From aseismic to seismic slip during fluid pressure oscillations on laboratory fault

A rise in fluid pressure decreases the effective normal stress acting on faults, facilitating reactivation. Within the upper crust, fluid pressure cyclic perturbations are common. They can originate from natural causes such as oceanic tides, seasonal hydrology, gas-rich magma ascent in volcanic edifices, etc.; or originate from anthropogenic causes due to fluid injections into deep geo-reservoirs. Recent numerical and field studies demonstrate that cyclic fluid pressure stimulations trigger less seismicity compared to monotonic injections. However, only few experimental studies are aimed at understanding the influence of fluid pressure oscillations on fault reactivation.

We investigated the role of fluid pressure cyclic perturbations by performing triaxial laboratory experiments on Fontainebleau sandstone. The samples were saw-cut at 30° from the maximum principal stress and the fault surfaces were polished with sand paper to impose a constant roughness. The experiments were conducted at 30 or 45 MPa confining pressure, with an initial fluid pressure of 10 MPa. Sample were then loaded at a constant axial velocity of 10^-4 or 10^-3 mm.s^-1. As soon as the sample behaved inelastically, oscillating pore pressure was imposed with a mean value of 10 MPa, a peak-to-peak amplitude between 0 and 8 MPa and a period of 102 s (macroscopically drained conditions). During deformation, the fault mechanical response and acoustic emission (AE) signals were monitored to investigate the effect of the fluid pressure oscillations. Our results show that: (1) Fluid pressure oscillations control the time distribution of the instabilities. We found that stick-slip and AE events occurred mainly at the peak of fluid pressure. (2) Fluid pressure oscillations promote seismic rather than aseismic slip. Indeed, both the effective maximum stresses and fault slip at the onset of stick-slip are reduced when increasing the oscillation amplitude. We found that the aseismic to seismic slip transition is caused by slip rate variations in phase with the fluid pressure. Indeed, with no fluid pressure oscillations, increasing the loading rate favours seismic slip. Our experiments demonstrate that in this drained case, cyclic fluid injections strongly affect fault stability and the resulting seismic events' time distribution.