{"id":163758,"date":"2013-10-06T15:19:58","date_gmt":"2013-10-06T18:19:58","guid":{"rendered":"http:\/\/revistapesquisa.fapesp.br\/?p=163758"},"modified":"2015-06-08T15:31:56","modified_gmt":"2015-06-08T18:31:56","slug":"why-the-earth-shakes-in-brazil-2","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/why-the-earth-shakes-in-brazil-2\/","title":{"rendered":"Why the earth shakes in Brazil"},"content":{"rendered":"

\"047_MapaCrosta_207NOVO_1\"<\/a><\/p>\n

Published in May 2013<\/em><\/p>\n

On October 8, 2010, the earth trembled\u00a0as never before in Mara Rosa,\u00a0a city of 10,000 in the northern region\u00a0of Goi\u00e1s State. The clock had\u00a0just struck 5 p.m. that Friday, and\u00a0people were preparing for the weekend, when\u00a0the ground shook so hard that it was difficult to\u00a0remain standing. Trees quivered, walls cracked\u00a0and tiles fell off houses. Less than a minute later,\u00a0this magnitude-5 earthquake, one of the strongest\u00a0on record in Brazil in the previous 30 years, had\u00a0traveled 250 km and reached Bras\u00edlia, where several\u00a0buildings had to be evacuated. \u201cMany people\u00a0in Mara Rosa thought the earth was opening up\u00a0and that the world was coming to an end,\u201d says\u00a0Lucas Barros, head of the Seismological Observatory\u00a0of the University of Bras\u00edlia (UnB). In the\u00a0weeks after the earthquake, Barros and his team\u00a0installed seismographs in Mara Rosa and neighboring\u00a0municipalities to track the reverberation\u00a0of the tremor. Within six months, 800 additional,\u00a0less intense earthquakes occurred and were\u00a0used to determine the direct cause of the earth\u2019s\u00a0instability in this region. Below Mara Rosa, at a\u00a0depth of approximately three kilometers, there\u00a0is a wide crack in the earth\u2019s crust, the most rigid\u00a0and outermost layer of the planet. Along this\u00a0fault, which is five kilometers long, rocks shifted\u00a0and caused the earth to tremble. \u201cWe had to hold\u00a0public hearings in Mara Rosa and Mutun\u00f3polis to\u00a0explain to people what was happening and what\u00a0they had to do to protect themselves,\u201d says Barros.<\/p>\n

The existence of this fault came as no surprise\u00a0to the UnB group: Mara Rosa and other municipalities\u00a0in northern Goi\u00e1s and southern Tocantins\u00a0State are located in a geologically unstable\u00a0region, i.e., the Goi\u00e1s-Tocantins seismic zone, in\u00a0which 10% of earthquakes in Brazil have been\u00a0concentrated. Some geologists attribute the high\u00a0frequency of earthquakes in this area\u2014a seismic\u00a0zone (one of nine demarcated in the country)\u00a0700 km long and 200 km wide\u2014to the proximity\u00a0of the Transbrasiliano Lineament, a long scar in\u00a0the earth\u2019s crust that crosses through Brazil and\u00a0continues into Africa on the other side of the Atlantic.\u00a0It is believed that the crust is weaker along\u00a0the lineament with a concentration of blocks of\u00a0cracked rock, which, under compression, may\u00a0move easily and produce earthquakes.<\/p>\n

However, not everyone agrees. In many cases\u00a0the location of the tremor is removed from this\u00a0series of faults, and along some stretches of the\u00a0lineament, no tremors have ever been detected.\u00a0Those who doubt the direct influence of the lineament\u00a0on earthquakes in this region believe that\u00a0there are deeper causes, such as those identified\u00a0by a group of researchers from the Institute of\u00a0Astronomy, Geophysics and Atmospheric Sciences\u00a0(IAG) of the University of S\u00e3o Paulo (USP),\u00a0based on a recent survey of the thickness of the\u00a0earth\u2019s crust in Brazil.<\/p>\n

