Velocity influence on the friction and wear of a single-asperity lab-fault

Understanding earthquakes mechanisms still represents a challenge, motivated by the large consequences of the numerous earthquakes occurring each year. The complexity of fault zones and fault behaviour requests to make some simplifications and to down-scale the studied system. In our work 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 behaviour of the down-scaled fault, especially when the velocity is changing. Moreover, mono-asperity experimental tests are an effective way to construct new friction laws for numerical simulations.

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. Under co-seismic conditions (contact size, contact normal stress) and with a rough track with presence of granular gouge, the lab-fault is submitted to different velocities (from 0.001 m/s up to 1m/s). A number of high-sampling-rate sensors are used to constrain the observation of the asperity-rough track contact during the simulated seismic events. Moreover, complete post-mortem analyses of the contact surfaces with optical microscopy, SEM and roughness images allow to quantify the mechanisms and to reconstruct friction scenarios in accordance with the time-series acquired during tests. A quasi 2D numerical twin is also created with the elementary discrete method software MELODY, in order to compare the different features observed on the pin or on the track.

In this present work, we focus on the change of wear mechanisms in the lab-fault due to changes in sliding velocity. Independently of the normal load applied, the Carrara white marble asperity-track system experiences weakening velocity due to frictional heating. The friction coefficient evolution during the co-seismic events and the post-mortem analyses put in contrast two regimes. At low velocity, the carbonate rock lab fault wears off at a constant rate producing a large amount of granular gouge. At high velocity, the contact surfaces are covered by viscous sintered material, which can be called mirror surfaces.