Poromechanics of stick-slip frictional sliding and strength recovery on tectonic faults

Pore fluids influence many aspects of tectonic faulting including frictional strength aseismic creep and effective stress during the seismic cycle. However, the role of pore fluid pressure during earthquake nucleation and dynamic rupture remains poorly understood. Here we report on the evolution of pore fluid pressure and porosity during laboratory stick-slip events as an analog for the seismic cycle. We sheared layers of simulated fault gouge consisting of glass beads in a double-direct shear configuration under true triaxial stresses using drained and undrained fluid conditions and effective normal stress of 5-10MPa. Shear stress was applied via a constant displacement rate, which we varied in velocity step tests from 0.1 to 30 mu m/s. We observe net pore pressure increases, or compaction, during dynamic failure and pore pressure decreases, or dilation, during the interseismic period, depending on fluid boundary conditions. In some cases, a brief period of dilation is attendant with the onset of dynamic stick slip. Our data show that time-dependent strengthening and dynamic stress drop increase with effective normal stress and vary with fluid conditions. For undrained conditions, dilation and preseismic slip are directly related to pore fluid depressurization; they increase with effective normal stress and recurrence time. Microstructural observations confirm the role of water-activated contact growth and shear-driven elastoplastic processes at grain junctions. Our results indicate that physicochemical processes acting at grain junctions together with fluid pressure changes dictate stick-slip stress drop and interseismic creep rates and thus play a key role in earthquake nucleation and rupture propagation.