In a paper published in February of this year\u00a0in Geophysical Research Letters, seismologist\u00a0Marcelo Assump\u00e7\u00e3o and geophysicist Victor\u00a0Sacek proposed a more complete, and for many\u00a0researchers more convincing, explanation of the\u00a0concentration of tremors in Goi\u00e1s and Tocantins\u00a0States. In certain areas of this seismic zone, the\u00a0earth\u2019s crust is thinner than in much of Brazil\u00a0and is bended due to the weight of the mantle;\u00a0in addition, the geological layer below the crust\u00a0is denser. Measurements of the intensity of the\u00a0gravitational field in these areas where the crust\u00a0is thin indicate that the mantle is thickening in\u00a0this region. The combination of these two layers\u00a0of rock\u2014the crust and the upper region of\u00a0the mantle, which physically behave as a single\u00a0and rigid structure that geologists call the lithosphere\u2014causes them to slowly rupture like a\u00a0bending tree branch. In this situation, the lithosphere\u00a0can break like a plastic ruler that bends\u00a0when you try to make the ends meet (see infographic<\/a>)<\/em>.<\/p>\n

\u201cThe lithosphere tends to sink where it is\u00a0more dense and to rise where it is not as dense,\u201d\u00a0explains Assump\u00e7\u00e3o, who is the coordinator of\u00a0the Brazil Seismograph Network that monitors\u00a0earthquakes in the country. \u201cThese tendencies\u00a0cause stress that produces faults and eventually\u00a0cause earthquakes to occur.\u201d<\/p>\n

During a conversation in his office in early\u00a0April, Sacek, co-author of the study,\u00a0picked up a paperback book to illustrate\u00a0what occurs in the stretch of the Goi\u00e1s-Tocantins\u00a0seismic zone where Mara Rosa is located.\u00a0\u201cLet\u2019s say that this book represents the lithosphere\u00a0of the region; there is an increase in the\u00a0load inside the lithosphere because there is a\u00a0higher proportion of mantle rocks [which are\u00a0more dense], and this will cause it to bend,\u201d he\u00a0explained as he held the book horizontally and\u00a0pressed on the sides, which caused it to bend\u00a0downward as though a brick were stuck to the\u00a0bottom cover. In this demonstration, the upper\u00a0part was subjected to forces of compression,\u00a0and the bottom to forces of distention.\u00a0\u201cAlthough it is rigid, the lithosphere has some\u00a0flexibility and withstands distortion to some\u00a0extent,\u201d Sacek says. \u201cBut after a certain point\u00a0it can bend and break.\u201d<\/p>\n

Several years ago, while analyzing a map of earthquake\u00a0locations in Brazil, Assump\u00e7\u00e3o realized that\u00a0most occurred in the Goi\u00e1s-Tocantins region, where\u00a0in 2004 geophysicist Jes\u00fas Berrocal, a former USP\u00a0professor, had identified a gravimetric anomaly. The\u00a0gravitational field there is unusually high in Brazil\u00a0for a plateau region with an average altitude of\u00a0300 to 400 meters. In these flat and relatively low\u00a0lands\u2014for example, there are no mountain ranges\u2014\u00a0there is no excess weight on the surface to account\u00a0for the bending of the lithosphere. Therefore, Assump\u00e7\u00e3o\u00a0concluded that this weight could only\u00a0be located below the ground, most likely in deep\u00a0regions such as the more superficial layers of the\u00a0mantle, as the crust is only 35 km thick.<\/p>\n

\"047_MapaCrosta_207NOVO_2\"<\/a>However, it was necessary to verify whether\u00a0this concept made sense and if the thickening of\u00a0the mantle could in fact cause the lithosphere to\u00a0arch. Assump\u00e7\u00e3o then asked Sacek, a specialist in\u00a0computer simulations, to develop a mathematical\u00a0model that would represent the geological layers\u00a0in this area of Goi\u00e1s and Tocantins and would\u00a0take into account all the forces acting on these\u00a0layers. Sacek developed a model that included\u00a0the effect of local forces originating a few dozen\u00a0kilometers away from the earthquake region due\u00a0to differences in relief, such as valleys, rivers and\u00a0hills, as well as to variations in the thickness of\u00a0the crust. In addition, the model considered the\u00a0effect of regional forces on a global scale that occur\u00a0thousands of miles away on the edges of the\u00a0blocks into which the lithosphere divides.<\/p>\n

