HighSTEPS. A high strain temperature pèressure and speed apparatus to study earthquake mechanics

We present a state of-the-art biaxial apparatus able to study both earthquake rupture nucleation and propagation at conditions typical of the seismogenic crust. The HighSTEPS, High Strain TEmperature Pressure Speed, apparatus simulates fault deformation in a wide range of slip velocities, i.e., from 10-5m/s to 0.25 m/s. Within this velocity range, it is possible to study, the rate-and-state friction, the fault dynamic weakening, and healing under unique boundary conditions, i.e., normal stress up to 100 MPa, confining pressure up to 100 MPa, pore fluid pressure up to 100 MPa and temperature up to 120 °C. The apparatus consists of a hydraulic system integrated with four linear motors. The hydraulic system allows for the application of normal stress, confining pressure and pore fluid pressure. The main peculiarity of this apparatus is the system of four linear motors that are mounted in series in order to apply shearing velocities up to 0.25 m/s, accelerations up to 10 m/s2 and shear stresses up to 200 MPa. Moreover, both experiments in sliding velocity control or shear stress control on the experimental faults are possible. Preliminary experiments on carbonate and silicate bearing rocks are coherent with the previous literature. The investigation of fault friction under a wide range of velocities, normal stresses, confining pressures and pore fluid pressures will provide insights into the mechanics of earthquakes and reduce the gap between natural and laboratory observations

Effect of glass on the frictional behavior of basalts at seismic slip rates

We performed 31 friction experiments on glassy basalts (GB) and glass-free basalts (GFB) at slip rates up to 6.5 m s−1 and normal stress up to 40 MPa (seismic conditions). Frictional weakening was associated to bulk frictional melting and lubrication. The weakening distance (Dw) was about 3 times shorter in GB than in GFB, but the steady state friction was systematically higher in GB than in GFB. The shorter Dw in GB may be explained by the thermal softening occurring at the glass transition temperature (Tg ~500°C), which is lower than the bulk melting temperature (Tm ~1250°C) of GFB. Postexperiment microanalyses suggest that the larger crystal fraction measured in GB melts results in the higher steady state friction value compared to the GFB melts. The effect of interstitial glass is to facilitate frictional instability and rupture propagation in GB with respect to GFB

Ductile flow in sub-volcanic carbonate basement as the main control for edifice stability:new experimental insights

Limestone in volcanic basements has been identified as a hazard in terms of edifice stability due to the propensity of calcite to decompose into lime and CO2 at high temperatures (>600 °C), causing a decrease in mechanical strength. To date, such hypotheses have been tested by experiments performed at ambient pressure. The present work determines the mechanical strength of limestone under sub-volcanic conditions of pressure and temperature and evaluates the effect of calcite decomposition. To this end, we use Mt. Etna as a case study, deforming sub-Etnean carbonate samples under triaxial compression using a Paterson deformation apparatus. We evaluate the effect of thermal decomposition of calcite on sample strength by comparing closed and open systems and measuring the permeability evolution under static conditions. Mechanical and micro-structural observations at a constant strain rate of 10-5 s-1 and at a confining pressure of 50 MPa indicate that the rocks are brittle up to and including 300 °C. At higher temperatures the deformation becomes macroscopically ductile, i.e., deformation is distributed throughout the sample. The brittle to ductile transition is accompanied by an irreversible permeability decrease from 10-17 to 10-19 m2 between 200 and 600 °C. We present new evidence that permanent change in permeability is due to ductile processes closing the initial pore space. Samples deformed at temperatures up to 900 °C do not contain any decarbonation products. At these temperatures, permeability is sufficiently low to permit CO2 pore pressures to increase, thereby increasing local CO2 fugacity, which in turn strongly limits the decarbonation reaction. We note that, for non-pure calcite rocks, permeability might be sufficient to allow decarbonation reactions to occur. As such, variability in lithologies may slightly influence the efficiency of decarbonation reactions. We conclude that, in a closed system, the instability of Mt. Etna is related to high temperature induced ductile flow of basement limestone rather than chemical/mineralogical changes. This may have important implication for the stability of volcanoes within carbonate-rich basement, as carbonates become significantly weak at high temperatures, which may increase the risk of sector collapse. © 2015 Elsevier B.V

Thermal weakening friction during seismic slip experiments and models with heat sources and sinks

