Pore pressure change during nucleation and slip along experimental faults

Fluid pressure variations within fault zones impact fault strength and have the potential to produce detectable geophysical signals that can help characterise fault dynamics. One key process impacting fluid pressure is pore volume variations (dilation or compaction) due to stress changes and inelastic deformation. Slip-induced dilation and compaction have been thoroughly documented in laboratory experiments, but their impact on pore pressure has not. In nature, we expect slip to be associated with stress variations, and there might be cumulated effects of poroelastic and inelastic pore pressure changes. In order to document such effects, we conducted laboratory rock friction experiments where fluid pressure was monitored in situ during sequences of quasi-static loading followed by dynamic slip event. The simulated fault was a 30 degrees saw-cut in a Westerly granite cylinder, saturated with water, tested under triaxial conditions. The low hydraulic diffusivity of the rock made the fault and wall rock transiently undrained during deformation. During quasi-static loading with no fault slip, we observed pore pressure rises that we interpret as poroelastic closure of the fault. During dynamic slip events, pore pressure systematically dropped, approximately in proportion to the drop in normal stress. A large contribution to the pore pressure drop is interpreted as poroelastic opening of the fault. Prior to stick-slip events, we detected systematic pore pressure decreases by up to around 1 MPa, correlated to the occurrence of inhomogeneous slip along the fault. Slip nucleation, inferred by kinematic inversion of local strain gauge data, is linked to local slip magnitudes of the order of 1 to 10 µm, and appears to lead to inelastic dilation. A stability analysis of fault slip including dilatant and poroelastic effects shows that poroelastic coupling tends to compensate normal stress variations, leading to faults operating under mostly constant effective normal stress if conditions are undrained.