By combining these factors, Sacek identified\u00a0an area in which the crust is weak; this region\u00a0coincided with the area where the majority of\u00a0earthquakes in Goi\u00e1s and Tocantins occur. Within\u00a0this large block, which is 200 km wide and five\u00a0km deep, the forces are so intense that they exceed\u00a0the limit of elasticity of the rocks, and these\u00a0forces split the rocks. \u201cThis model even explains\u00a0the depth of the earthquakes, which are generally\u00a0less than five km from the surface,\u201d says Sacek.<\/p>\n

He and Assump\u00e7\u00e3o believe that this mechanism\u2014the bending in the area where the crust is\u00a0thinnest\u2014can also explain the high frequency of\u00a0earthquakes in other regions of the country, such\u00a0as the Pantanal Basin and the Porto de Ga\u00fachos\u00a0seismic zone in Mato Grosso State, where in 1955\u00a0the strongest earthquake ever recorded in Brazil\u00a0occurred, at a magnitude of 6.2 on the Richter\u00a0scale. Earthquakes with magnitudes greater than\u00a0five are rare in Brazil and occur on average every\u00a0five years. However, even minor earthquakes can\u00a0frighten people who are unaccustomed to living\u00a0with them and are unprepared for such events.\u00a0There is a lack of information on how to prepare\u00a0for tremors, and the weakest houses cannot\u00a0withstand small earthquakes that would cause\u00a0little damage to the buildings in a large city. On\u00a0December 9, 2007, a 4.9 magnitude quake damaged\u00a0several houses in the village of Cara\u00edbas,\u00a0located near Itacarambi in the northern part of\u00a0Minas Gerais State. A wall collapsed and killed\u00a0a child. \u201cThis is the only death ever recorded in\u00a0Brazil that was directly caused by an earthquake,\u201d\u00a0says geologist Cristiano Chimpliganond of UnB.<\/p>\n

The bending of the crust also explains the\u00a0earthquakes in another seismic zone of Brazil:\u00a0the margin of the continental shelf between Rio\u00a0Grande do Sul and Espirito Santo States. At a distance\u00a0of 100 to 200 km from the coast, the seabed\u00a0drops off suddenly. On this step, the ocean depth\u00a0increases from 50 to 2,000 meters. The sediment\u00a0that the rivers carry to the sea accumulates at\u00a0the end of this step, increasing the weight on the\u00a0crust. Assump\u00e7\u00e3o believes that this excess weight\u00a0causes the earthquakes that are detected in this\u00a0region, as a result of mechanisms similar to those\u00a0that may be occurring in Goi\u00e1s and Tocantins.\u00a0The difference here is that the excess weight is\u00a0not below the crust but above it.<\/p>\n

In 2011, Assump\u00e7\u00e3o and colleagues from Universidade\u00a0Estadual Paulista (Unesp), the S\u00e3o Paulo\u00a0Institute for Technological Research (IPT), and\u00a0Petrobras studied an earthquake that occurred\u00a0in April 2008, 125 kilometers south of the city\u00a0of S\u00e3o Vicente, on the coast of S\u00e3o Paulo State.\u00a0The quake was felt as far away as the city of S\u00e3o\u00a0Paulo. The point of origin of the tremor was at\u00a0the end of the step of the continental shelf, and\u00a0the characteristics of its seismic waves seem to\u00a0confirm the concept that the event was triggered\u00a0by excess sediment.<\/p>\n

The development of these models of the\u00a0cause of the tremors in Brazil was made\u00a0possible only through the discovery of variations\u00a0in the thickness of the crust within its\u00a0borders. Assump\u00e7\u00e3o and colleagues from UnB,\u00a0the Federal University of Rio Grande do Norte\u00a0(UFRN) and the National Observatory (ON) collected\u00a0information on the thickness of the crust\u00a0at nearly a thousand sites in South America, both\u00a0on land and at sea. Of this total, approximately\u00a0200 measurements were taken in the past 20\u00a0years with funding from FAPESP and the federal\u00a0government. On the map that summarizes\u00a0these data, published in the Journal of South\u00a0American Earth Sciences, the researchers indicated\u00a0the regions where the crust is thicker or\u00a0thinner. \u201cThe thickness of the crust is one of the\u00a0most important parameters for understanding\u00a0the tectonics [the forces and movements of the\u00a0geological layers] of a region,\u201d says seismologist\u00a0Jordi Juli\u00e0 from UFRN.<\/p>\n

