The Stability Transition from Stable to Unstable Frictional Slip with
Finite Pore Pressure
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.