Investigations of the reservoir-triggered seismicity of Nova Ponte and Irapé, Brazil

Abstract

The rise of large hydropower plants with extensive reservoirs has raised the need for a deeper understanding of reservoir-triggered seismicity, a phenomenon that causes significant challenges for safe reservoir management. Reservoir-triggered seismicity is an increasingly relevant topic of interest throughout the world. The global distribution cases involve a large number of hydroelectric power plants as well as future planned dams and reservoirs, with many documented cases for more than six decades. In this scenario, Brazil has experienced 29 cases of RTS in which the mechanism of magnitude mR 4.0 Nova Ponte and mR 3.0 Irape is not well understood. The M4.0 RTS mechanism at Nova Ponte is still important to understand, the second-largest known case of Reservoir reservoir-triggered earthquake in Brazil. The main event occurred in the year 1998 along predominantly reverse faulting and roughly oriented NE-SW. But it still remains to be understood whether it occurred by state pore pressure or undrained response of excess loading of the reservoir is such low permeable rocks that are needed to trigger RTS. If the pore pressure diffusion could not propagate deep into vertical low permeable rocks it can take hundreds of years to reach the depth of the earthquakes. So, in such above conditions, the two major events of triggered earthquakes occurred in 1995 and 1998 just 1.5 years after the impoundment of the reservoir when the water reached the highest level. Thus, in this study, we made an attempt in this area to explore the causal mechanism by creating the 3D Reservoir model and its effect on the subsurface both in terms of pore pressure and stress changes with the help of numerical modeling. We numerically simulate the poromechanical subsurface response to reservoir impoundment using a 3D model that includes three geological rock layers down to 10 km depth. From the proposed potential nodal planes of the 1995 M3.5 earthquake, we show that the earthquake has most likely occurred on a vertical, E-W- oriented strike-slip fault with a reverse-displacement component. The deviatoric stresses generated by the water column loading on the surface, superimposed by undrained pore pressure enhancement in deep low-permeability layers can explain the fault reactivation. We find that for the 1998 M4.0 earthquake to occur, conductive flow pathways with permeability as high as 6.6·10-15 m2 should exist to transmit pore pressure to a deep critically oriented fault. The analysis owned by us raises the importance of accounting for coupled poromechanical mechanisms controlling fault stability, hydromechanical properties of different rock layers, and realistic shape of the reservoir to accurately assess the potential for reservoir-triggered seismicity. We remarked that our findings can explain the triggered seismicity by the undrained response of excess loading of the reservoir filling due to pore pressure perturbation such as in deep underground low permeable rocks. The second case study Irapé, in Brazil, is a prominent RTS site where seismicity surged after reservoir filling, with a maximum event of magnitude 3.0 in May 2006, 6 months after the start of reservoir impoundment. Despite more than a decade has passed since the seismicity occurred, the factors governing these earthquakes and their connection to subsurface rock properties remain poorly understood. Here, we attempt to understand the potential causes of RTS at Irapé dam, which is the highest dam in Brazil with 208 m, and the second highest in South America. Permeability and porosity measurements of cylindrical cores from hard and intact rock samples, which have been extracted near the RTS zone, by pitting 10 cm from the surface, reveal a lowpermeability rock. Porosity values range from 6.340 to 14.734%. Only 3 out of the 11 tested samples present permeability higher than the lowest measurable value of the apparatus (0.002 mD), with the highest permeability being 0.0098 mD. The undrained response of the low-permeability rock placed below the reservoir results in an instantaneous increase in pore pressure and poroelastic stress changes due to elastic compression, which brings potential faults located below the reservoir closer to failure conditions. According to our analytical calculations, the increase in 136 m of the reservoir-water level caused a 0.54 MPa pore pressure buildup at the depth of the M3.0 earthquake, i.e., 3.88 km, resulting in an increase of 0.82 MPa in the vertical effective stress and a decrease of 0.34 MPa in the horizontal effective stress. These changes resulted in an increase in the deviatoric stress that led to fault destabilization, causing the RTS. The laboratory measurements and analytical calculations corroborate the hypothesis that the initial seismic activity was induced by the undrained subsurface response to the reservoir loading at Irapé.