27 Feb 2024
 | 27 Feb 2024
Status: this preprint is open for discussion.

Earthquakes triggered by the subsurface undrained response to reservoir-impoundment at Irapé, Brazil

Haris Raza, George Sand França, Eveline Sayão, and Victor Vilarrasa

Abstract. The necessity to reduce carbon emissions to mitigate climate change is accelerating the transition from fossil fuels to renewable energy sources. Specifically, hydropower, in particular, has emerged as a prominent and safe renewable energy source but entails reservoir-triggered seismicity (RTS). This phenomenon causes significant challenges for safe reservoir management. 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, just six 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 low-permeability 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 Magnitude 3.0 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.

Haris Raza, George Sand França, Eveline Sayão, and Victor Vilarrasa

Status: open (until 15 Apr 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2024-166', Giacomo Medici, 14 Mar 2024 reply
  • RC1: 'Comment on egusphere-2024-166', Anonymous Referee #1, 03 Apr 2024 reply
Haris Raza, George Sand França, Eveline Sayão, and Victor Vilarrasa
Haris Raza, George Sand França, Eveline Sayão, and Victor Vilarrasa


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Short summary
To achieve Paris Agreement goals, emissions reduction is prioritized. Hydropower, a key renewable, faces challenges, like reservoir-triggered seismicity (RTS). Core samples show 6.34–14.734 % porosity, max 0.0098 mD permeability. A 136m reservoir rise causes 0.54 MPa pore pressure increase. Vertical stress rises 0.82 MPa, horizontal drops 0.34 MPa. Irapé's RTS links to the undrained response of reservoir loading, These facts urge sustainable energy strategies and future development of dams.