Frictional behavior of quartz gouge during slide-hold-slide considering normal stress oscillation

Slide-hold-slide (SHS) test is an essential experimental approach for studying the frictional stability of faults. The origin SHS framework was established based on a consistent constant normal stress, which cannot truly reflect the stress disturbance around fault zones. In this paper, we conducted a series of 'dynamic SHS tests', which includes normal stress oscillations in the relaxation stage with different oscillation amplitudes and frequencies on synthetic quartz gouge using a double direct shear assembly. The experimental results reveal that the amplitude of the normal load oscillation has a remarkable effect on the frictional relaxation and healing patterns. However, the frequency of the normal load oscillation has a minor effect. Additionally, the shear loading rate is proportional to the normal loading rate during the relaxation stage, and the normal stiffness of the quartz layer remains nearly constant under various loading conditions. The creep rate during the hold phase is not obviously affected by the normal load oscillation, while the precursory slip is also sensitive to the oscillation amplitude. This study provides insights into the evolution of frictional stability in discontinuities and is beneficial for controlling relative disasters in fault zones.