I designed state-of-the-art laboratory experiments to shed light on the effect of fluid pressure on fault rheology and frictional stability. I performed experiments using a word class apparatus (BRAVA) at the HP-HT laboratory installed at INGV in Rome, with fluid pressure ranging from sub hydrostatic to near lithostatic, and showed that the friction stability parameter (a-b) evolves with increasing fluid pressure from velocity strengthening (indicative of aseismic creep) to velocity neutral behaviour. Furthermore, the critical slip distance, Dc, dramatically decreases as the pore fluid pressure increases. These observations indicate that fluid overpressure can facilitate earthquake nucleation since it controls the evolution of fault zone rheology (Scuderi and Collettini, 2016). However, while this study was the first to show the effect of fluid pressure on the friction stability parameters, these results did not show clear evidence of velocity weakening behavior, which is required to nucleate a seismic instability. To dig further into the physical mechanism at the origin of fluid driven fault slip, I have since developed a new experimental approach that consists in deforming simulated fault gouge under constant applied stress field (i.e. creep experiments), which is inferred for intraplate faults, and monitored the evolution of fault slip during fluid pressure build up. Remarkably, and for the first time, my results showed that, when the Coulomb-Mohr criterion for fault reactivation is satisfied, a frictional instability can spontaneously nucleate on a velocity strengthening fault gouge due to fluid pressurization (Scuderi et al., 2017). I proposed that under the stress conditions of induced seismicity the second order frictional variations, as evaluated by the RSF approach, are overcome by the weakening induced by the fluid pressure. Based on mechanical data and microstructural analysis, I proposed a microphysical mechanism that relates fault zone deformation with the evolution of the stress state through an energy balance to explain the observed nucleation of slip instability.
Another aspect that was developed within FEAT was to understand the physical processes during fault zone deformation using non-destructive techniques, such as ultrasonic wave propagation, which may prove effective in the future to implement systems of early warning. By matching the fault rheological properties with the elastic surrounding I was able to reproduce, for the first time, the spectrum of fault slip behaviors in the laboratory and investigate the underlying physical mechanisms. By analyzing ultrasonic wave propagation during stick-slip frictional sliding, and developing a new technique that uses coda waves, I have shown that P-wave velocity changes before the earthquake stress drop can be clearly detected within laboratory fault zones. The results, that have been published in Nature Geoscience and Nature Communication, have built the ground for a better understanding of the physical mechanisms at the origin of fault slip. For each experiment, we have collected the resulting fault zone and analyzed to characterize the evolution of shear fabric in granular fault gouge from stable sliding to stick slip and understand the implications for fault slip mode. This is a long-standing problem in fault mechanics with implication in identifying the record of past co-seismic slip and evaluate the seismic hazard. I have shown that once shear localizes along narrow slip zones the slip behavior is controlled by the elastic interaction between the fault zone and the surrounding. This suggest that a single fault segment can experience a spectrum of slip behaviors, from aseismic creep, slow earthquakes to dynamic rupture, depending on the evolution of fault rock frictional properties and elastic conditions of the loading system.
