To specify quantitatively the effect of pressure and water weakening on the flow strength of feldspar we performed triaxial creep experiments in a gas deformation apparatus at temperatures of 1000–1150°C, confining pressures of 100–450 MPa, and axial stresses of 10–400 MPa, resulting in strain rates of ∼6 × 10⁻⁷ to 3 × 10⁻³ s⁻¹. Dense samples with a grain size of ∼3 μm were prepared by hot‐isostatic pressing of anorthite glass powder. Hydrous samples contain about 0.33 ± 0.14 wt % H₂O and dry specimens 0.0005–0.02 wt % H₂O. The estimated residual glass content of wet samples is <2 vol %. Samples deformed by grain boundary diffusion‐controlled creep at low stresses and dislocation creep at stresses ≳150 MPa. We estimate an activation volume of V ≈ 24 cm³mol⁻¹ for anhydrous samples deforming in diffusion creep. For wet samples, deformed in hydrous conditions with varying buffers fixing oxygen fugacity, the activation volume is about 38 cm³ mol⁻¹. Creep rate of hydrous anorthite aggregates depends on water fugacity raised to a power of r = 1.0 ± 0.3, suggesting hydrolysis of oxygen bonds. Considering the effect of activation volume and water fugacity on extrapolation of constitutive laws to conditions prevailing in the continental lower crust, viscosities of hydrous feldspar aggregates increase by a factor of <3.

Surface roughness ubiquitously prevails in natural faults across various length scales. Despite extensive studies highlighting the important role of fault geometry in the dynamics of tectonic earthquakes, whether and how fault roughness affects fluid-induced seismicity remains elusive. Here, we investigate the effects of fault geometry and stress heterogeneity on fluid-induced fault slip and associated seismicity characteristics using laboratory experiments and numerical modeling. We perform fluid injection experiments on quartz-rich sandstone samples containing either a smooth or a rough fault. We find that geometrical roughness slows down injection-induced fault slip and reduces macroscopic slip velocities and fault slip-weakening rates. Stress heterogeneity and roughness control hypocenter distribution, frequency–magnitude characteristics, and source mechanisms of injection-induced acoustic emissions (AEs) (analogous to natural seismicity). In contrast to smooth faults where injection-induced AEs are uniformly distributed, slip on rough faults produces spatially localized AEs with pronounced non-double-couple source mechanisms. We demonstrate that these clustered AEs occur around highly stressed asperities where induced local slip rates are higher, accompanied by lower Gutenberg–Richter b -values. Our findings suggest that real-time monitoring of induced microseismicity during fluid injection may allow identifying progressive localization of seismic activity and improve forecasting of runaway events.

Semi‐brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid‐levels of the Earth's crust. Using a Paterson gas‐deformation apparatus, we performed 67 conventional triaxial deformation experiments on dry samples of Carrara marble in the temperature range of T = 20–800°C; confining pressures (P_C) were 30, 50,100, 200, and 300 MPa, and strain rates () were 10⁻³, 10⁻⁴, 10⁻⁵, and 10⁻⁶ s⁻¹. Axial strains (ε) were ≲0.12. The measured (differential) stress, , changes with strain at most applied conditions. At ε = 0.05, both stress and the hardening coefficient (h), that is, the rate of increase of stress with strain, increase as T decreases and P_C increases. At T ≲ 400°C, h is quite large and the sensitivity of on is low, while both are sensitive to increasing pressure. In this temperature range, the mechanical behavior of the marble is very similar to that exhibited by high‐strength, high‐ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Twinning and dislocation motion are abundant in the samples, as are inter‐ and intracrystalline microfractures. The concurrent activation of these deformation mechanisms leads to complex relationships of and h with the applied T ‐ P_C ‐ conditions. This behavior suggests that peak stresses for calcite rocks deforming by semi‐brittle processes will occur at P_C ‐ T conditions of the middle crust, and that they might be more strongly influenced by total strain rather than by strain rate.