Fault gouge composition controls preparatory processes and recurrence of laboratory earthquakes: From time-Predictable failure to aseismic slip

Fault zone heterogeneity takes several forms, such as surface roughness, lithological variations, and the intermixing of mineral phases with contrasting frictional properties within the fault core. Each is thought to affect earthquake nucleation and arrest, yet their effects on preparatory processes before failure and on earthquake recurrence remains insufficiently understood. Here, we conducted direct shear experiments on binary mixtures of soda-lime glass beads and quartz powder, systematically varying the gouge composition from 100 to 0 wt% glass beads at 313 ± 15 kPa normal stress. Frictionally contrasting phases are intermixed at the grain scale, making the mixtures spatially homogeneous but compositionally heterogeneous. Homogeneous glass beads, 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, compositionally mixed gouges accelerate more gradually and exhibit shorter dilational phases with small amplitudes. Furthermore, preparatory slip starts earlier in the interseismic period and evolves towards an intermittent critical stress state prior to failure, which is associated with increasing deviation from time-predictable recurrence and a growing fraction of aseismically accommodated slip. The transition from stick-slip to stable sliding occurs within a narrow compositional range (80–85 wt% glass beads), with peak slip velocities differing by approximately 3.8 orders of magnitude between the two regimes. These results demonstrate that gouge composition strongly affects preparatory processes, and predictability of stick-slip recurrence in the laboratory and potentially in nature.