We investigate how fluid pre-conditioning (FP) influences the reactivation and nucleation behavior of a critically stressed laboratory fault during fluid pressurization. Six reactivation experiments were performed on a saw-cut cylindrical sample of Rotondo granite confined at 20 MPa. Fault-parallel and axial deformation were monitored using distributed strain sensing (DSS) with optical fibers, complemented by active and passive measurements from sixteen piezoelectric transducers to track P-wave velocity variations associated with fluid migration and to detect acoustic emissions. FP substantially modifies fault reactivation behavior. Pre-conditioned tests exhibit a smoother and more spatially distributed reduction in effective normal stress at the onset of rapid pressurization, consistent with a relatively drained response at the fault scale. During the rapid pressurization ramp, FP tests show a subtle directional dependence in accelerated deformation, with slip preferentially developing where effective stress is reduced more strongly. This behavior is consistent with preferential fluid pathways inferred from ultrasonic velocity variations measured in separate fault-flooding characterization experiments. In contrast, non-pre-conditioned rapid pressurization is more consistent with a relatively locally undrained response, in which stronger effective-stress gradients preserve shear-resistance heterogeneity and promote slip barriers, foreshock activity, and more complex nucleation dynamics. These observations demonstrate that injection-driven reactivation is governed not only by pore-pressure magnitude but by the spatiotemporal distribution of permeability and effective stress established before and during rapid pressurization. Our results provide experimental constraints for models coupling pressurization regime, permeability heterogeneity, and rupture nucleation, with implications for safer geoenergy operations.
Bianchi et al. (Sat,) studied this question.