\"047_MapaCrosta_207NOVO_3\"<\/a>This map is the most comprehensive and detailed\u00a0compilation ever made for the Brazilian\u00a0crust. The thickness at all of these points was\u00a0obtained by combining data captured by three\u00a0methods that use seismic waves to determine the\u00a0structure of the geological layers through which\u00a0they pass. The most accurate method, which is also\u00a0the most costly, is seismic refraction, in which\u00a0researchers record along hundreds of kilometers\u00a0tremors caused by controlled explosions (see Pesquisa\u00a0FAPESP No. 184<\/a>). The other two methods\u00a0are based on the monitoring of earthquakes that\u00a0occur throughout the world each year.<\/p>\n

In general, the thickness of the crust in Brazil\u00a0is similar to that in other continents, an average\u00a0of 40 km, measured at sea level. However, there\u00a0are some regions in the country where the crust\u00a0thins out to less than 35 km. One of these areas\u2014a 1,000-km strip that extends from the Pantanal\u00a0in Mato Grosso do Sul to Goi\u00e1s and\u00a0Tocantins\u2014has not yet been well\u00a0delineated because little seismic\u00a0information is available about the\u00a0region. In the northeast, where the\u00a0team of Reinhardt Fuck from UnB\u00a0performed most of the seismic refraction\u00a0experiments, there is less\u00a0uncertainty.<\/p>\n

The province of Borborema, a rocky\u00a0block on which almost all the states\u00a0in the northeast sit, is the largest area\u00a0in Brazil with the thinnest crust and\u00a0has the highest frequency of tremors\u00a0in the country. At some points in\u00a0this region, the crust is thinner than\u00a030 km. This thinning appears to have\u00a0occurred between 136 million and 65\u00a0million years ago, when South America\u00a0separated from Africa.<\/p>\n

One of the thickest regions is located under the\u00a0Amazon forest on the border between Roraima,\u00a0Amazonas and Par\u00e1 States. The crust, which is\u00a0up to 45 kilometers thick, is one of the oldest\u00a0pieces in Brazil, at more than 2.5 billion years\u00a0old. \u201cThese oldest regions tend to have thicker\u00a0crust,\u201d says Assump\u00e7\u00e3o.<\/p>\n

However, the thickest stretch of crust in the\u00a0country lies in a relatively new region, the Paran\u00e1\u00a0Basin, which began forming 460 million years\u00a0ago. In the interior of S\u00e3o Paulo, near the Paran\u00e1\u00a0River, the crust is as thick as 46 km. Assump\u00e7\u00e3o\u00a0offers two possible reasons for this thickness.The first, suggested by several studies, is that\u00a0under the Paran\u00e1 Basin there may be a block of\u00a0older crust, called the Paranapanema Craton,\u00a0that is billions of years old. The second reason\u00a0involves the intense volcanic activity in this area\u00a0130 million years ago. For some unknown reason,\u00a0the mantle beneath the Paran\u00e1 Basin became abnormally\u00a0hot, forming an area that geologists call\u00a0a thermal plume. This plume may have partially\u00a0melted the deep layers of the earth, producing\u00a0basaltic magma that spilled over the surface and\u00a0created one of the largest volcanic provinces\u00a0on the planet. The rocks produced strips of red\u00a0earth and very fertile soil. Some of the material\u00a0produced in the process remained underneath,\u00a0and when the mantle cooled, it fused to the lower\u00a0portion of the crust, increasing its thickness.<\/p>\n

Together with researchers from Chile and China,\u00a0Assump\u00e7\u00e3o expanded the mapping of the\u00a0crust to the Andes. Under this mountain range,\u00a0the thickness of the crust ranges from 35 km on\u00a0the border between Peru and Ecuador to 75 km\u00a0on the Bolivian high plain. This maximum thickness\u00a0is similar to the thickness observed in other\u00a0relatively recent mountainous regions, such as\u00a0the Himalayas. In general, there is a direct correlation\u00a0between the altitude of land and the\u00a0thickness of crust. \u201cThe higher the topography,\u00a0the thicker the crust,\u201d Assump\u00e7\u00e3o explains. \u201cFor\u00a0altitudes above 3,000 meters, it is normal for the\u00a0crust to be as thick as 70 kilometers.\u201d<\/p>\n

