Influence of Viscous Lubricant on Nucleation and Propagation of Frictional Ruptures

Fluids are pervasive in the Earth's crust and saturate fractures and faults. The combination of fluids and gouge layers developing along faults can generate fluids of different viscosities. Such viscous fluids were found to influence the reactivation, frictional stability of faults, and eventually the dynamics of propagating earthquake ruptures. We reproduced laboratory earthquakes on analog material (PMMA) to study the influence of viscous lubricant on fault frictional stability, rupture nucleation, and propagation under mixed lubrication conditions. Experiments were conducted in dry conditions, and with fluids presenting a viscosity ranging from 1 to 1,000 mPa.s. Through photoelasticity, high‐frequency strain gauge sensors, and accelerometer measurements, we obtained new insights about the influence of lubricant on a characteristic nucleation length, rupture propagation velocity, and local slip and slip rate evolution during the reproduced frictional ruptures. Our experiments show that the presence of a lubricant generating mixed lubricated conditions along the fault induces, (a) a reduction of the frictional resistance, (b) an increase in nucleation length, (c) a decrease in the fracture energy. In addition, the larger the viscosity of the fluids, the larger the reduction of frictional strength and the increase in the nucleation length. Moreover, ruptures occurring under mixed lubricated conditions showed a pulse‐like rather than crack‐like behavior, suggesting that viscous lubrication can induce the transition from crack‐like to pulse‐like rupture along natural faults. We demonstrate, supported by existing theory, that this transition is mainly governed by the stress acting on the fault at the onset of nucleation, which is drastically reduced in presence of a lubricant. Fluids naturally permeating the Earth's crust or being injected for reservoir stimulation can promote fault reactivation, resulting in natural or so‐called induced earthquakes. It is important to comprehend how the presence of these fluids, as well as their properties, affect the activation and magnitude of seismic events. In particular, we investigated the role of fluid viscosity under controlled experimental conditions through laboratory experiments on analog material. We show that for the same applied normal stress, faults under lubricated conditions (i.e., where a thin film of viscous lubricant had been spread) can be activated at much lower shear stresses than dry faults. At the same time, the generated events will be much smaller in magnitude (i.e., smaller stress drop) and propagate more stably. Frictional strength of lubricated artificial interfaces is dramatically reduced under mixed lubrication conditionsNucleation length of rupture under lubricated conditions is larger than the one of ruptures under dry conditionsViscous lubrication (under mixed conditions) can affect the nature of ruptures that would propagated pulse‐like rather than crack‐like Frictional strength of lubricated artificial interfaces is dramatically reduced under mixed lubrication conditions Nucleation length of rupture under lubricated conditions is larger than the one of ruptures under dry conditions Viscous lubrication (under mixed conditions) can affect the nature of ruptures that would propagated pulse‐like rather than crack‐like