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

Porefluids influence many aspects of tectonic faulting including frictional strength aseismiccreep and effective stress during the seismic cycle. However, the role of porefluid pressure during earthquakenucleation and dynamic rupture remains poorly understood. Here we report on the evolution of porefluidpressure and porosity during laboratory stick-slip events as an analog for the seismic cycle. We sheared layers ofsimulated fault gouge consisting of glass beads in a double-direct shear configuration under true triaxialstresses using drained and undrainedfluid conditions and effective normal stress of 5–10 MPa. Shear stress wasapplied via a constant displacement rate, which we varied in velocity step tests from 0.1 to 30μm/s. We observenet pore pressure increases, or compaction, during dynamic failure and pore pressure decreases, or dilation,during the interseismic period, depending onfluid boundary conditions. In some cases, a brief period of dilationis attendant with the onset of dynamic stick slip. Our data show that time-dependent strengthening and dynamicstress drop increase with effective normal stress and vary withfluid conditions. For undrained conditions,dilation and preseismic slip are directly related to porefluid depressurization; they increase with effectivenormal stress and recurrence time. Microstructural observations confirm the role of water-activated contactgrowth and shear-driven elastoplastic processes at grain junctions. Our results indicate that physicochemicalprocesses acting at grain junctions together withfluid pressure changes dictate stick-slip stress drop andinterseismic creep rates and thus play a key role in earthquake nucleation and rupture propagation