipality of Itacarambi, northern Minas Gerais, was responsible for Brazil’s first earthquake fatality, a five-year-old child, and it toppled nearly every house in the neighborhood.

Rapid response
Three monitors at the Seismology Center receive satellite or internet data from the 80 seismological stations. Another three track activity on the IAG website and on the Center’s Facebook page for reports of earthquakes in Brazil. Since 2015, the website has received over 700 reports, most of them from residents of the Southeast and Northeast.

It was in this way that the IAG team learned of the tremors in Jurupema, a district with 2,000 residents in the municipality of Taquaritinga, Sao Paulo State, in early 2017. “The most frightening thing is the bursting sound from below ground and the noise that seems to come from the sewer pipes,” says business owner Paulo César Andreguetto, 46, who works in Jurupema. In response, the researchers installed the first sensor in Jurupema in April, and three more in subsequent months, since the tremors have become frequent; in late May, there had already been more than 100. They have been low-intensity—less than 2.0—but because they occur near the surface, they frighten the residents, rattle windows and knock pictures off of walls.

“The tremors in Jurupema occur more frequently when it rains more,” observes José Roberto Barbosa, a technician at the Seismology Center who is in charge of installing two sensors in Taquaritinga. He says the provisional hypothesis is that the recently drilled water extraction wells are playing a role in the earthquakes, as happened in Bebedouro, another city in inland Sao Paulo (see Pesquisa FAPESP Issue No. 170). The drilling could intensify the fracturing of the basaltic rocks beneath the surface. Because of these wells, it is presumed, when it rains, the water percolates more easily and in greater quantity through the cracks in the rocks, acting as a lubricant and helping release the stresses built up in those cracks. “The wells have anticipated an earthquake that shouldn’t happen for another 100 years or so,” Barbosa speculates.

Project
Focal mechanism in Brazil with waveform modelling (No. 14/26015-7); Grant Mechanism Doctoral research grant; Principal Investigator  Marcelo Sousa de Assumpção (USP); Grant recipient Fabio Luiz Dias; Investment R$59,667.09.

Scientific article
ASSUMPÇÃO, M. et alIntraplate stress field in South America from earthquake focal mechanismsJournal of South American Earth Sciences. v. 71. p. 278-95. 2016.

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