Rotary shear experiments on glass bead aggregates: Stick-slip statistics and parallels with natural seismicity

Abstract

The goal of predicting earthquakes remains elusive despite decades of instrumental observations and research, and a much longer historical record. Even the practice of seismic hazard analysis is a topic of heated debate, in part due to our inability to accurately determine the rate and size distribution of earthquakes that a fault can produce. The main reason for these deficiencies is the lack of a validated, physics-based theory of earthquakes. Heuristic attempts to discover patterns in seismicity based on its phenomenology have produced ambiguous and sometimes contradicting results, due to the relatively short instrumental record of big earthquakes compared to their rate of occurrence. From a geodynamics perspective, earthquakes are bursts of energy release as the lithosphere is loaded due to the motion of tectonic plates at rates of a few centimeters per year. Similar behavior, known as crackling, is observed when shearing granular aggregates. Loosely packed particles behave collectively as a fluid, giving rise to small instabilities only. At a critical packing fraction, the size distribution of the instabilities approaches power law scaling. This suggests that the aggregate is at a phase transition and that long-range correlations are a key characteristic of its macroscopic behavior. Above the critical packing fraction, the collective behavior of the particles is similar to that of a solid. In that solid-like regime, the aggregates alternate between power law distributed event sizes and quasi-periodic stick-slip. A significant number of laboratory studies have employed granular media to explore the dynamics of critical systems in the context of seismicity and fault gouge rheology. These studies have been performed either at low normal stress (< 1 MPa) or to limited shear displacements (< 50 mm), and often under dry conditions. It is not known whether the macroscopic behavior of granular aggregates remains the same under higher normal stress and larger displacements, or in the presence of pressurized water. If not, is it possible to determine what mechanisms are responsible for the change? The rotary shear experiments presented in this thesis expand the envelope of the experimentally tested conditions up to 8 MPa normal stress and 165 mm of shear displacement, \textit{simultaneously}. This enabled us to infer the emergence of correlations under these conditions, through changes in the statistics of granular avalanches. Because the elevated stress conditions do not allow direct visual observation of the glass bead samples, a specially developed AE monitoring system was used to detect and locate the source of crackling. The findings of this thesis highlight the importance of emergent, long range correlations in sheared granular media, as a function of experimental conditions. We infer that the key parameter that determines the scaling of avalanche statistics is the packing fraction, which in turn depends on normal stress, wear rate, and particle size distribution.