{"id":182233,"date":"2015-04-10T15:08:50","date_gmt":"2015-04-10T18:08:50","guid":{"rendered":"http:\/\/revistapesquisa.fapesp.br\/?p=182233"},"modified":"2015-07-03T13:17:15","modified_gmt":"2015-07-03T16:17:15","slug":"heading-northwest","status":"publish","type":"post","link":"https:\/\/revistapesquisa.fapesp.br\/en\/heading-northwest\/","title":{"rendered":"Heading northwest"},"content":{"rendered":"

\"062-063_GEO_230\"<\/a>Fragments 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\u2014the name geologists use to refer to the rocky block that comprises about 540,000 square kilometers and encompasses much of the states of Cear\u00e1, Rio Grande do Norte, Para\u00edba, Pernambuco, Alagoas and Sergipe\u2014causes subtle extensions and contractions at different points on the surface and raises the risk of local tremors. \u201cThe province is under pressure from all sides,\u201d says geophysicist Giuliano Sant\u2019Anna Marotta of the Seismological Observatory at the University of Bras\u00edlia (UnB), principal author of the paper. \u201cThe situation is similar to what happens when we squeeze a rubber eraser.\u201d Some points contract while others stretch; some parts sink as others rise.<\/p>\n

There is no cause for alarm, however. The movement of pieces of the geological province, where much of Brazil\u2019s 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\u2014the 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. \u201cWe knew that the Borborema Province was moving, and now we\u2019ve been able to quantify the maximum velocity of this type of occurrence,\u201d says geologist Francisco Hil\u00e1rio Bezerra of the Federal University of Rio Grande do Norte (UFRN), a co-author of the scientific paper.<\/p>\n

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<\/a><\/em>). 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\u2014which does not sink, rise or move horizontally\u2014always shows the same coordinates over a given time interval.<\/p>\n

Within the tectonic plate<\/strong>
\nIn 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. \u201cIdeally, we should have data for at least three consecutive years,\u201d says Jo\u00e3o Francisco Galera M\u00f4nico of S\u00e3o Paulo State University (Unesp) at Presidente Prudente, an expert in geodetic studies using GPS who was also involved in the study.<\/p>\n

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\u2019s surface. Large-magnitude tremors occur in areas located near the edges of the plates, where there are large geological faults\u2014fissures in the crust that mark the zone of contact between the end of one plate and the beginning of another\u2014such 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.<\/p>\n

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. \u201cThe Borborema Province has many shear zones,\u201d 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\u2014a fault that cuts through the interior of Cear\u00e1 State to the Potiguar Basin\u2014had 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.<\/p>\n

Scientific article<\/em>
\nMAROTTA G. S. et al<\/em>. Strain rates estimated by geodetic observations in the Borborema Province, Brazil<\/a>. Journal of South American Earth Sciences<\/strong>.\u00a0V. 58, p. 1\u20138. Mar. 2015.<\/p>\n","protected":false},"excerpt":{"rendered":"Part of the Earth\u2019s crust is moving a few millimeters each year","protected":false},"author":13,"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],"coauthors":[101],"class_list":["post-182233","post","type-post","status-publish","format-standard","hentry","category-science","tag-geology"],"acf":[],"_links":{"self":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/182233","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\/13"}],"replies":[{"embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/comments?post=182233"}],"version-history":[{"count":0,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/posts\/182233\/revisions"}],"wp:attachment":[{"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/media?parent=182233"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/categories?post=182233"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/tags?post=182233"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/revistapesquisa.fapesp.br\/en\/wp-json\/wp\/v2\/coauthors?post=182233"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}