Velocityweakening

Velocity-weakening refers to a constitutive behavior of frictional interfaces in which the shear resistance decreases as slip velocity increases, over a range of velocities. This contrasts with velocity-strengthening, where friction rises with velocity. Velocity-weakening is a key ingredient in models of fault slip and earthquake rupture because it can produce stick-slip instability: as a fault accumulates shear stress, any small slip that increases velocity reduces friction, causing acceleration and transient slips that can grow into earthquakes.

In rock friction experiments and rate-and-state formulations, velocity-weakening occurs under conditions of intermediate to high temperature, low to moderate confining pressure, and depending on mineralogy and fault gouge. Mechanisms include flash heating at asperities, frictional melting, thermal pressurization of pore fluid, and wear-material evolution, which reduce shear strength with faster sliding.

Rate-and-state friction models describe friction mu as a function of slip rate V and a state variable theta. In the velocity-weakening branch, mu decreases with increasing V, potentially producing unstable slip if the weakening rate surpasses any velocity-strengthening stabilizing effects. The critical slip distance Dc and the balance of timescales determine whether slip grows into an earthquake or remains creeping.

Implications: velocity-weakening provides a plausible mechanism for earthquake nucleation and rupture propagation along faults. It is not universal; some faults or portions exhibit velocity-strengthening behavior, enforcing stable sliding. Real faults are influenced by fluid pressure, thermal effects, healing and aging, and material heterogeneity, complicating simple classifications.