Understanding the triggering mechanisms of reservoir-triggered seismicity at Nova Ponte, Brazil, through hydro-mechanical modeling

Reservoir impoundment is usually accompanied by induced/triggered seismicity. The rise in the number of planned hydropower plants requires improving the understanding of the causes of this induced/triggered seismicity, which eventually could serve to propose mitigation measures to reduce the induced/triggered-seismicity risk. We investigate the case of reservoir-triggered seismicity at Nova Ponte, Brazil, where triggered seismicity started shortly after reservoir impoundment, with the maximum magnitude of M3.5 when reaching the highest water level on the dam, and followed by delayed seismicity, with the largest earthquake being a M4.0 about 4.5 years after impoundment. We have built a hydro-mechanical fully-coupled numerical model reproducing the T shape of the reservoir and including the three geological layers placed below the reservoir down to 10 km depth. Simulation results serve to identify the nodal plane, from the two nodal planes of the proposed focal mechanism, which nucleated the seismicity of the M3.5 earthquake: a vertical, E-W-oriented strike-slip fault with a reverse-displacement component. The initial seismicity was triggered by the undrained response of the subsurface to the loading of the reservoir. We also find that the delayed seismicity was triggered by pore pressure diffusion, bringing a critically oriented vertical fault to failure conditions. The vertical permeability to allow the pore pressure perturbation to reach the depth of the M4.0 earthquake, i.e., 3 km depth, in 4.5 years is 6.6·10^-15 m², two to three orders of magnitude higher than the expected permeability of the host rock, a low-permeability mica-schist. We contend that hydro-mechanical models are a useful tool to understand the triggering mechanisms of reservoir-triggered seismicity.