Fault reactivation by fluid injection: Controls from stress state and injection rate

We studied the influence of stress state and fluid injection rate on the reactivation of faults. We conducted experiments on a saw-cut Westerly granite sample under triaxial stress conditions. Fault reactivation was triggered by injecting fluids through a borehole directly connected to the fault. Our results show that the peak fluid pressure at the borehole leading to reactivation depends on injection rate. The higher the injection rate, the higher the peak fluid pressure allowing fault reactivation. Elastic wave velocity measurements along fault strike highlight that high injection rates induce significant fluid pressure heterogeneities, which explains that the onset of fault reactivation is not determined by a conventional Coulomb law and effective stress principle, but rather by a nonlocal rupture initiation criterion. Our results demonstrate that increasing the injection rate enhances the transition from drained to undrained conditions, where local but intense fluid pressures perturbations can reactivate large faults

Micromechanics of rock damage and its recovery in cyclic loading conditions

Under compressive stress, rock ``damage'' in the form of tensile microcracks is coupled to internal slip on microscopic interfaces, such as preexisting cracks and grain boundaries. In order to characterise the contribution of slip to the overall damage process, we conduct triaxial cyclic loading experiments on Westerly granite, and monitor volumetric strain and elastic wave velocity and anisotropy. Cyclic loading tests show large hysteresis in axial stress-strain behaviour that can be explained entirely by slip. Elastic wave velocity variations are observed only past a yield point, and show hysteresis with incomplete reversibility upon unloading. Irrecoverable volumetric strain and elastic wave velocity drop and anisotropy increase with increasing maximum stress, are amplified during hydrostatic decompression, and decrease logarithmically with time during hydrostatic hold periods after deformation cycles. The mechanical data and change in elastic properties are used to determine the proportion of mechanical work required to generate tensile cracks, which increases as the rock approaches failure but remains small, up to around 10\% of the net dissipated work per cycle. The pre-rupture deformation behaviour of rocks is qualitatively compatible with the mechanics of wing cracks. While tensile cracks are the source of large changes in rock physical properties, they are not systematically associated with significant energy dissipation and their aperture and growth is primarily controlled by friction, which exerts a dominant control on rock rheology in the brittle regime. Time-dependent friction along preexisting shear interfaces explains how tensile cracks can close under static conditions and produce recovery of elastic wave velocities over time

Micromechanical controls on the brittle-plastic transition in rocks

The rheology of rocks transitions from a localized brittle behaviour to distributed plastic behaviour with increasing pressure and temperature. This brittle-plastic is empirically observed to occur when the material strength becomes lower than the confining stress, which is termed Goetze's criterion. Such a criterion works well for most silicates but is not universal for all materials. We aim to determine the microphysical controls and stress-strain behaviour of rocks in the brittle-plastic transition. We use a micro-mechanical approach due to Horii and Nemat-Nasser, and consider representative volume elements containing sliding wing-cracks and plastic zones. We find solutions for frictional slip, plastic deformation and crack opening at constant confining pressure, and obtain stress-strain evolution. We show that the brittle-plastic transition depends on the confining stress, fracture toughness and plastic yield stress but also critically on the friction coefficient on preexisting defects. Materials with low friction are expected to be more brittle, and experience transition to fully plastic flow at higher pressure than anticipated from Goetze's criterion. The overall success of Goetze's criterion for the brittle-plastic transition in rocks is likely arising from the low toughness, high strength, and medium friction coefficient character of most rock forming minerals

Rupture and afterslip controlled by spontaneous local fluid flow in crustal rock

Shear rupture and fault slip in crystalline rocks like granite produce large dilation, which impacts the spatiotemporal evolution of fluid pressure in the crust during the seismic cycle. To explore how fluid pressure variations are coupled to rock deformation and fault slip, we conducted laboratory rock failure experiments under upper crustal conditions while monitoring acoustic emission locations and in situ fluid pressure. Our results show the existence of two separate faulting stages: an initial shear rupture propagation phase, associated with large dilatancy and stabilised by local fluid pressure drops, followed by a sliding phase on the newly formed fault, promoted by local fluid pressure recharge from the fault walls. This latter stage had not been previously recognised and can be understood as fluid-induced afterslip, co-located with the main rupture patch. Upscaling our laboratory results to the natural scale, we expect that spontaneous fault zone recharge could be responsible for early afterslip in locally dilating regions of major crustal faults, independently from large-scale fluid flow patterns

The fracture energy of ruptures driven by flash heating

We present a model for dynamic weakening of faults based on local flash heating at microscopic asperity contacts coupled to bulk heating at macroscopic scale. We estimate the fracture energy G associated with that rheology and find that for constant slip rate histories G scales with slip δ as math formula at small slip, while math formula at large slip. This prediction is quantitatively consistent with data from laboratory experiments conducted on dry rocks at constant slip rate. We also estimate G for crack-like ruptures propagating at constant speed and find that math formula in the large slip limit. Quantitative estimates of G in that regime tend to be several orders of magnitude lower than seismologically inferred values of G. We conclude that while flash heating provides a consistent explanation for the observed dynamic weakening in laboratory experiments with kinematically imposed slip, its contribution to the energy dissipation during earthquakes becomes negligible for large events when considering the elastodynamic coupling between strength and slip evolution

