José Roberto Barbosa

The tailings dam failure of 5 November 2015 in SE Brazil and its preceding seismic sequence

The collapse of a mine tailings dam and subsequent flood in SE Brazil on 5 November 2015 was preceded by a small-magnitude seismic sequence. In this report, we explore the spatiotemporal associations between the seismic events and the accident and discuss their possible connection. We also analyze the signals generated by the turbulent mudflow, as recorded by the Brazilian Seismographic Network (RSBR). In light of our observations, we propose as possible contributing factor for the dam collapse either ground shaking and/or soil liquefaction triggered by the earthquakes. The possibility of such a small-magnitude earthquake contributing to the collapse of a tailings dam raises important concerns regarding safety and related legislation of dams in Brazil and the world.

Seismicity patterns and focal mechanisms in southeastern Brazil

The seismicity of SE Brazil is studied with a recent deployment of additional temporary digital and analog seismographic stations. Regional and local stations have allowed the determination of three new composite focal mechanisms for the earthquake series of Betim in 1992/93, Formiga in 1993, and the reservoir-induced events of Nova Ponte in 1995, all in Minas Gerais state. The focal mechanism data in the southern part of the Sao Francisco craton and adjacent Brasilia fold belt indicate both normal and reverse faulting with a common E-W to NE-SW orientation of the maximum horizontal stress. Far from the continental margin, the focal mechanisms are consistent with theoretical estimates of stress directions from finite-element modelings of the forces driving the South American plate. Near the Serra do Mar coastal range, the stress pattern seems more complex. A selection of the earthquake catalog, using threshold magnitudes varying in time to yield a data set with spatially uniform coverage, indicates two main seismic areas: 1) the offshore continental shelf , and 2) the southern part of the Brasilia fold belt and the Sao Francisco craton. The high topography areas of the Serra do Mar and Serra da Mantiqueira (in the Ribeira fold belt) and the Parana basin are much less seismically active.