Frictional properties of megathrust fault gouges at low sliding velocities: new data on effects of normal stress and temperature

Friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions. We investigated the influence of in situ effective stresses and temperatures on the frictional properties of (simulated) fault gouges, prepared either from Nankai ODP material or illite shale, at sliding velocities approaching those relevant for earthquake nucleation and SSEs. Biaxial (double direct shear) experiments were performed at room temperature, normal stresses of 5e30 MPa, and sliding velocities of 0.16e18 mm/s. All materials exhibited velocity strengthening under these conditions, along with an increase in the friction coefficient and slip hardening rate with increasing normal stress. Illite gouge showed increased velocity strengthening towards higher normal stresses. The effect of temperature was investigated by means of ring shear experiments on illite gouge at 200e300 C, an effective normal stress of 170 MPa, a pore-fluid pressure of 100 MPa and sliding velocities of 1e100 mm/s. These experiments showed a transition from velocity strengthening to velocity weakening at w250 C. Our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and SSEs at low effective normal stresses.