Decades of seismological observations have highlighted the variability of foreshock occurrence prior to natural earthquakes, making thus difficult to track how earthquakes start. Here, we report on three stick-slip experiments performed on cylindrical samples of Indian metagabbro under upper crustal stress conditions (30-60 MPa). Acoustic emissions (AEs) were continuously recorded by 8 calibrated acoustic sensors during the experiments. Seismological parameters of the detected AEs (-8.8 <= Mw <= -7 ) follow the scaling law between moment magnitude and corner frequency that characterizes natural earthquakes. AE activity always increases towards failure and is found to be driven by along fault slip velocity. The stacked AE foreshock sequences follow an inverse power-law of the time to failure (inverse Omori), with a characteristic Omori time c inversely proportional to normal stress and nucleation length. AEs moment magnitudes increase towards failure, as manifested by a decrease in b-value from ~ 1 to ~ 0.5 at the end of the nucleation process. During nucleation, the averaged distance of foreshocks to mainshock continuously decreases, highlighting the fast migration of foreshocks towards the mainshock epicenter location, and stabilizing at a distance from the latter compatible with the predicted Rate-and-State nucleation size. Finally, the seismic component of the nucleation phase is orders of magnitude smaller than that of its aseismic component, which suggests that foreshocks are the byproducts of a process almost fully aseismic. Seismic/aseismic energy release ratio continuously increases during nucleation, which starts as a fully aseismic process and evolves towards a cascading process.

We propose a numerical model of laboratory earthquake cycle inspired by a set of experiments performed on a triaxial apparatus on sawcut Carrara marble samples. The model couples two representations of rock matter: rock is essentially represented as an elastic continuum, except in the vicinity of the sliding interface, where a discrete representation is employed. This allows to simulate in a single framework the storage and release of strain energy in the bulk of the sample and in the loading system, the damage of rock due to sliding, and the progressive production of a granular gouge layer in the interface. After independent calibration, we find that the tribosystem spontaneously evolves towards a stick-slip sliding regime, mimicking in a satisfactory way the behaviour observed in the lab. The model offers insights on complex phenomena which are out of reach in experiments. This includes the variability in space and time of the fields of stress and effective friction along the fault, the progressive thickening of the damaged region of rock around the interface, and the build-up of a granular layer of gouge accommodating shear. We present in detail several typical sliding events, we illustrate the fault heterogeneity, and we analyse quantitatively the damage rate in the numerical samples. Some limitations of the model are pointed out, as well as ideas of future improvements, and several research directions are proposed in order to further explore the large numerical dataset produced by these simulations.

Small transient stress perturbations are prone to trigger (micro)seismicity. In the Earth’s crust, these stress perturbations can be caused by various sources such as the passage of seismic waves, forcing by tides, or hydrological seasonal loads. A better understanding of the dynamic of earthquake triggering by stress perturbations is essential in order to improve our understanding of earthquake physics and our consideration of seismic hazard. Here, we study an experimental sandstone-gouge-filled fault system undergoing combined far field loading and periodic stress perturbations (of variable amplitude and frequency) at crustal pressure conditions. Microseismicity — in the form of acoustic emissions (AE) — strains, and stresses, are continuously recorded in order to study the response of microseismicity as a function of loading rate, amplitude and frequency of a periodic stress perturbation. The observed AE distributions do not follow the predictions of a Coulomb failure model taking into account both constant loading and oscillation-induced strain rates. A susceptibility of the system’s AE response to confinement pressure amplitude is estimated, which showcases a linear relation between confinement pressure amplitude and the AE response amplitude, observations which agree with recent higher frequency experimental results on dynamic triggering. The magnitude-frequency distribution of AEs is also computed. Oscillations in Gutenberg-Richter b-value are observed in experiment catalogues but are not quantified. Our experiments may help complement our understanding of the influence of low inertia stress phenomena on the distribution of seismicity, such as observations of dynamic triggering and seismicity modulation by solid earth tides or seasonal loading.

During an earthquake, fault slip weakening is often explained by frictional heating phenomena, generally promoting melt production on the fault surface. Here, we investigate the influence of melt production at the scale of the grains composing a fault gouge. We use a modern version of Discrete Element Modelling (DEM) able to deal with realistic grain shapes, and couple it with a Multibody Meshfree Approach able to provide a satisfactory proxy for the mechanical behaviour of molten grains. Frictional sliding of solid grains and viscous shearing of molten grains are monitored during simulations. Our results confirm the natural tendency of granular gouge to localize deformation in a thin layer, and thus to trigger local melt production. We also show that the appearance of melt is likely to enhance this localization, and might create a positive feedback to its own production. We propose guidelines for the future writing of a friction model inspired by these simulation results.