Frictional Resistance within the Wake of Frictional Rupture Fronts

Frictional resistance to slip, tau, is determined by the real area of contact, A, and the shear strength of the contacts forming the frictional interface. We perform simultaneous high-speed local measurements of t and A at the tail of propagating rupture fronts. Rate dependence is investigated over 2 orders of magnitude of local slip velocities which reach up to similar to 1 m/s. A critical slip velocity is observed that signifies a transition in the frictional behavior: enhanced velocity weakening of A and t. These measurements enable us to infer the contact shear strength, an otherwise elusive quantity, and show that the contact shear strength persistently increases with slip rate. This, surprisingly, contrasts with expected contact softening at the high temperatures induced by rapid sliding.