But there are exceptions. In northern Argentina,\u00a0where the Andes rise to more than 4,000\u00a0meters, the crust is less than 55 km thick. Again,\u00a0researchers provide two possible explanations: Either\u00a0the crust was already abnormally thin before\u00a0the Andes were formed or four million years ago\u00a0it became so thick and hot that it lost a portion of\u00a0its deepest layers, an event called delamination.<\/p>\n

On the border between Peru and Ecuador, where\u00a0the altitude is greater than 3,000 meters, the thickness\u00a0is only 35 km. In this case, the crust seems\u00a0to be supported by the motion in the currents of\u00a0the deeper layers of the mantle, which, although\u00a0rocky, behave like a highly viscous fluid in geological\u00a0time, with a flow of a few centimeters per year.\u00a0The stren gth of these rising currents is capable of\u00a0suspending the crust and pushes up the crust one\u00a0to two kilometers in the mountains. The opposite\u00a0can also occur: The downward flow can pull the\u00a0crust down in some regions, as Sacek and Naomi Ussami,\u00a0a geophysicist at USP, observed in the Mara\u00f1on\u00a0Basin between Ecuador, Peru and Colombia.<\/p>\n

\"The

eduardo cesar<\/span>The Andes Range:\u00a0This region has\u00a0the thickest crust\u00a0in South America,\u00a0at up to 75 km thickeduardo cesar<\/span><\/p><\/div>\n

Despite two decades of work, research in this\u00a0field in South America is still lagging. The United\u00a0States and Europe developed detailed maps of\u00a0crustal thickness in the late 1990s. \u201cThe state of\u00a0mapping the crust varies with per capita income\u00a0across countries,\u201d says Assump\u00e7\u00e3o. \u201cWe are only\u00a0better than Africa.\u201d<\/p>\n

In Brazil, the major research institutions in the\u00a0area joined forces two years ago and created the\u00a0Seismograph Network of Brazil, which includes\u00a050 seismic stations; the goal is to increase that\u00a0number to 80. Thus, researchers hope to better\u00a0monitor the country and increase the resolution\u00a0of the map. As more earthquakes are observed,\u00a0additional details of the thickness of the crust can\u00a0be identified, and with greater detail, models can\u00a0be produced to more accurately predict areas at\u00a0risk for large-magnitude earthquakes. \u201cSeismology\u00a0cannot predict earthquakes, and even if it could,\u00a0it would be unable to prevent them,\u201d says Barros.\u00a0\u201cSo we have to learn to live with them and protect\u00a0ourselves from them.\u201d<\/p>\n

Project<\/strong>
\nTectonic, climatic and erosional evolution in convergent margins: A\u00a0numerical approach (No. 2011\/10400-0<\/a>); Grant mechanism<\/strong> Postdoctoral\u00a0research grant; Coord<\/strong>. Victor Sacek – IAG\/USP; Investment<\/strong>\u00a0R$153,896.91 (FAPE SP).<\/p>\n

Scientific articles<\/em>
\nASSUMP\u00c7\u00c3O, M. and Sacek, V. Intra-plate seismicity and flexural\u00a0stresses in central Brazil<\/a>. Geophysical Research Letters<\/strong>. v. 40 (3),\u00a0p. 487-91. 16 February In 2013.
\nASSUMP\u00c7\u00c3O, M. et al<\/em>. Crustal thickness map of Brazil: Data compilation\u00a0and main features<\/a>. Journal of South American Earth Sciences<\/strong>.\u00a0v. 43, p. 74-85. April 2013.
\nASSUMP\u00c7\u00c3O, M. et al<\/em>. Models of crustal thickness for South America\u00a0from seismic refraction, receiver functions and surface wave dispersion<\/a>.\u00a0Tectonophysics<\/strong>. 2013 (online).<\/p>\n","protected":false},"excerpt":{"rendered":"Seismologists propose a new explanation for earthquakes in Brazil","protected":false},"author":14,"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":[240,241],"coauthors":[103,105],"class_list":["post-163758","post","type-post","status-publish","format-standard","hentry","category-science","tag-geology","tag-history"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/163758","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\/14"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=163758"}],"version-history":[{"count":0,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/163758\/revisions"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=163758"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=163758"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=163758"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=163758"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}