• Oscillating normal loads reduce the coefficient of friction of rough rock joints, with the reduction degree increasing at higher shear rates and decreasing at higher oscillation frequencies.
• A constant ratio of shear rate to oscillation frequency (v/f) yields a constant friction reduction, indicating a rate-frequency coupled behavior.
• A time lag between peak normal load and peak shear load is identified; the lag increases with shear rate and decreases with static COF.
• Lower creep rates combined with higher oscillation frequencies may promote instability in creeping fault zones, offering insights for seismic hazard assessment.