Seismicity Induced by the Cajati Open Pit Mine, SE Brazil: a case of stress concentration, weakness zone, and favorable geometry

Fluid injection and reservoir-induced seismicity have been studied for decades. However, seismicity induced by crustal unloading in open-pit mines are seldomly reported. We describe a case associated with a large mine in SE Brazil exploiting carbonate rocks. The pit is 1.4 km long, 0.8 km wide, 300 m deep. 440 Mton of rock have been extracted. Eleven earthquakes (magnitudes 2.0 to 3.3 mb) have occurred since 2009. The largest event (3.2 Mw in 2015) caused expressive cracks in the mine benches. Relocated epicenters, using correlated P- and S-waves at regional stations, align in a single NNW-SSE oriented, 0.5 km long fault beneath the pit major axis. Focal mechanism shows reverse faulting, as expected for crustal unloading, with Coulomb stress increase of 2 MPa. In the coastal ranges of SE Brazil, low-velocities at lithospheric depths suggest stress concentration in the upper crust. The coast-parallel NE-SW P axis is consistent with stress rotation due to continent/ocean transition. Aeromagnetic data shows a NNW-SSE regional fault crossing the mine area. The Cajati mine is a classic case where several factors combine to induce seismicity: upper crustal high stresses, favorable orientation of a previous weak zone, and large Coulomb stress changes from unloading.