Microearthquakes and the Balbina Lake, A Possible Case of Induced Seismicity in the Amazon Area

Since 1979 the ELETRONORTE-Centrais Eletricas do Norte do Brasil S/A through the Seismological Observatory of the University of Brasilia started a large seismic program to monitor its main reservoirs located in the Amazon region. By the end of 1979 the first seismic station was deployed at Balbina area almost eight years before the impoundment of the lake. During this period a routine monitoring using a single station showed a very low level of seismicity surrounding the Balbina reservoir area. A local seismicity at Balbina Lake following its impoundment suggested a reservoir-induced seismicity (RIS) origin for these tremors. The epicentral region is located outside the lake area, almost 17 km western of the dam. Four shocks with magnitude between 3.0 to 3.4 were detected during the main activity period that was observed in 1990.