Reservoir-triggered seismicity at the highest Brazillian dam

Irapé Reservoir has the highest brazillian dam, with 208 meter high, and is located at Minas Gerais State, in the Jequitinhonha river. Since September 2003 a 3-component broadband digital seismic station is in opperation at the area and more three 3-component short period digital stations were installed nine months before the impoundment of the lake. During this two years pre-impoundment monitoring no local event was detected at the area of the lake. The filling of the reservoir started in December 7, 2005, and was followed by microearthquakes just one day after. Until October 2006 more than 300 events were detected by this local network. The largest microearthquake occurred in May 14, 2006 with 3.0 magnitude and was felt by the officials of the power plant. A preliminary hypocentral determination was performed for these events showing that the seismicity is located in a small area, with hypocenters below the lake or at its margins near the dam. The clear time correlation between the start of impoundment of the lake and the occurence of seismicity shows a causative relationship for these seismicity. Also the spatial distribution of the epicenters let us to conclude that this is another case of reservoir- triggered seismicity in Brazil of the rapid or initial response type.