Stick-slip failure in granular experiments: the role of low-frequency vibrational modes

Abstract:

The intermittent nature of strain at geological faults is a consequence of the loss of rigidity of the gouge. Due to the granular nature of the gouge, we take insight from granular physics in order to characterize the role of gouge in the generation of seismic events. In static granular packings, the loss of rigidity has been observed to be associated with an excess in low-frequency vibrational modes. The same holds true in both molecular and colloidal glasses, suggesting that excess low-frequency vibrational modes are a general feature in the failure of disordered solids. Therefore, we seek a relationship between these low-frequency modes and spatiotemporal patterns of failure by studying an experimental analog to a geological fault. Specifically, we measure the acoustic emissions from jammed, quasi-2D granular packings under shear. We use a split-bottom annular shear geometry, driven with a linear-ramp torque in order to generate stick-slip behavior, and observe intervals of both periodic and aperiodic slip. We investigate how low-frequency emissions are associated with both local and global slip events of various magnitudes and directions. Twelve piezoelectric acoustic receivers, an imaging system, and high-resolution stress/strain sensors enable direct comparison of acoustic properties as the system approaches failure. The use of photoelastic grains enables local measurement of changes to the internal stresses.