Effect Of Loading Perturbation On Frictional Behavior: An Experimental Study

Earthquakes result from instability of active faults under tectonic stress. Beside the slowly and steady changed tectonic stress caused by plate movement and other tectonic processes, transient or periodic loading processes also exist in the crust, such as stress imposed by solid Earth tides, loading and unloading caused by change of water level in reservoirs, and static and dynamic stress changes caused by large earthquake. Action of such transient and periodic loading on faults can change stress state on the faults, thus affect seismicity on the faults, advancing or delaying occurrence of strong earthquakes. Studying the frictional response to loading perturbation in laboratory is helpful to understand the impact of stress perturbation on earthquake triggering.We performed a series of frictional experiments using a servo-controlled biaxial loading machine with three granodiorite block direct shear configuration. In the experiments, a small-amplitude sine wave of loading or displacement was modulated to normal loading and shear loading to simulate shear stress perturbation and normal stress perturbation on a fault, respectively. The effects of perturbation amplitude, average normal stress and perturbation period on stick-slip instability were analyzed based on the experimental results. The main results we obtained are as follows.(1) Under constant normal stress and loading point velocity in shear direction, the sample shows regular stick-slip behavior, i.e. stick-slip events with similar stress drop and period, as the displacement gradually increases. After the displacement perturbation is modulated to shear loading perturbation, stress drop and period of stick-slip events tends to be scattered. The experimental results show that the effect of perturbation amplitude on friction is very evident. The scattered degree of stress drop and period of stick-slip events increases with increasing perturbation amplitude, and the correlation between the timing of stick-slip and the perturbation also increases with increasing perturbation amplitude, and the critical amplitude for producing significant impact is around 0.05MPa. Normal stress imposes some effect on results of loading perturbation. The tendency that the stress drop and period of stick-slip events are scattered with increasing perturbation amplitude is more obvious at higher normal stress, and the correlation between the timing of stick-slip and the perturbation also increases with increasing normal stress at the same perturbation amplitude. The effect of perturbation period on friction is not evident.(2) After the loading perturbation was modulated to normal direction, the stick-slip rule was disrupted. The stress drop and period response to amplitude change was quite distinct, and it became gradually scattered as the perturbation amplitude increased. The critical period differentiating whether the impact on instability time is obvious or not was also around 0.05MPa. In this perturbation direction, stress drop response to amplitude change was evident in all normal stress state. And for same amplitude the discrete degree of stress drop was increased obviously as normal stress increased, indicating that normal stress itself also affected the effects of loading perturbation in normal direction. As the perturbation in shear direction, the effect of period was also not obvious, and the critical period did not exist either.(3) Both shear and normal direction perturbation could affect the fault frictional behavior, but there was some difference in two types of perturbation. Perturbation in normal direction, the fault normal stress is changed, thus affecting the fault inherent strength, so this impact was overall. When disturbance was modulated in shear direction, the system critical stiffness unchanged, and the perturbation was local. Under low normal stress condition, stick-slip instability response to amplitude disturbance in normal direction was stronger than shear direction, while under high normal stress condition, the response to amplitude perturbation was similar in two directions.(4)The experiment result shows that the stress perturbation not only changes the occurrence time of strong earthquakes (advance or postpone), then affects the recurrence interval of strong earthquakes, but also changes the earthquake magnitude obviously, particularly when the normal stress perturbation might trigger strong earthquakes, whose magnitude might be higher than the characteristic magnitude of the faults. When discussing the seismic activity trend based on Coulomb failure stress changes, those conclusions should be concerned.