Two blocks in frictional contact, are supported by a multitude of microscopic contacts whose real contact area, A, is much smaller than the nominal one. We measure the spatial and temporal behavior of A along a rough spatially extended interface. Using high speed imaging we record, both slow nucleation processes as well as rapid, crack like events that eventually lead to overall sliding of an entire frictional interface. We report that a discrete sequence of crack-like precursors propagating within the interface is excited by a slowly increasing shear force applied to the blocks, when the shear force is applied to a single edge of the sample. The precursors are triggered at shear stresses well bellow those usually associated with the static friction coefficient. They increase systematically in length and significantly redistribute the real area of contact. Thus when the critical shear force for sliding is reached, the initially uniform contact area along the interface has already evolved to one that is highly non-uniform in space. These results suggest a fundamentally new view of the processes leading to frictional motion with ramifications to earthquake dynamics and material failure.

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