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Large earthquakes take place on mature faults with hundreds of meters to kilometres of cumulative slip. At shallow depths, the fault zone is generally composed of non-cohesive rock wear products, often referred to as gouge. Seismic and aseismic slip occur in this fault gouge and fracture/brecciation of the wall rock and damage zone can add to the fault gouge as part of the wear process. Gouge thickness generally increases linearly with the cumulative fault shear displacement and laboratory work shows that gouge tends to stabilize fault frictional stability. Previous works show that frictional stability of simulated fault gouge varies as a function of shear displacement. The stability evolution is interpreted as a consequence of the degree of shear localisation within the simulated fault gouge: the more the deformation is localized, the more the fault slip is unstable. This implies that for bare rock surfaces, unstable behaviour is expected as the deformations are forced to be localized at the interface between the two sheared surfaces.On natural faults at large shear displacement (or for faults having a high gouge production rate), a competition must take place between 1) the localization of the deformation at rock-on-rock surfaces, 2) the delocalization of deformation due to gouge production and wall rock brecciation, 3) fault zone lithification and frictional healing and 4) shear localization within the gouge and wear material. The competition and interaction between these phenomena are modulated by cumulative fault slip, temperature and fluid chemistry. In turn, this competition may influence the frictional stability of faults with increasing shear displacement, and thus, their potential seismic activity.To characterise the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to large displacement. Two initially intact rocks were chosen as starting material: a high porosity Fontainebleau sandstone and a low porosity quartzite. These samples represent very different resistances to abrasion (i.e., wear production with slip) for the same initial mineral composition (< 95% quartz), which allows us to investigate wear and wear rate on fault stability. Additionally, simulated quartz gouge was tested for comparison. Mechanical data are analysed within the rate-and-state framework, and post-mortem microscopic analyses of the sample were performed. For initially bare surface experiments a threshold shear displacement is required to transition from stable to unstable sliding. Stick-slip events (laboratory earthquakes) evolve systematically as a function of fault zone shear displacement. The inversion of the rate-and-state parameters shows that shear displacement has a dominant influence on both (a-b) and Dc. For all the faults tested, (a-b) decreases with increasing shear displacement. For high wear rates and simulated gouge, Dc decreases with increasing shear displacement. However, for low wear rate faults, Dc is constant within the tested shear displacement. These results demonstrate that, under the tested boundary conditions, fault stability varies systematically with fault maturity and in particular that shear displacement and strain localization are the dominant parameters controlling fault slip stability.

Dynamic changes in the stress field during the seismic cycle of tectonic faults can control frictional stability and the mode of fault slip. Small perturbation in the stress field, like those produced by tidal stresses can influence the evolution of frictional strength and fault stability with the potential of triggering a variety of slip behaviors. However, an open question that remains still poorly understood is how amplitude and frequency of stress changes influence the triggering of an instability and the associated slip behavior, i.e. slow or fast slip.Here we reproduce in the laboratory the spectrum of fault slip behaviors, from slow-slip to dynamic stick-slip, by matching the critical fault rheologic stiffness (kc) with the surrounding stiffness (k). We investigate the influence of normal stress variations on the slip style of a quartz rich fault gouge at the stability boundary, i.e. k/kc slightly less than one, by adopting two techniques: 1) instantaneous step-like changes and 2) sinusoidal variations in normal stress. For the latter case, modulations of normal stress were chosen to have amplitudes greater, less or equal to the typical stress drop observed during unperturbed experiments. Also, the period was varied to be greater, less or equal to the typical recurrence time of laboratory slow-slip events. During the experiments, we continuously record ultrasonic wave velocity to monitor the microphysical state of the fault. We find that frictional stability is profoundly affected by variation in normal stress giving rise to a variety of slip behaviors. Furthermore, during strain accumulation and fabric development, changes in normal stress permanently influence the microphysical state of the fault gouge increasing kc and producing a switch from slow to fast stick-slip. Our results indicate that perturbations in the stress state can trigger a variety of slip behaviors along the same fault patch. These results have important implications for the formulation of constitutive laws in the framework of rate- and state- friction, highlighting the necessity to account for the microphysical state of the fault in order to improve our understanding of frictional stability.

