Laboratory and theoretical studies provide insight into the mechanisms that control earthquake nucleation, when fault slip velocity is slow (< 0 .001 cm/s), and dynamic rupture when fault slip rates exceed centimeters per second. The application of these results to tectonic faults requires information about fabric evolution with shear and its impact on the mode of faulting. Here we report on laboratory experiments that illuminate the evolution of shear fabric and its role in controlling the transition from stable sliding (v ~0.001 cm/s) to dynamic stick slip (v > 1 cm/s). The full range of fault slip modes was achieved by controlling the ratio K = k/kc, where k is the elastic loading stiffness and kc is the fault zone critical rheologic stiffness. We show that K controls the transition from slow-and-silent slip (K > 0.9) to fastand- audible (K < 0.7, v = 3 cm/s, slip duration 0.003 s) slip events. Microstructural observations show that with accumulated strain, deformation concentrates in shear zones containing sharp shear planes made of nanoscale grains, which favor the development of frictional instabilities. Once this fabric is established, fault fabric does not change significantly with slip velocity, and fault slip behavior is mainly controlled by the interplay between the rheological properties of the slipping planes and fault zone stiffness.
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