Propagation of premonitory slip leading up to system-size rupture events on laboratory faults in porous sandstone: Effects of loading rate and confining pressure

Premonitory slip and migrating foreshocks transitioning into an accelerating unstable rupture are commonly observed in experiments and less frequently in nature, but what controls their spatiotemporal evolution remains unclear. In this context, we conduct a series of displacement-driven triaxial compression experiments on porous sandstone samples containing a saw-cut fault under conditions of varying load point velocities (1 to 10 μm/s), confining pressures (35 to 75 MPa) and constant pore pressure (5 MPa). Integrating far-field mechanical and displacement measurements, near-fault strain gauge arrays, and a dense network of piezoelectric transducers, we find that local premonitory slip always occurs above a threshold stress, showing a crack-like propagating front with a slow speed up to 2 cm/s. Premonitory slip is accompanied by migrating small-magnitude precursory Acoustic Emissions (AEs) with dominantly shear-enhanced compaction source mechanisms transitioning to double-couple when approaching slip events. The transition from local premonitory slip to system-size slip event occurs once the premonitory slip front crosses the entire fault, followed by the emergence of system-size slip event with an accelerating rupture front in the opposite direction. Premonitory slip and precursory AE rates accelerate progressively, culminating in slow (< 5 µm/s slip rates) or fast (1 to 10 mm/s) slip events. With increasing load point velocities, average premonitory slip rates increase at reduced precursory time spans, leading to fast slip events. Increasing confining pressure causes increasing premonitory slip and off-fault precursory AEs, but does not affect premonitory slip rates. Precursory slip scales with co-seismic slip, and is predominately aseismic. Our results imply that local variations in loading conditions at slow slip and rupture velocities will affect spatiotemporal evolution of premonitory slip and potentially associated foreshock activity.