{"id":250307,"date":"2017-12-12T13:45:33","date_gmt":"2017-12-12T15:45:33","guid":{"rendered":"http:\/\/revistapesquisa.fapesp.br\/?p=250307\/"},"modified":"2017-12-14T16:20:42","modified_gmt":"2017-12-14T18:20:42","slug":"tension-beneath-the-earth","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/tension-beneath-the-earth\/","title":{"rendered":"Tension beneath the Earth"},"content":{"rendered":"
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Josenildo Ten\u00f3rio \/ Estad\u00e3o Content<\/span><\/a> Residents of Jo\u00e3o C\u00e2mara, in the state of Rio Grande do Norte in early December 1986, after a magnitude 5.3 quakeJosenildo Ten\u00f3rio \/ Estad\u00e3o Content<\/span><\/p><\/div>\n

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 S\u00e3o Paulo (USP), which includes the university\u2019s 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)<\/a>, of which the Center is a member. The RSBR\u2019s 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.<\/p>\n

Based on data from the RSBR, previous studies and analysis of waves generated by earthquakes, researchers from USP, the State University of Western Par\u00e1 and the University of California, Davis, identified the types and direction of the stresses that cause the movement of geologic faults\u2014cracks in the large surface blocks of rock\u2014in South America. The fracturing of these rocky blocks of crust\u2014the upper surface layer of the Earth\u2014releases energy that is expressed in the form of earthquakes.<\/p>\n

The researchers studied the horizontal and vertical movements of the geologic faults\u2014known as focal mechanisms\u2014that 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 <\/em>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\u2019s outermost layers, and thus complement the mathematical models used to describe these phenomena.<\/p>\n

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<\/em> Issue No. 224<\/a>).<\/p>\n

\u201cFocal mechanisms reveal the direction of the forces that caused the geologic faults to move, but not the causes of the faults,\u201d explains geophysicist Fabio Luiz Dias, a USP researcher currently working at the National Observatory in Rio de Janeiro and co-author of the study. \u201cPrior to our study, it was not possible to determine the focal mechanism of these tremors because the existing methods were limited.\u201d 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\u2019s neighborhoods.<\/p>\n

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. \u201cWe found that most of the earthquakes in the Southeast and the Pantanal wetlands are generated by stresses that coincide with this east-west compression,\u201d Dias says.<\/p>\n

Horizontal compression of crustal rocks also explains the 3.6 magnitude tremor on the morning of January 6, 2006 in the municipality of Tel\u00eamaco Borba, in Paran\u00e1 State. \u201cIdentification 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,\u201d Dias points out. In search of more information, a team led by geophysicist Marcelo Assump\u00e7\u00e3o, 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.<\/p>\n

According to the study in South American Earth Sciences<\/em>, the entire equatorial region of Brazil\u2014from the state of Rio Grande do Norte to the mouth of the Amazon River\u2014is subject to the same type of geological stress: \u201cthe overlapping of a compression parallel<\/p>\n

to the coast, running east-west with an extension due to the contrasting density of the continental and oceanic crusts,\u201d as Dias describes. This type of stress caused a 4.3 magnitude tremor in Vargem Grande, Maranh\u00e3o State\u2014the largest-magnitude quake in Brazil in the past four years, in January 2017. \u201cThe 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,\u201d Assump\u00e7\u00e3o notes.<\/p>\n

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\u2019s mantle in the Amazon Region. Conversely, in the states of Cear\u00e1 and Rio Grande do Norte, the crust is being stretched due to the proximity of the coast and the effect of gravity, according to Assump\u00e7\u00e3o. He says that the continental crust tends to stretch out in the direction of the ocean, \u201ccausing tensile stress.\u201d In the Northeast, in addition to the fact that the crust is thinner, the lithosphere\u2014the layer consisting of the crust and the upper layer of the mantle\u2014is also thinner than it is farther inland in Brazil, which facilitates earthquakes. \u201cIn 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.\u201d<\/p>\n

