Velocity-dependent frictional properties of fault gouges in the Upper Rhine Graben under hydrothermal conditions: Implications for induced seismicity

The Upper Rhine Graben (URG) is a tectonically active area that has been extensively investigated for its geothermal energy potential. However, modification of fluid pressures in subsurface reservoir for geothermal energy production can affect the regional stress field inducing seismicity that causes high public and social concern as well as economic losses, e.g., in Landau, Insheim, Soultz-sous-Fôrets, Rittershofen, Strasbourg, and Basel. The effective frictional strength and stability of faults depend on the nature of stimulation, the reservoir conditions, and subsurface fault rock characteristics. It is crucial that we explore the complex relationships between these factors and the frictional stability of faults for safe geothermal energy operations. In this study, to understand the seismic potential of faults in geothermal reservoir rocks, we investigated how the frictional behaviour of fault gouges from the URG area varies when stimulated with fluids at different temperatures and pressures using hydrothermal rotary shear friction experiments.

Our data are fit by rate-and-state friction laws (RSF) and the mechanical results are supplemented with microstructural observations to identify the active deformation mechanisms. We also analyzed porosity, grain size, shape, and mineralogy of fault gouges employing scanning electron microscopy and energy-dispersive spectroscopy. Simulated fault gouges were prepared from Muschelkalk, Buntsandstein, Rotliegend, and crystalline basement rocks (granite and gneiss), and velocity-stepping tests were conducted at temperatures from 20 to 250 ºC, effective normal stresses of 60 and 75 MPa, pore fluid pressures of 40 and 50 MPa, and slip velocities 0.3 to 100 µm/s, depending on the fault gouge type. Moreover, X-ray diffraction (XRD) was performed on the fault gouge samples to determine their mineralogical composition, which significantly influences the mechanical behavior of the gouges.

We observed differences in gouge sliding strength and frictional character as a function of both sliding velocity and temperature. Preliminary mechanical results show a strong temperature dependent steady-state strength during initial sliding, with friction coefficients in the range of 0.38 – 0.9. All the fault gouges exhibit stable velocity-strengthening (aseismic) behavior, except those derived from Rotliegend and granite, which show a transition from velocity strengthening to velocity weakening with increasing sliding velocity at T>200 ºC. The rate-and-state parameters (a, b, and D_c) for Rotliegend and granite show a transition from a velocity-neutral to velocity- and strain-weakening behavior at temperatures between 200 and 250 ºC. This transition enhances mechanical instability and creates conditions more favorable for earthquake nucleation. In contrast, the Muschelkalk and Buntsandstein samples revealed velocity-strengthening and strain-hardening behavior, favouring aseismic creep over dynamic rupture, which we interpret to be mainly due to the presence of small amounts of weak hydrous minerals and amorphous content. These results indicate that the Rotliegend and crystalline basement rocks (granite) are more prone to induced seismicity than Muschelkalk and Buntsandstein. Our findings provide vital insights into the understanding of fault behavior at regional scales, allowing constraint input for seismic models, and strengthen the connection to numerical models.