Throughout our project, we made significant progress in understanding fluid-induced earthquakes and their interactions within geological formations, resulting in the publication of 32 peer-reviewed papers that contributed valuable insights to the scientific community.
In this endeavor, we developed a fluid-induced simulator ("FIRST) capable of reaching pressures up to 200 MPa and temperatures of 200°C. This simulator, equipped with 16 acoustic emission sensors, electrical conductivity measurement capabilities, and up to 16 strain gauges, allowed for comprehensive monitoring of seismic activity (Noel et al., 2021). Additionally, we constructed a 3-meter-long biaxial apparatus housing analog samples (Paglialunga et al., 2021). Upgrades to a High-pressure (400 MPa) and high-temperature (1000°C) gas medium triaxial apparatus (TARGET) and a High-speed biaxial friction machine (HighSTEPS) (slip up to 50 cm/s) enabled us to replicate fluid-induced earthquakes across the entire seismic cycle (Violay et al., 2021; Meyer et al., 2022).
In Work Package 1, we developed mechanism-based constitutive models describing faults’ frictional behavior and porosity permeability development during inter-seismic periods. Conducting high-temperature and high-pressure experiments over extended periods, we measured the seismic and transport properties of rock. Utilizing the high-temperature - high-pressure creep apparatus ‘TARGET’ and the newly developed “FIRST” apparatus, we measured:
1. The porosity and permeability of faults and surrounding matrix (Orellana et al., Sci rep 2022; Acosta, Maye, and Violay, JGR, 2020; Meyer and Violay Gji 2023).
2. The seismic velocities to measure the crack density and aspect ratio evolution during the earthquake cycle (Acosta and Violay RMRE, 2020; Paglialunga et al., GRL., 2021).
3. We developed a new setup for electrical conductivity to monitor time-dependent changes in pore fluid chemistry and real-time fluid-rock interactions (Lazari and Violay, in prep, Giorgetti and Violay in prep).
In Work Package 2, we clarified the role of pore fluids on fault stability and earthquake nucleation. Using ‘FIRST,’ we reproduced miniature-scale injection experiments and measured induced acoustic emissions and their characteristics. Tests with different injection rates, volumes, and fluid physical properties were performed (Cornelio and Violay, GRL 2020; Acosta et al., GRL, 2019). Independently measuring frictional rate parameters while controlling machine stiffness, pore pressure, and normal and shear stresses under brittle and semi-brittle conditions (Noel et al., 2019a, b; Noël, Passelègue, and Violay, JGR, 2021), we observed and modeled the spontaneous nucleation precursors and evolution of frictional parameters.
In Work Package 3, we experimentally tracked lubrication processes during co-seismic sliding, reproducing co-seismic slip in a laboratory. We obtained unprecedented datasets on frictional dynamic weakening to understand the energy budget of fluid-induced earthquakes (Paglialunga et al., EPSL 2022, and EPSL 2024). Investigating the thermodynamic properties of fluid and fluid viscosity, along with pore fluid pressure, facilitated seismic source interpretation (Acosta et al., 2018, Nature comm; Cornelio et al., 2019 Nat comm, Cornelio et al., 2019, JGR, Cornelio GRL, 2020, Cornelio and Violay, GJI, 2020, Paglialunga et al., JGR, 2023).
Finally, we proposed an earthquake mitigation strategy from deep geothermal reservoirs, based on stress preconditioning, as a proactive approach to managing seismic risks associated with geothermal energy extraction (Fryer et al., 2023).
