Asperities in sliding surfaces as frictional bridges. Negative friction

The sliding surfaces of faults and shear fractures in rocks are rarely smooth. They possess local undulations or asperities that act as distributed points of sliding disturbance. These asperities work as bridges connecting the opposite sliding surfaces and creating forces that non-linearly depend on deformation. These forces can even act in the direction of sliding. The mechanism of generating these forces is the local dilation caused by the fact that the sliding makes some asperities of one surface climb over the asperities of the opposite surface in the presence of the external compressive stress. Therefore, the dilation creates an effective friction angle which is the original (microscopic) friction angle controlling friction between the surfaces of the opposite asperities plus the inclination angle between the tangent line to the asperity surfaces at the point of contact and the direction of the movement. The inclination angle depends upon the position of the contact between the asperities making the effective friction angle non-linearly dependent upon the displacement. After passing the point of maximum dilation, the inclination angle becomes negative. Therefore, depending upon the surface friction angle, the effective friction angle can become negative. This creates the effect of local negative stiffness, where the energy is supplied by the convergence of the opposite faces of the fault/fracture (after the top of the contacting asperity is passed) in the presence of the external pressure.

 

The development of the zones of negative friction which are the zones of energy release can be accompanied by microseismisity (one zone - one signal) generated without involving local fracturing. Understanding this mechanism will contribute to refining of the existing monitoring methods of faults/fracture movement.

Acknowledgement. The authors acknowledge support from the Australian Research Council through project DP190103260.