Auto-acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration

Granular shear flows are intrinsic to many geophysical processes, ranging from landslides and debris flows to earthquake rupture on gouge-filled faults. The rheology of a granular flow depends strongly on the boundary conditions and shear rate. Earthquake rupture involves a transition from quasi-static to rapid shear rates. Understanding the processes controlling the transitional rheology is potentially crucial for understanding the rupture process and the coseismic strength of faults. Here we explore the transition experimentally using a commercial torsional rheometer. We measure the thickness of a steady shear flow at velocities between 10(-3) and 10(2) cm/s, at very low normal stress (7 kPa), and observe that thickness is reduced at intermediate velocities (0.1-10 cm/s) for angular particles, but not for smooth glass beads. The maximum reduction in thickness is on the order of 10% of the active shear zone thickness, and scales with the amplitude of shear-generated acoustic vibration. By examining the response to externally applied vibration, we show that the thinning reflects a feedback between internally generated acoustic vibration and granular rheology. We link this phenomenon to acoustic compaction of a dilated granular medium, and formulate an empirical model for the steady state thickness of a shear-zone in which shear-induced dilatation is balanced by a newly identified mechanism we call auto-acoustic compaction. This mechanism is activated when the acoustic pressure is on the order of the confining pressure, and results in a velocity-weakening granular flow regime at shear rates four orders of magnitude below those previously associated with the transition out of quasi-static granular flow. Although the micromechanics of granular deformation may change with greater normal stress, auto-acoustic compaction should influence the rheology of angular fault gouge at higher stresses, as long as the gouge has nonzero porosity during shear.