Influência de estruturas brasilianas na sismicidade desencadeada por reservatório na Usina Hidrelétrica de Irapé, Minas Gerais, Brasil

Abstract

RESUMO:Inaugurada em 2006, a Usina Hidrelétrica de Irapé possui a mais alta barragem do país, com 210 m, que vem sendo monitorada pelo Observatório Sismológico da Universidade de Brasília desde o começo do enchimento de seu reservatório. Apresentando sismicidade causada pelo aumento da tensão crustal logo após o início do represamento, a Sismicidade Desencadeada por Reservatório da Usina Hidrelétrica de Irapé permanece como um importante objeto de estudo para o entendimento deste fenômeno. O presente trabalho se propôs a estudar a sismicidade registrada de janeiro a maio de 2006, assim como a caracterizar a geologia ao redor da Usina Hidrelétrica de Irapé com o intuito de identificar e descrever as estruturas que permitam melhor compreender os eventos sísmicos. A orientação espacial dos eventos sísmicos obtidos pela análise das polaridades e da forma de onda condiz com aquelas dos lineamentos de relevo. As estruturas encontradas podem ser associadas a uma maior, de direção principal NE/SW, com dezenas de quilômetros, que seria responsável pela captura dos rios Jequitinhonha e Araçuaí, interpretada neste trabalho como uma estrutura relacionada a uma fase tardia do Evento Brasiliano. O diagrama de solução do mecanismo focal gerado utilizando um método de inversão da forma de onda no domínio do tempo indica que a direção dos possíveis planos de falhas está correlacionada ao principal direcionamento das estruturas rúpteis na área de trabalho. PALAVRAS-CHAVE: reativação sísmica de estruturas geológicas; estudos de sismicidade desencadeada por reservatório; Usina Hidrelétrica de Irapé.ABSTRACT: Inaugurated in 2006, the Irapé Hydroelectric Plant has the highest dam in Brazil, with 210 m. The Seismological Observatory of the University of Brasilia is monitoring this structure since the beginning of its reservoir filling. The reservoir triggered seismicity caused by the increase of crustal tension just after the start of the remains like an important object of study for understanding this phenomenon. This paper aims at studying the seismicity recorded from January to May 2006, as well as to characterize the geology around the Hydroelectric Plant Irapé in order to identify and describe the structures that help for better understanding the seismic events. The spatial orientation of the seismic events obtained by analyzing polarity and waveform is consistent with the guidance of prominent relief lineaments. These faults can be found attached to a larger structure with main NE/SW direction and tens of kilometers, which would be responsible for capturing Jequitinhonha and Araçuaí rivers. This great lineament is interpreted herein as a structure related to a late stage of the Brasiliano-Panafrican event. The solution of focal mechanism, created using an inversion method of the waveform in the time domain, indicates that the direction of the possible plans are correlated to main failure of brittle structures targeting the area of work.

Earthquakes triggered by the subsurface undrained response to reservoir-impoundment at Irapé, Brazil

“…The regional geology at the eastern part of the São Francisco Craton in the State of Minas Gerais follows a N-S direction (Almeida, 1977). Silva et al (2014) also mentioned that the repetition of a structural trend in the NE-SW direction originates from the geological and geophysical structuring of the crust. This trend makes it feasible to assume the existence of a N-S vertical mature fault that could become destabilized by small changes in the effective stress.…”

“…1975). The analysis of seismograms went through a double-checks routine (Silva et al, 2014). Velocity models were adopted based on a deep seismic refraction survey in combination with local geological interpretations and studies of the crustal structure in south-eastern Brazil to locate seismic events in the Irapé area (Assumpção et al, 2012).…”

“…Velocity models were adopted based on a deep seismic refraction survey in combination with local geological interpretations and studies of the crustal structure in south-eastern Brazil to locate seismic events in the Irapé area (Assumpção et al, 2012). The local velocity model consists of a superficial 4.8 km-thick layer with a P-wave velocity (Vp) of 4.5 km/s, representing the mica-schist to graphite-schist rocks from surface, and a second layer from schist to crystalline basement rocks with a thickness of 11.2 km with P-wave velocity (Vp) of 6 km/s (Marshak et al, 2006;Silva et al, 2014). earthquake have not been resolved yet (Silva et al, 2014).…”

“…The local velocity model consists of a superficial 4.8 km-thick layer with a P-wave velocity (Vp) of 4.5 km/s, representing the mica-schist to graphite-schist rocks from surface, and a second layer from schist to crystalline basement rocks with a thickness of 11.2 km with P-wave velocity (Vp) of 6 km/s (Marshak et al, 2006;Silva et al, 2014). earthquake have not been resolved yet (Silva et al, 2014).…”

Influence of the Three Gorges Reservoir Impoundment on Xiannvshan Fault Activity

“…D. Gough and W. Gough [5] studied the RIS at Lake Kariba and stated that either the release of preexisting stress due to water loading or the increase in pore pressure resulted in the earthquakes, but they thought that the former mechanism was more likely the cause of the earthquakes. Silva et al [6] combined geophysical observations and hydrogeological data to evaluate local structural effects on reservoir-triggered earthquake magnitudes; however, they only considered qualitative structural impacts without quantitative geometric modeling. K. Gahalaut and V. K. Gahalaut [7] used satellite imaging to detect fault activity changes before and after impoundment, but their understanding of underlying mechanisms was still inadequate.…”