Influence of viscous fluids in the nucleation of spontaneous frictional ruptures.

Studying the nucleation phase of earthquakes is of great importance for attempting a prediction of earthquake motions. Despite the difficulties linked to the observation of the nucleation in laboratory experiments, it was showed how such complex process can be described by a first quasi-static phase (aseismic slip) followed by an acceleration phase and subsequent dynamic rupture propagation. However, these observations were found in dry conditions. Given the systematic presence of fluids in the Earth crust, as well as the increase of injection-induced seismicity, it is important to study their influence on such nucleation processes, in terms of nucleation length () and acceleration profiles.

Here we present results of laboratory stick-slip experiments performed under dry and lubricated conditions. In our experiments two PMMA samples were loaded in a direct shear configuration, with applied normal load ranging between 1.75 MPa and 3.5 MPa, and an increasing shear load leading to spontaneous ruptures nucleation. Fluids with different viscosities (mixtures of glycerol and water in different proportions; water, 60% glycerol - 40% water, 85% glycerol - 15% water, 99% glycerol) were distributed on the fault interface to study their influence in the nucleation and propagation processes. The rupture evolution was captured by strain gauges (acquiring at high frequency ~2 MHz) located along the fault, coupled with a high-speed camera, allowing high resolution photoelasticity measurements.

Our experiments highlight that (i) faults under mixed lubricated conditions show lower peak strength than dry faults (with higher viscosity leading to lower peak strength values), (ii) nucleation length increases with lubricant and with lubricant viscosity, (iii) viscous lubricant influences the rupture mode; rupture nucleating under dry conditions propagate in a crack-like manner while the ones nucleating under lubricated conditions propagate in a pulse-like manner.