Interseismic deformation transients and precursory phenomena: Insights from stick-slip experiments with a granular fault zone

Abstract

The release of stress in the lithosphere along active faults shows a wide range of behaviors spanning several spatial and temporal scales. It ranges from short-term localized slip via aseismic slip transients to long-term distributed slip along large fault zones. A single fault can show several of these behaviors in a complementary manner often synchronized in time or space. To study the multiscale fault slip behavior with a focus on interseismic deformation transients we apply a simplified analog model experiment using a rate-and-state-dependent frictional granular material (glass beads) deformed in a ring shear tester. The analog model is able to show, in a reproducible manner, the full spectrum of natural fault slip behavior including transient creep and slow slip events superimposed on regular stick-slip cycles (analog seismic cycles). Analog fault slip behavior is systematically controlled by extrinsic parameters such as the system stiffness, normal load on the fault, and loading rate. Accordingly, interseismic creep and slow slip events increase quantitatively with decreasing normal load, increasing stiffness and loading rate. We observe two peculiar features in our analog fault model: (1) Absence of transients in the final stage of the stick-slip cycle ("preseismic gap") and (2) "scale gaps" separating small interseismic slow (aseismic) events from large (seismic) fast events. Concurrent micromechanical processes, such as dilation, breakdown of force chains and granular packaging affect the frictional properties of the experimental fault zone and control interseismic strengthening and coseismic weakening. Additionally, interseismic creep and slip transients have a strong effect on the predictability of stress drops and recurrence times. Based on the strong kinematic similiarity between our fault analog and natural faults, our observations may set important constraints for time-dependent seismic hazard models along single faults.