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Abstract:
On August 14, 2011, an earthquake of magnitude 3.2 occurred near a hydroelectric reservoir and open pit limestone mines in Ijaci, SE Brazil. The mainshock was felt (IV-V MM), and was followed by three other events of similar magnitudes within three months. We study the seismicity near Ijaci, and its possible relationship with the reservoir impoundment, the mining activity, and the lowering of the water table. We use data of a local 7-station seismic network, and piezometer measurements. More than 1000 events were detected, of which 500 were classified as tectonic and the rest explosion in four limestone quarries that operate close to the reservoir and daily feed one of the largest cement plants in Brazil. The local karst relief sustains a large aquifer, arising at the bottom of the pits, and is responsible for an average outflow of 350 m3/hour at Mina Sul pit. The mining activities and the filling of the Funil reservoir occurred in the end of 2002. Focal mechanisms determined by polarities and waveform inversion indicate reverse faults and a compressive tectonic regime in the Ijaci area. Piezometer measurements showed no direct connection between the lake and the bottom of the pit; the pumping that lowers the water table has little influence on the stress regime. The event locations, focal mechanisms, volume of extracted material, and seismicity behavior suggest an association of the seismic activity with the extraction of limestone, which is now equivalent to an approximate 60 million tons (5.9x10N) of load relief.
