Effect of partial asymmetric friction on sliding over fractures

Asymmetric friction refers to different friction forces resisting sliding in opposite directions. This property is found to arise from material anisotropy that can form coupling between normal and shear stress. It is plausible that such material anisotropy can emerge as a result of fractures of asperities or micro-cracks that precedes shear fractures. This study considers a model with multiple blocks where some blocks have asymmetric friction, while others have conventional symmetric friction. This assembly is driven by an external surface that connects to each block through leaf springs in the spirit of the Burrige-Knoppof model. The driving force is considered to undergo harmonic oscillations. It is found that asymmetric friction can induce intermittent regions of tension and compression that can form fractures normal (tension) or parallel (compression) to the sliding surface. Asymmetric friction produces velocity spectra with a frequency falloff of 1/f², where f is the frequency; these are observed in sliding over shear fracture produced by strain localisation in compression. Opposite to this, symmetric friction leads to the falloff of 1/f observed in sliding over pre-existing fractures or faults. This distinguishes sliding over pre-existing faults from sliding over fractures formed in compression. This model can shed light on the mechanisms of fault sliding and production of geological fractures observed near the faults, as well as to complement the micromechanics-based asperity models in the emerging fields such as surface morphing.