Frictional Response of Clay-rich Sandstone to Pore-Pressure Oscillation Throughout Interseismic Periods

During interseismic periods a fault at depth can experience non-constant effective normal stress due to fluctuations in the pore-fluid pressure. Pore-pressure oscillations may influence the healing capability of the fault and ultimately affect its reactivation. Thus, studying the behaviour of faults during interseismic periods is a critical factor in understanding the seismicity. Triaxial tests were conducted using saw-cut (45^o) samples of Pennant Sandstone to investigate the influence of pore-pressure oscillations during slide-hold-slide (SHS) tests (t_h = 900 – 7300s) on its frictional behaviour and fault reactivation. The cylindrical samples were hydrostatically compacted at 30 MPa and pore-pressurized with argon gas at 5, 10 and 18 MPa resulting in effective normal stress (σ’_n) 25, 20 and 12 MPa, respectively. Then the saples were deformed at a constant shear displacement rate ≈ 4.5 μm/s. To overcome the displacement hardening tendency of the sample geometry, we servo-controlled the confining pressure so that the resolved normal stress on the sliding surface is kept constant. Experimental observations revealed a significant influence of pore-pressure oscillation on the frictional behaviour resulting in an increase in both frictional healing and creep relaxation. Moreover, this effect was enhanced as the effective normal stress was increased further. To understand better the underling mechanism(s) that influences these time-dependent processes we coupled the frictional results with permeability measured using the oscillating pore pressure method during the SHS tests. Finally, we tested how the pore-pressure oscillation affected the fault reactivation by conducting creep experiments at constant shear stress while the fault was brought to reactivation via progressive increase in fluid pressure. Our results demonstrated how non-constant effective normal stress history during interseismic periods deeply affects the fault behaviour, with important implications for natural and human-induced seismicity.