Evolution from a clean surface to a mature gouge interface in a seismic fault – asperity system through the lens of pin-on-disk experiments

Understanding earthquakes mechanisms still represents a challenge, motivated by the large consequences of the numerous earthquakes occurring each year. A number of uncertainties remain concerning the complexity of the fault structure, the constitutive properties of materials or the fault rheology. To address those points, we borrow from the tribological approach the pin-on-disk experiment so that the two rough surfaces in contact through a series of asperities fault concept is downscaled to a single asperity sliding on a rough surface. The single asperity response to shearing induced by sliding and the evolution of friction are studied closely to understand the different stages undergoing by the asperity and the consequences on the fault behaviour during co-seismic events.

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 pieces are made in the same carbonate rock (Carrara white marble) with controlled roughness. The experimental downscaled fault is submitted to co-seismic conditions: contact size of 0.1-5 mm, contact normal stress of 10-200 MPa, sliding velocity of 0.01-1 m/s, and sliding distance of 10 - 60 m. A number of high-sampling-rate sensors are used to constrain the observation of the asperity contact during the simulated seismic events. Complete post-mortem analyses of the wear tracks with optical microscopy, SEM and roughness images allow to quantify the regime features and to reconstruct friction scenarios in accordance with the time-series acquired during tests.

Independently of the velocity and the normal load applied, the friction coefficient exhibits a clear transition between an idealized lab conditions regime and a mature interface with the formation of granular gouge, as a function of the sliding distance. Within the same regime (clean surface, intermediate, mature gouge), velocity weakening and hardening due to higher loading are pointed out. We propose to focus on the clean surface to mature gouge transition and on the stability of the mature gouge interface regime to address the fault rheology and the role of asperities in seismic weakening.