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[1] We investigate the physics of laboratory earthquake precursors in a biaxial shear configuration. We conduct laboratory experiments at room temperature and humidity in which we shear layers of glass beads under applied normal loads of 2-8 MPa and with shearing rates of 5-10 μm/s. We show that above~3 MPa load, acoustic emission (AE), and shear microfailure (microslip) precursors exhibit an exponential increase in rate of occurrence, culminating in stick-slip failure. Precursors take place where the material is in a critical state-still modestly dilating, yet while the macroscopic frictional strength is no longer increasing.

[1] Recent work in medical nonlinear acoustics has led to the development of refined experimental method to measure material elastic nonlinear (anelastic) response. The technique, termed dynamic acoustoelastic testing, has significant implications for the development of a physics-based theory because it provides information that existing methods cannot. It provides the means to dynamically study the velocity-strain and attenuation-strain relations through the full wave cycle in contrast to most methods that measure average response. The method relies on vibrating a sample at low frequency in order to cycle it through different levels of stress-strain. Simultaneously, an ultrasonic source applies pulses and the change in wave speed and attenuation as a function of the low frequency strain is measured. We report preliminary results in eleven room-dry rock samples. In crystalline rock, we expect that the elastic nonlinearity arises from the microcracks and dislocations contained within individual crystals. In contrast, sedimentary rocks may have other origins of elastic nonlinearity, currently under debate. A large quadratic elastic nonlinearity is observed in Berkeley blue granite, presumably due to microcracks and dislocation-point defect interactions. In sedimentary rocks that include limestones and sandstones we observe behaviors that can differ markedly from the granite, potentially indicating different mechanical mechanisms. We further observe changes in measured nonlinear coefficients that are wave-strain amplitude dependent. Ultimately we hope that the new approach will provide the means to quantitatively relate material nonlinear elastic behavior to the responsible features, that include soft bonds dislocations, microcracks, and the modulating influences of water content, temperature and pressure.

Among the most fascinating, recent discoveries in seismology are the phenomena of dynamically triggered fault slip, including earthquakes, tremor, slow and silent slip—during which little seismic energy is radiated—and low frequency earthquakes. Dynamic triggering refers to the initiation of fault slip by a transient deformation perturbation, most often in the form of passing seismic waves. Determining the frictional constitutive laws and the physical mechanism(s) governing triggered faulting is extremely challenging because slip nucleation depths for tectonic faults cannot be probed directly. Of the spectrum of slip behaviors, triggered slow slip is particularly difficult to characterize due to the absence of significant seismic radiation, implying mechanical conditions different from triggered earthquakes. Slow slip is often accompanied by nonvolcanic tremor in close spatial and temporal proximity. The causal relationship between them has implications for the properties and physics governing the fault slip behavior. We are characterizing the physical controls of triggered slow slip via laboratory experiments using sheared granular media to simulate fault gouge. Granular rock and glass beads are sheared under constant normal stress, while subjected to transient stress perturbation by acoustic waves. Here we describe experiments with glass beads, showing that slow and silent slip can be dynamically triggered on laboratory faults by ultrasonic waves. The laboratory triggering may take place during stable sliding (constant friction and slip velocity) and/or early in the slip cycle, during unstable sliding (stick‐slip). Experimental evidence indicates that the nonlinear‐dynamical response of the gouge material is responsible for the triggered slow slip.

[1] 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 À3 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.Citation: van der Elst, N. J., E. E. Brodsky, P.-Y. Le Bas, and P. A. Johnson (2012), Auto-acoustic compaction in steady shear flows: Experimental evidence for suppression of shear dilatancy by internal acoustic vibration,