Premonitory Slip, Rupture Propagation, and Frictional Sliding on Laboratory Faults in Porous Sandstone: Effects of Loading Rate and Confining Pressure

To investigate spatiotemporal evolution of premonitory slip and foreshock activity, 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 (1–10 μm/s), confining pressures (35–75 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 observe a transition from premonitory slip to rupture events and subsequent frictional sliding. Local premonitory slip 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. Premonitory slip and precursory AEs display progressively accelerating processes, culminating in slow (<5 μm/s slip rates) or fast (1–10 mm/s) slip events. With increasing load point velocities, average premonitory slip rates increase at reduced precursory time spans, leading to fast slip events. We separate an initial rupture propagation phase from a subsequent frictional sliding phase for a slip event, highlighting that macroscopic slip and stress drop associated with rupture propagation generally account for a fraction <30% of total slip and stress drop. Our results imply that local variations in loading conditions at these slow slip and rupture velocities will affect spatiotemporal evolution of premonitory slip and associated foreshock activity.