Pore fluids are ubiquitous throughout the lithosphere and are commonly cited as the cause of slow-slip and complex modes of tectonic faulting. We investigate the role of fluids for slow-slip and the frictional stability transition and find that the mode of fault slip is mainly unaffected by pore pressures. We shear samples at effective normal stress (σ’n) of 20 MPa and pore pressures Pp from 1 to 4 MPa. The lab fault zones are 3 mm thick and composed of quartz powder with median grain size of 10 µm. Fault permeability evolves from 10-17 to 10-19 m2 over shear strains up to 26. Under these conditions, dilatancy strengthening is minimal. Slow slip may arise from dilatancy strengthening at higher fluid pressures but for the conditions of our experiments slip rate-dependent changes in the critical rate of frictional weakening are sufficient to explain slow-slip and the stability transition to dynamic rupture.

The dynamics of the fluid flow within faults plays a critical role in the evolution of fault strength through the seismic cycle. The key processes that control how fluids affect fault slip behavior are the evolution of fault porosity and fluid recharge during slip that, in turn, determine dilational strengthening or compaction weakening. Despite the significance of these processes, high-fidelity lab measurements that include the evolution of porosity, fluid pressure and frictional properties are sparse. Here, we report such data for drained and undrained velocity-stepping experiments from 3 to 300 µm/s on natural fault gouges from the seismogenic zone of injection well 16A (2050 - 2070m) of the Utah FORGE EGS site. We conducted a suite of experiments under fixed normal stresses (44 MPa) and pore fluid pressures (13, 20, 27 MPa) corresponding to pore fluid factors between 0.3 and 0.65. We carefully monitor the volumetric strain and show that the dilatancy coefficient of the material ranged from 5 to 12 x 10-4, and showed minor sensitivity to fluid boundary conditions. In some cases, we see that larger slip velocities cause a transition from dilatancy strengthening to compaction weakening via fluid pressurization. Fluid pressure diffusion across the fault evolves during shear suggesting that permeability asymmetry, up to 4 orders-of-magnitude, is required to explain the interaction between fault stress, dilation and fluid diffusion. We posit that the spatial-temporal pattern of pore connectivity creates a spectrum of fault drainage conditions, ultimately controlling the mode of fault slip.

The frictional velocity dependence and healing behavior of subduction fault zones play key roles in the nucleation of stick-slip instabilities at convergent margins. Diagenetic to low-grade metamorphic processes such as pressure solution are proposed to be responsible for the change in frictional properties of fault materials along plate interfaces; pressure solution also likely contributes to the acceleration of healing according to previous studies. Here, we report velocity-step experiments using rocks collected from ancient subduction fault zones, the Lower Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two mélanges preserve paleotemperature records corresponding to the updip and downdip limits of the seismogenic zone and deformation recording a lower versus higher degree of pressure solution. Our data show that the Lower Mugi mélange sample exhibits velocity-weakening to velocity-neutral behavior under low normal stress, and the Makimine mélange sample shows velocity-strengthening behavior under high normal stress. This is consistent with the slip behavior observed at the depths they have been subducted to along the plate interface. We also perform a series of slide-hold-slide experiments under different hydrothermal conditions using the Lower Mugi mélange sample to evaluate the role of pressure solution in fault healing and its dependency on temperature. The results show that healing rates increase in tests operated at higher temperatures. The microstructures related to pressure solution found in the postexperimental gouges support the idea that the elevated healing rate can be related to pressure solution.

The Delaware Basin in West Texas and Southeast New Mexico has experienced a proliferation in seismic activity since 2016. The seismic activity is primarily due to subsurface injection of wastewater into both shallow and deep reservoirs. However, the precise mechanisms connecting pore-fluids to seismic activity is not well understood. To shed light on these processes, we measure rate-state friction and poromechanical properties of rocks sampled from the Delaware Mountain Group (DMG) at pressures and stresses representative of in-situ conditions. Experiments were conducted inside a pressure vessel and loaded in a true-triaxial stress state. The samples exhibit velocity-strengthening behavior and transition to a velocity-neutral behavior with increasing slip. We also measure frictional healing and demonstrate that the healing rates are consistent with those measured from quartz-feldspathic-rich rocks. Fault acceleration produces a transient increase in layer thickness (i.e, dilatancy), which in turn, reduces the local pore-pressure and causes dilatancy strengthening. Broadly speaking, the frictional and poromechanical data indicate that shallow faults within the DMG should favor aseismic creep as opposed to unstable slip. Hence, alternative mechanisms to an increase in pore-pressure being the direct causative agent to seismicity in the DMG need to be considered. We propose that seismicity in the DMG could be caused by a slip-weakening mechanism via a transition to velocity weakening behavior associated with shear localization at higher shear strains. Alternatively, seismic activity in the DMG could be a byproduct of aseismic creep as opposed to being triggered directly by the advancement of a pore-pressure front.