Michael Rudolf , Matthias Rosenau, Onno Oncken

The exact principles of earthquake recurrence and magnitude are currently unknown which is why earthquake hazard assessment relies on statistical models combined with numerical simulations. A component of seismic and aseismic slip is the frictional character of a fault. We shear fused glass beads with a narrow particle size distribution of 300-400µm at stresses of 5-20kPa and with low shear rates of less than 1mm/s. As a result, we show that characteristic slip events emerge, ranging from fast and large slip to small scale oscillating creep and stable sliding. In particular we observe small scale slip events that occur immediately before large scale slip events for a specific set of experiments. Similar to natural faults we find a separation of scales by several orders of magnitude for slow events and fast events. Enhanced creep and transient dilatational events pinpoint that the granular analogue is close to failure. From slide-hold-slide tests, we find that the rate-and-state properties are in the same range as estimates for natural faults and fault rocks. The fault shows velocity weakening characteristics with a reduction of frictional strength between 0.8 to 1.3 \% per e-fold increase in sliding velocity. Furthermore, the slip modes that are observed in the normal shear experiments are in good agreement with analytical solutions. Our findings highlight the influence of micromechanical processes on macroscopic fault behaviour. The comprehensive dataset associated with this study can act as a benchmark for numerical simulations and alleviate the understanding of observations of natural faults.