Experiments that systematically explore rock friction under crustal earthquake conditions reveal that faults undergo abrupt dynamic weakening. Processes related to heating and weakening of fault surfaces have been invoked to explain pronounced velocity weakening. Both contact asperity temperature Ta and background temperature T of the slip zone evolve significantly during high-velocity slip due to heat sources (frictional work), heat sinks (e.g., latent heat of decomposition processes), and diffusion. Using carefully calibrated High-Velocity Rotary Friction experiments, we test the compatibility of thermal weakening models: (1) a model of friction based only on T in an extremely simplified, Arrhenius-like thermal dependence; (2) a flash heating model which accounts for the evolution of both V and T; (3) same but including heat sinks in the thermal balance; and (4) same but including the thermal dependence of diffusivity and heat capacity. All models reflect the experimental results but model (1) results in unrealistically low temperatures and model (2) reproduces the restrengthening phase only by modifying the parameters for each experimental condition. The presence of dissipative heat sinks in stage (3) significantly affects T and reflects on the friction, allowing a better joint fit of the initial weakening and final strength recovery across a range of experiments. Temperature is significantly altered by thermal dependence of (4). However, similar results can be obtained by (3) and (4) by adjusting the energy sinks. To compute temperature in this type of problem, we compare the efficiency of three different numerical approximations (finite difference, wavenumber summation, and discrete integral)

Effect of glass on the frictional behavior of basalts at seismic slip rates

We performed 31 friction experiments on glassy basalts (GB) and glass-free basalts (GFB) at slip rates up to 6.5 m s−1 and normal stress up to 40 MPa (seismic conditions). Frictional weakening was associated to bulk frictional melting and lubrication. The weakening distance (Dw) was about 3 times shorter in GB than in GFB, but the steady state friction was systematically higher in GB than in GFB. The shorter Dw in GB may be explained by the thermal softening occurring at the glass transition temperature (Tg ~500°C), which is lower than the bulk melting temperature (Tm ~1250°C) of GFB. Postexperiment microanalyses suggest that the larger crystal fraction measured in GB melts results in the higher steady state friction value compared to the GFB melts. The effect of interstitial glass is to facilitate frictional instability and rupture propagation in GB with respect to GFB

Effect of Fluid Viscosity on Earthquake Nucleation

Injection of fluids in geo-reservoirs can reduce the effective stresses at depth, lubricating the nearby faults, promoting slip and, potentially, earthquakes. High-viscous fluids are often used during hydraulic fracturing and production phases in geo-reservoirs. Here, we performed dedicated experiments to study the influence of fluid viscosity on earthquake nucleation. We performed frictional sliding experiments at 30 and 50 effective normal stresses and fluids viscosity ranging from 1 to 1,226 mPa s and modeled them with a rate-and-state friction law. In the presence of fluid, the state variable is defined as the ability of the fluid to flow. Our results showed that static friction slightly decreases with increasing viscosity, the dynamic friction is governed by the dimensionless Sommerfeld number (S = 6 eta VL/(sigma'H-n(2))). Moreover, we observed that the (a - b) parameters of the rate-and-state friction law decrease with increasing viscosity down to (a - b) < 0, possibly promoting unstable slip and earthquake nucleation. Plain Language Summary In the last 30 years, the exponential worldwide increase of human-induced seismicity has become an important issue in solid earth sciences. Most of the induced seismicity is due to engineering operations in deep geo-reservoirs for hydrocarbon production, CO2 storage, wastewater disposal, and the exploitation of geothermal resources. While the reactivation of faults at the origin of this seismicity has been extensively studied, the influence of fluid properties including its viscosity has been overlooked, even if the viscosity of injected fluids spans from that of water to that of honey. In this study, we discuss the influence of fluid viscosity on the nucleation of earthquakes in fluid-permeated experimental faults and on induced earthquakes. Our experimental observations suggest that the viscosity of the fluid does not influence the fault strength. Instead, the viscosity of the fluid controls the behavior of the fault from stable to unstable sliding

Porosity evolution at the brittle-ductile transition in the continental crust: Implications for deep hydro-geothermal circulation

International audienceRecently, projects have been proposed to engineer deep geothermal reservoirs in the ductile crust. To examine their feasibility, we performed high-temperature (up to 1000 °C), high-pressure (130 MPa) triaxial experiments on granite (initially-intact and shock-cooled samples) in which we measured the evolution of porosity during deformation. Mechanical data and post-mortem microstuctural characterisation (X-ray computed tomography and scanning electron microscopy) indicate that (1) the failure mode was brittle up to 900 °C (shear fracture formation) but ductile at 1000 °C (no strain localisation); (2) only deformation up to 800 °C was dilatant; (3) deformation at 900 °C was brittle but associated with net compaction due to an increase in the efficiency of crystal plastic processes; (4) ductile deformation at 1000 °C was compactant; (5) thermally-shocking the granite did not influence strength or failure mode. Our data show that, while brittle behaviour increases porosity, porosity loss is associated with both ductile behaviour and transitional behaviour as the failure mode evolves from brittle to ductile. Extrapolating our data to geological strain rates suggests that the brittle-ductile transition occurs at a temperature of 400 ± 100 °C, and is associated with the limit of fluid circulation in the deep continental crust

Porosity and permeability reduction in conduit wall rock. A transition from localised cataclastic pore collapse to distributed viscous flow

Porosity and permeability reduction in conduit wall rock. A transition from localised cataclastic pore collapse to distributed viscous flow