Seismic fault slip affected by pore pressure and cyclic normal stress – deduced by lab investigations

Slip characteristics of tectonic faults are highly correlated with earthquake risks. However, the stress conditions in-situ are not static, because tides and seismic waves produce dynamic stress disturbances. The effect of fluids also needs to be considered. The fault slip evolution considering both, stress perturbation and fluid pressure is poorly investigated in the laboratory.

We performed direct shear tests on saw-cut granite joints using a shear box device with external syringe pump. The lower part of the specimen was driven by constant load point velocity, and static/dynamic normal loads were applied to the upper part. LVDTs recorded horizontal and vertical movements: fault slip and vertical dilatancy, respectively. The impact of two factors are studied in the experiment: pore fluid pressure and applied normal stress oscillation amplitude.

In conclusion, static pore fluid pressure reduces effective normal stress and shear stiffness of the sheared fault. Under constant normal stress, the reduction in fault shear stiffness caused by fluids synchronously competes with the reduction in critical stiffness (K_c) as the effective normal stress decreases. The stick-slip events are most intensive under low fluid pressure and high normal stress. Under oscillating normal stress, as the normal stress oscillation amplitude increases, the overall fault shear strength weakens continuously. Frictional strengthening and aseismic slips always occur in the normal stress loading stage. Normal stress unloading leads to multi-step stick-slip behavior of the sheared fault. The fault normal deformation is controlled by both normal loading/unloading and asperity overriding. Increasing pore pressure and superimposed normal stress magnitudes lead to more dramatic shear stress changes, but the degree of seismic slip is reduced.