Like the Northeast, where quakes occur more frequently, the Central-West has areas where the Earth\u2019s crust is thinner. The difference is that, in the central region, the mantle\u2014the denser layer beneath the crust\u2014is shallower than it is in other regions. As a result, the lithosphere\u2014made up of the crust and the outer mantle\u2014bends and cracks, generating earthquakes (see Pesquisa FAPESP<\/em> Issue No. 207<\/a>).<\/p>\n

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Jo\u00e3o Miranda \/O Tempo \/Folhapress<\/span><\/a> A 4.0 magnitude tremor destroyed the balcony of this house in Montes Claros, Minas Gerais, in December 2012Jo\u00e3o Miranda \/O Tempo \/Folhapress<\/span><\/p><\/div>\n

Earthquake risk<\/strong>
\nThe data from the RSBR indicated the regions at greatest risk of being hit by earthquakes: the states of Cear\u00e1 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. \u201cHere, almost no one plans to build houses or other buildings because of the potential for earthquakes, which can occur anywhere,\u201d says geophysicist Lucas Vieira Barros, a professor at the University of Bras\u00edlia (UnB). The destructive power of an earthquake comes not just from the magnitude of the tremor. Building quality and the population\u2019s capacity to respond can increase or reduce the impact.<\/p>\n

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. \u201cAn 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,\u201d Barros says. For this reason, a tremor of the same magnitude could occur in the same place many years later.<\/p>\n

That prospect could be worrisome for regions like the municipality of Porto dos Ga\u00fachos, 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\u20146.3\u2014leveled the Italian city of L\u2019Aquila and killed nearly 300 people.<\/p>\n

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 Cara\u00edbas, located in the municipality of Itacarambi, northern Minas Gerais, was responsible for Brazil\u2019s first earthquake fatality, a five-year-old child, and it toppled nearly every house in the neighborhood.<\/p>\n

\"\"<\/a>Rapid response<\/strong>
\nThree monitors at the Seismology Center receive satellite or internet data from the 80 seismological stations. Another three track activity on the IAG website<\/a>\u00a0and on the Center\u2019s Facebook page<\/a>\u00a0for reports of earthquakes in Brazil. Since 2015, the website has received over 700 reports, most of them from residents of the Southeast and Northeast.<\/p>\n

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. \u201cThe most frightening thing is the bursting sound from below ground and the noise that seems to come from the sewer pipes,\u201d says business owner Paulo C\u00e9sar 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\u2014less than 2.0\u2014but because they occur near the surface, they frighten the residents, rattle windows and knock pictures off of walls.<\/p>\n

\u201cThe tremors in Jurupema occur more frequently when it rains more,\u201d observes Jos\u00e9 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<\/em> Issue No. 170<\/a>). 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. \u201cThe wells have anticipated an earthquake that shouldn\u2019t happen for another 100 years or so,\u201d Barbosa speculates.<\/p>\n

Project<\/strong>
\nFocal mechanism in Brazil with waveform modelling (No. 14\/26015-7<\/a>); Grant Mechanism<\/strong>\u00a0Doctoral research grant; Principal Investigator\u00a0 <\/strong>Marcelo Sousa de Assump\u00e7\u00e3o (USP); Grant recipient<\/strong>\u00a0Fabio Luiz Dias; Investment<\/strong>\u00a0R$59,667.09.<\/p>\n

Scientific article<\/em>
\nASSUMP\u00c7\u00c3O, M.\u00a0et al<\/em>.\u00a0Intraplate stress field in South America from earthquake focal mechanisms<\/a>.\u00a0Journal of South American Earth Sciences<\/strong>. v. 71. p. 278-95. 2016.<\/p>\n","protected":false},"excerpt":{"rendered":"Characterization of geologic faults explains earthquakes in Brazil","protected":false},"author":605,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"footnotes":""},"categories":[159],"tags":[240,235],"coauthors":[1610],"class_list":["post-250307","post","type-post","status-publish","format-standard","hentry","category-science","tag-geology","tag-physics"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/250307","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/users\/605"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=250307"}],"version-history":[{"count":0,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/250307\/revisions"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=250307"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=250307"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=250307"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=250307"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}