Fault stress evolution and aftershocks during frictional sliding in the lab
Stick-slip events are commonly viewed as a laboratory analog for earthquakes because they produce series of repeatable events with gradual stress increase and rapid release. However, stick-slip events are impacted by finite fault sizes and boundary conditions in the lab. Large stress drops and long recurrence intervals are a notable difference between the lab and natural seismicity which consist of sequences of fore, main and aftershocks.
I will discuss: 1) strategies and problems with relating lab tests to natural faults; 2) how fault roughness affects seismicity clustering during stick-slip; and 3) how gouge compositional heterogeneity changes stress and acoustic emission localization. We conducted series of triaxial compression tests on faulted granite samples with different surface roughness. The lab tests suggest that larger roughness promotes less slip during mainshocks but higher aftershock productivity which scales with residual strain energy. Smooth faults, on the other hand, promote more slip in large events with few aftershocks. The characteristics of acoustic emission sequences in the lab and aftershocks clustering in nature are statistically indistinguishable. We conclude that roughness and heterogeneity govern slip stability and seismic energy partitioning during these tests.
A second series of experiments on fault gouge with compositional variation further highlights the importance of fault heterogeneity. We conducted direct shear tests on mixtures of velocity-weakening soda-lime spheres and velocity-strengthening quartz fault gouge, during which we systematically varied composition from 100% to 0% velocity-weakening material. Homogeneous velocity-weakening faults, as one end member of the gouge mixtures, produce quasi-periodic stick-slip with pronounced pre-failure dilation, late-stage rapid slip acceleration, and time and slip predictable recurrence. In contrast, heterogeneous faults with gouge mixtures accelerate more gradually and exhibit shorter dilational phases with small amplitudes. Preparatory slip starts earlier in the interseismic period and evolves towards an intermittent, critical stress state prior to failure which is associated with increasingly less predictable stick-slip. The transition from stick-slip to stable sliding regimes occurs within a narrow compositional range (80%–85%), with peak slip velocities increasing by approximately 3.8 orders of magnitude between the two regimes.
Taken together these results suggest that heterogeneity strongly affects the predictability of stick-slip events and promotes space-time clustering similar to natural