Despite the numerous advantages of storing CO2 into basalts by dissolving carbon dioxide into water prior to its injection, the large amount of H2O required for this operation poses an increased risk of fluid overpressure into the fault/fracture networks, and renders the seismicity analysis pivotal to upscale this storage method to voluminous basaltic occurrences diffused worldwide.To deepen our knowledge on the frictional strength, stability, and the healing properties of basalt-built faults, we carried out friction tests on basalts from Mt. Etna using the biaxial deformation machine, BRAVA, and the rotary-shear apparatus, SHIVA (HP-HT laboratory of INGV-Rome, Italy). Specimens were selected for their relative abundance of olivine and pyroxene crystals, i.e. the main sources of divalent cations in silicate rocks necessary to trap CO2 into basalts.Experiments were performed both on synthetic powdered samples and bare surfaces, at room-dry and water drained-saturated conditions, at room temperature and pressure. Bare surfaces consisted in basalt slabs and hollowed cylinders, which were mounted on BRAVA and SHIVA apparatus, respectively. Samples were subjected to 5 to 30 MPa normal stress (σn) for powdered samples and in the range 5 to 10 MPa for bare surfaces.At the investigated normal stresses, frictional sliding data obey Byerlee’s law for friction, with the friction coefficient µ = 0.59 – 0.78. Differences in μ mainly reflect sample variability, different experimental configurations, sample geometry, and, to a lesser extent, the boundary conditions (dry/wet). However, in detail, basalt slabs are generally characterized by the highest friction coefficient and hollow cylinders exhibit a slight increase in friction coefficient with increasing shear displacement, due to the progressive slip hardening resulting from gouge production during frictional sliding.Velocity step increases were conducted on BRAVA after steady values of friction were attained (~ 6.5-7.5 mm for gouge and ~ 3 mm slip for bare surfaces) and consisted in velocity sequences from 0.1 to 300 µm s-1, with ~ 500 μm displacement for each step. Rate-and-state friction experiments show opposite mechanical behavior between bare surfaces and synthetic fault gouge: while bare surfaces experience a transition from rate-weakening at low sliding velocity (V) to rate-strengthening behavior at higher V without any clear dependence on the applied σn, gouge revealed a negative trend of (a-b) with shear velocity at σn > 5 MPa and a velocity-weakening behavior at V ≥ 30 µm s-1, regardless of experimental conditions. We ascribe this different behavior to shear delocalization owing to frictional wear production in bare surfaces, and to shear localization accompanied by grain size reduction along the Riedel R1 and boundary B shear zones in fault gouges, as also confirmed by microstructural analysis.The velocity weakening behavior of fault gouge, coupled with the fast healing rates retrieved from slide-hold-slide experiments (500 µm displacement cumulated at V = 10 µm/s followed by hold times from 30 to 3000 s), define high strength zones that are potentially seismogenic. Conversely, velocity strengthening behavior of bare surfaces promotes aseismic creep at V ≥ 100 µm s-1, regardless of the faster restrengthening compared to fault gouge.

Giant rockslides creep for centuries and then can fail catastrophically posing major threats to society. There is growing evidence that creeping landslides are widespread worldwide and extremely sensitive to hydrological forcing, especially in climate change scenarios. Rockslide creep is the results of progressive rock failure processes, leading to rock damage accumulation, permeability enhancement and strain localization within basal shear zones similar to tectonic faults. As shear zone accumulate strain, they become thicker and less permeable, favoring the development of perched aquifers. Since then, the creep behavior of mature rockslides becomes dominated by hydro-mechanical interaction with external triggers, e.g. rainfall and snowmelt. However, the mechanisms regulating the slow-to-fast transition toward their catastrophic collapse remain elusive, and statistical and simplified mathematical models used for collapse prediction are usually unable to account for the full spectrum of observed slip behaviors.Here we couple laboratory experiments on natural rockslide shear zone material, sampled from high quality drillcores, and in situ observations (groundwater level and surface displacement) to investigate the mechanism of rockslide response to short-term pore pressure variations within basal shear zones at the Spriana rockslide (Italy). Using a biaxial apparatus within a pressure vessel, we characterized the strength and permeability of the phyllosilicate-rich shear zone material at in situ stress, as well as the rate and state frictional properties for shear rates typical of the slow-to-fast transition of real rockslides. Then we carried out non-conventional pore pressure-step creep experiments, in which shear stress is maintained at subcritical levels and pore pressure is increased stepwise while monitoring shear zone slip and dilatancy until runaway failure.Our results, that are quantitatively consistent with in situ monitoring observations, provide a scale-independent demonstration that short-term pore pressure variations originate a full spectrum of creep styles, modulated by slip-induced undrained conditions. Shear zones respond to fluid pressure increments by impulsive acceleration and dilatancy, causing spontaneous deceleration followed by sustained steady-rate creep. Increasing fluid pressure results in high creep rates and eventual collapse. Laboratory experiments quantitatively capture the in situ behavior of giant rockslides, providing physically-based basis to improve forecasting models for giant mature rockslides in crystalline rocks.