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Pore pressure reduction in sandstone reservoirs generally leads to small elastic plus inelastic strains. These small strains (0.1%-1.0% in total) may lead to surface subsidence and induced seismicity. In current geomechanical models, the inelastic component is usually neglected, though its contribution to stress-strain behavior is poorly constrained. To help bridge this gap, we performed deviatoric and hydrostatic stress cycling experiments on Slochteren sandstone samples from the seismogenic Groningen gas field in the Netherlands. We explored in situ conditions of temperature (T = 100°C) and pore fluid chemistry, porosities of 13% to 26% and effective confining pressures (≤320 MPa) and differential stresses (≤135 MPa) covering and exceeding those relevant to producing fields. We show that at all stages of deformation, including those relevant to producing reservoirs, 30%-50% of the total strain measured is inelastic. Microstructural observations suggest that inelastic deformation is largely accommodated by intergranular displacements at small strains of 0.5%-1.0%, with intragranular cracking becoming increasingly important toward higher strains. The small inelastic strains relevant for reservoir compaction can be described by an isotropic, Cam-clay plasticity model. Applying this model to the depleting Groningen gas field, we show that the in situ horizontal stress evolution is better represented by taking into account combined elastic and inelastic deformation than it is by representing the total deformation behavior using poroelasticity (up to 40% difference). Therefore, inclusion of the inelastic contribution to reservoir compaction has a key role to play in future geomechanical modelling of induced subsidence and seismicity.

Understanding the internal mechanisms controlling fault friction is crucial for understanding seismogenic slip on active faults. Displacement in such fault zones is frequently localized on highly reflective (mirrorlike) slip surfaces, coated with thin films of nanogranular fault rock. We show that mirror-slip surfaces developed in experimentally simulated calcite faults consist of aligned nanogranular chains or fibers that are ductile at room conditions. These microstructures and associated frictional data suggest a fault-slip mechanism resembling classical Ashby-Verrall superplasticity, capable of producing unstable fault slip. Diffusive mass transfer in nanocrystalline calcite gouge is shown to be fast enough for this mechanism to control seismogenesis in limestone terrains. With nanogranular fault surfaces becoming increasingly recognized in crustal faults, the proposed mechanism may be generally relevant to crustal seismogenesis.

We report ring shear experiments on simulated calcite fault gouges performed at fixed temperatures (T) within the range from 20°C to 600°C. The experiments were performed wet, using pore fluid pressures (Pf) of 10 ≤ Pf ≤ 60 MPa. One series of experiments employed a constant effective normal stress ( σneff) of 50 MPa, while in a second series, σneff was sequentially stepped from 30 to 100 MPa. In all experiments, sliding velocity (v) was stepped in the range from 0.03 to 100 µm/s. The results showed stable, velocity‐strengthening behavior at 20°C, but velocity weakening at 100°C to 550°C (for all v steps to <3 µm/s), which was frequently accompanied by stick slip. At 600°C, velocity strengthening occurred. Microstructural observations suggest increasing importance of ductile deformation with increasing temperature, as reflected by a localized shear band structure at 20°C giving way to a pervasive, shear plane‐parallel grain shape fabric at 600°C. Using existing flow equations for dense calcite polycrystals, we show that dislocation and/or diffusion creep of 10–30 µm‐sized bulk gouge grains likely played a role in experiments performed at T ≥ 400°C. We suggest that the observed velocity‐weakening behavior can be explained by a slip mechanism involving dilatant granular flow in competition with creep‐controlled compaction. Our results have important implications for the breadth of the seismogenic zone in limestone terrains and for the interpretation of natural fault rock microstructures. Specifically, while samples sheared at 400–550°C exhibited essentially brittle/frictional mechanical behavior (stick slip), the corresponding microstructures resembled that of a mylonite.

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Fault slip and healing history may crucially affect the fault seismogenic potential in the earthquake nucleation regime. Here we report direct shear friction tests on simulated gouges derived from a carbonate fault breccia, and from a clay/carbonate fault‐core gouge, retrieved from a surface exposure of the Longmenshan Fault Zone (LFZ) which hosted the 2008 Wenchuan earthquake. The experiments were conducted under dry and hydrothermal conditions, at temperatures up to 140°C, at an effective normal stress of 50 MPa, and involved sequential velocity‐stepping (VS), slide‐hold‐slide (SHS), and velocity‐stepping stages. Dry tests performed on breccia‐derived samples showed no dependence of (quasi) steady state friction (μss) on SHS or VS history, and a log linear relation between transient peak healing (Δμpk) and hold time, or classical “Dieterich‐type” healing behavior. By contrast, all experiments conducted under hydrothermal conditions were characterized by “non‐Dieterich” healing behavior. This included (1) an increase in μss upon resliding after a hold period and (2) an increase in friction rate parameter (a − b), after SHS testing. Comparison with previous results suggests that the healing behavior seen in our wet tests may be attributed to solution transfer processes occurring during hold periods. Our findings imply that the shallow portions of faults with carbonate/clay‐rich cores (e.g., the LFZ) can heal much faster than previously recognized, while the upper limit of the seismogenic zone may migrate to deeper levels during interseismic periods. These effects have important implications for understanding the seismic cycle in tectonically active carbonate terrains.