P- and S-wave precursors to lab earthquakes under variable drainage and fluid pressurization

Fault slip emerges from coupled frictional and hydromechanical processes, yet forecasting stress evolution remains challenging when fluid pressurization, drainage, and dilatancy modulate effective normal stress. We present laboratory double-direct shear experiments on quartz-rich natural fault gouge conducted under dry, 100% humidity, and constant fluid-pressure boundary conditions. Throughout quasi-periodic and irregular seismic cycles, we continuously acquire active-source ultrasonic waveforms transmitted across the gouge layer and derive cycle-resolved acoustic observables: P- and S-wave velocity changes and amplitude-based transmissivity metrics (band-limited RMS).

For our range of conditions, the acoustic properties exhibit robust, two-stage precursory evolution. During the early interseismic phase, acoustic transmissivity increases with contact stress, consistent with progressive asperity contact growth and rising contact stiffness (“healing”). This is followed by a late interseismic-to-preseismic transition characterized by gradual bulk velocity reduction, interpreted as distributed inelastic creep and microcrack growth within the gouge. For the 100% humidity condition, velocity changes track slip velocity, peaking as the fault locks and decreasing prior to dynamic slip. Under constant external fluid pressure, partial drainage and localized undrained behavior further modulate both elastic velocity and transmissivity through shear-induced porosity changes and associated pore-pressure transients. Localized slip regions can show transient acoustic velocity increases consistent with dilatancy hardening, while the bulk response trends toward overall velocity decrease as failure approaches.

We develop a mechanistic poromechanical framework that links the ultrasonic observables to evolving contact stiffness, porosity, and effective stress, providing a physical basis for interpreting travel-time and amplitude changes under fluid pressurization. As an additional validation, a lightweight sequence model trained on the acoustic observables can reconstruct cycle-scale shear-stress evolution and event timing, demonstrating that the acoustic measurements encode the state of the fault. These results highlight the role of fault zone elastic properties for detection of precursory processes prior to earthquake failure and illuminate the processes that occur during the preparatory stages of earthquake nucleation for fluid-saturated fault systems.