<p>Dynamic disturbances certainly reduce shear strength of rock joints, yet the mechanism needs deeper explanation. We investigate the shear behavior of a rough basalt joint by conducting laboratory shear experiments. Constant and superimposed oscillating normal loads are applied at the upper block. Meanwhile, the bottom block moves at a constant shear rate. We investigate the shear behavior by: 1) altering the normal load oscillation frequency with a same shear rate, 2) altering the shear rate with a same normal load oscillation frequency, and 3) altering the normal load oscillation frequency and shear rate simultaneously with a constant ratio. The results show that the oscillating normal load reduces the coefficient of friction (COF). The reduce degree of COF increases with higher shear rate, decreases when increasing normal load oscillation frequency, and keeps constant if the special ratio, <i>v/f</i> (shear rate divided by normal oscillation frequency), is constant. Moreover, we identify a time lag between peak normal load and peak shear load. And the lagging proportion increases with higher shear rate, and decreases with larger static COF. Our results imply that a lower creep rate with a higher normal load oscillation frequency easily destabilizes the creeping fault zones.</p>