Fault Weakening During Short Seismic Slip Pulse Experiments: The Role of Pressurized Water and Implications for Induced Earthquakes in the Groningen Gas Field

High-velocity friction experiments on simulated fault gouges sheared at high normal stress and to low displacement are particularly relevant to induced seismicity, which is becoming an important topic in fault mechanics. Using a new, improved set-up, which allows simulation of fault stress and fluid pressure (P) conditions approaching in-situ reservoir values, we performed ring-shear experiments on simulated fault gouges prepared from the source-, reservoir-, and caprock-formations of the Groningen gas field. Pre-sheared gouges were subjected to a rotational slip pulse reaching similar to 1.0 m/s peak velocity and 13-16 cm total displacement at effective normal stresses (sigma(e)(n)) of 5-31 MPa and P up to 5 MPa, using water or dry nitrogen as pore fluid. All water-saturated gouges show strong dynamic weakening within a few cm of slip, with the lowest dynamic friction (0.2-0.4) measured at the highest sigma(e)(n). By contrast, the weakening was subtle in experiments using nitrogen. Our analyses focus on the high-P experiments, which are more realistic and show a distinct dependence of constitutive parameters (e.g., slip-weakening rate) on sigma(e)(n), in the form of empirical linear, power-law or exponential relations. The results provide much-needed constraints for numerical modeling of induced rupture propagation in the Groningen field. Based on temperature- and P-measurements made in near-direct contact with the active shear band, and using "post-mortem" microstructures, we exclude previously-proposed dynamic weakening mechanisms (e.g., flash heating or thermal pressurization) and suggest that water pressurization at heated asperity or grain contacts explains the weakening seen in our high-P experiments. In the Groningen gas field of the Netherlands, induced seismic events have occurred since the 1990s. The strongest event occurred in 2012 with a magnitude of M-W 3.6. How faults slip during such small magnitude earthquakes (i.e., magnitude 3-4) is not clear, despite several recent studies. We conduct laboratory experiments to simulate the pulse-like (i.e., short but rapid) fault slip behavior characteristic for small magnitude human-induced seismicity, using a newly designed experimental set-up enabling monitoring of the slip behavior under stress and displacement closer to conditions relevant to faults in Groningen. We find that the different rock types present in the Groningen field offer various levels of resistance to rupture of small earthquakes, which is crucial information for modeling earthquakes and understanding seismic hazard and risk. In addition, based on observations of fluid pressure, temperature and dilatation during the experiments, we propose a fault weakening mechanism occurring on the micrometer scale, that can explain our observations.