Federico Ciardo, Antonio Pio Rinaldi

Fluid injection into underground formations reactivates preexisting geological discontinuities such as faults or fractures. In this work, we investigate the impact of injection rate ramp-up present in many standard injection protocols on the nucleation and potential arrest of dynamic slip along a planar pressurized fault. We assume a linear increasing function of injection rate with time, up to a given time tc after which a maximum value Qm is achieved. Under the assumption of negligible shear-induced dilatancy and impermeable host medium, we solve numerically the coupled hydro-mechanical model and explore the different slip regimes identified via scaling analysis. We show that in the limit when fluid diffusion time scale tw is much larger than the ramp-up time scale tc, slip on an ultimately stable fault is essentially driven by pressurization at constant rate. Vice versa, in the limit when tc/tw ≫ 1, the pressurization rate, quantified by the dimensionless ratio (Qm tw / tc Q∗), does impact both nucleation time and arrest distance of dynamic slip. Indeed, for a given initial fault loading condition and frictional weakening property, lower pressurization rates delay the nucleation of a finite-sized dynamic event and increase the corresponding run-out distance approximately proportional to (Qm tw / tc Q∗)^(-0.472). On critically stressed faults, instead, the ramp-up of injection rate activates quasi-static slip which quickly turn into a run-away dynamic rupture. Its nucleation time decreases non-linearly with increasing value of (Qm tw / tc Q∗) and it may precede (or not) the one associated with fault pressurization at constant rate only.