Microstructural and Frictional Consequences of Slip Velocity Variations: Insights from a single-asperity lab-fault experiment

Understanding earthquakes mechanisms still represents a challenge. The complexity of both the fault zone and the fault behaviour requires to make some simplifications and to downscale the studied system.

 In our work, we aim at creating a down-scaled experimental fault model where the behaviour of the asperities and the shearing of the granular gouge are both considered. In order to do so, we borrow from the tribological approach the pin-on-disk experiment: the original experimental apparatus consists in a centimetric pin with a hemispherical extremity representing the fault asperity while a large flat rotating disk stands for the opposite surface of the experimental fault. Both parts are made in the same carbonate rock with controlled roughness. Co-seismic conditions (Velocity in the range [0.001 – 1]  m/s, Normal stress in the range [4 – 400] MPa) are applied during the different experimental tests. A number of high-sampling-rate sensors are used to constrain the observation of the asperity-track contact during the simulated seismic events. Moreover, complete post-mortem analyses of the contact surfaces allow to quantify the mechanisms and to reconstruct friction scenarios in accordance with the time-series acquired during tests.

In a previous work, we determined the conditions most representative for a mature lab-fault. In the present study, we focus on the changes in wear and friction behaviours in the lab-fault linked to the slip velocity variations and the presence of granular gouge. Wear is mostly dependent on the slip velocity and the granular gouge layer thickness obtained at the mature conditions appears as an optimal thickness to limit wear. Here, velocity weakening is observed, with dramatic consequences on the microstructure of the contact surfaces. SEM and optical images show evidences of the combination of high stresses and heating on the first layer of minerals of the contact zone.