Seismicity Induced by a Large Open Pit Mine in SE Brazil: Combination of Stress Concentration and Crustal Weakness.

Seismicity induced by water reservoir and fluid injection are widely known phenomena and have been studied for several decades. However, seismicity induced by crustal unloading in large open-pit mines are seldomly reported in the literature. Here we describe a case of seismicity associated with the large open-pit Cajati mine in SE Brazil, which has been operating for more than 40 years. The mine exploits carbonate rocks of a Mesozoic alkaline intrusion complex. The pit is 1.4 km long and 0.75 km wide reaching 300 m depth. The estimated amount of extracted rock is about 350 Mton. Nine earthquakes with magnitudes in the range 2.0-3.2 Mw have been recorded since 2009 by stations of the Brazilian Seismic Network (RSBR), some of them felt with intensities IV MM in nearby towns. The 2015 mainshock (3.2 Mw) caused expressive cracks in the mine benches, with up to 10 cm displacement. Epicenter relocation of the six largest events, using correlated P- and S-waves at regional distances, show that all events occurred in a single NNW-SSE oriented, 0.5 km long rupture aligned with the major axis of the pit, in agreement with the main trend of the micro-seismicity recorded by the local mine network. Focal mechanism was determined with two techniques: ISOLA envelope and FMNEAR using three stations at 100 to 160 km distance. Both methods show a reverse faulting mechanism with nodal planes oriented NW-SE to NNW-SSE.  This is consistent with the expected mechanism for crustal unloading in open-pit mines. The Cajati mine is located in the Ponta Grossa Arch, in the coastal ranges of SE Brazil, a region with low-velocities at lithospheric depths. This suggests lithospheric thinning that concentrates stresses in the upper crust. In addition, the NE-SW P axis orientation is parallel to the coast line, a pattern that favors concentration of the regional stresses due to continental/oceanic structural transition. Aeromagnetic data shows a clear NNW-SSE regional fault crossing the mine area. The Cajati mine-induced seismicity is a classic case where several positive factors contribute to the inducing mechanism: a) high stresses in the upper crustal, b) favorable orientation of a previously existing weak zone (related to the NNW-SSE fault during emplacement of the alkaline body), and large Coulomb stress changes of about 4 MPa from unloading of the vertical stresses.