Low-Frequency Measurements of Seismic Moduli and Attenuation in Antigorite Serpentinite

Laboratory measurements of seismic moduli and attenuation in antigorite serpentinite at a confining pressure of 200 MPa and temperatures up to 550 °C provide new results relevant to the interpretation of geophysical data in subduction zones. A polycrystalline antigorite specimen was tested via forced oscillations at small strain amplitudes and seismic frequencies (millihertz to hertz). The shear modulus has a temperature sensitivity, ∂G/∂T, averaging −0.017 GPa/K. Increasing temperature above 500 °C results in more intensive shear attenuation ( urn:x-wiley:grl:media:grl58579:grl58579-math-0001) and associated modulus dispersion, with urn:x-wiley:grl:media:grl58579:grl58579-math-0002 increasing monotonically with increasing oscillation period and temperature. This “background” relaxation is adequately captured by a Burgers model for viscoelasticity and possibly results from intergranular mechanisms. Attenuation is higher in antigorite ( urn:x-wiley:grl:media:grl58579:grl58579-math-0003 at 550 °C and 0.01 Hz) than in olivine ( urn:x-wiley:grl:media:grl58579:grl58579-math-0004 below 800 °C), but such contrast does not appear to be strong enough to allow robust identification of antigorite from seismic models of attenuation only

Insight into the microphysics of antigorite deformation from spherical nanoindentation.

The mechanical behaviour of antigorite strongly influences the strength and deformation of the subduction interface. Although there is microstructural evidence elucidating the nature of brittle deformation at low pressures, there is often conflicting evidence regarding the potential for plastic deformation in the ductile regime at higher pressures. Here, we present a series of spherical nanoindentation experiments on aggregates of natural antigorite. These experiments effectively investigate the single-crystal mechanical behaviour because the volume of deformed material is significantly smaller than the grain size. Individual indents reveal elastic loading followed by yield and strain hardening. The magnitude of the yield stress is a function of crystal orientation, with lower values associated with indents parallel to the basal plane. Unloading paths reveal more strain recovery than expected for purely elastic unloading. The magnitude of inelastic strain recovery is highest for indents parallel to the basal plane. We also imposed indents with cyclical loading paths, and observed strain energy dissipation during unloading-loading cycles conducted up to a fixed maximum indentation load and depth. The magnitude of this dissipated strain energy was highest for indents parallel to the basal plane. Subsequent scanning electron microscopy revealed surface impressions accommodated by shear cracks and a general lack of dislocation-induced lattice misorientation. Based on these observations, we suggest that antigorite deformation at high pressures is dominated by sliding on shear cracks. We develop a microphysical model that is able to quantitatively explain Young's modulus and dissipated strain energy data during cyclic loading experiments, based on either frictional or cohesive sliding of an array of cracks contained in the basal plane. This article is part of a discussion meeting issue 'Serpentinite in the earth system'.This work was supported by the Natural Environment Research Councilthrough grant no. NE/M016471/1 to L.N.H. and N.B., and by the European Research Councilunder the European Union’s Horizon 2020 research and innovation programme (project RockDEaF, grant agreement no. 804685)

On the scale dependence in the dynamics of frictional rupture: constant fracture energy versus size-dependent breakdown work

Potential energy stored during the inter-seismic period by tectonic loading around faults is released during earthquakes as radiated energy, heat and fracture energy. The latter is of first importance since it controls the nucleation, propagation and arrest of the seismic rupture. On one side, fracture energy estimated for natural earthquakes (breakdown work) shows a clear slip-dependence. On the other side, recent experimental studies highlighted that, fracture energy is a material property limited by an upper bound value corresponding to the fracture energy of the intact material independently of the size of the event. To reconcile these contradictory observations, we performed stick-slip experiments in a bi-axial shear configuration. We analyzed the fault weakening during frictional rupture by accessing to the near-fault stress-slip curve through strain gauge array. We first estimated fracture energy by comparing the measured strain with the theoretical predictions from Linear Elastic Fracture Mechanics and a Cohesive Zone Model. By comparing these values to the breakdown work obtained from the integration of the stress-slip curve, we show that, at the scale of our experiments, fault weakening is divided into two stages; the first one consistent with the estimated fracture energy, and a long-tailed weakening corresponding to a larger energy not localized at the rupture tip, increasing with slip. Through numerical simulations, we demonstrate that only the first weakening stage controls the rupture initiation and that the breakdown work induced by the long-tailed weakening can enhance slip during rupture propagation and allow the rupture to overcome stress heterogeneity along the fault. We conclude that the origin of the seismological estimates of breakdown work could be related to the energy dissipated in the long-tailed weakening rather than to the one dissipated near the tip