A central problem in studies of fault interaction and earthquake triggering is that of quantifying changes in frictional strength and the constitutive response caused by dynamic stressing. We imposed normal stress vibrations on creeping laboratory shear zones to investigate the process of dynamic weakening and the conditions under which resonant frictional behavior occurs. Layers of quartz powder were sheared at room temperature in a double-direct shear geometry at normal stress $bar{sigma}$n = 25--200 MPa, vibration amplitude A = 0.1--10 MPa, period T = 0.1--200 s, and loading rate V = 1--1000 ¿m/s. Frictional response varied systematically with A, T, and V. Small-amplitude, short-period vibrations had no effect on frictional strength, but large-amplitude, short-period vibrations reduced shear zone strength by about 1%. Intermediate periods caused phase lags between shear strength and imposed vibrations. During long-period vibrations, frictional strength varied sinusoidally, in phase with vibrations and with an amplitude consistent with a constant coefficient of friction. Our data show that friction exhibits a critical vibration period, as predicted by theory. At long periods, the Dieterich (aging) friction law, with the Linker and Dieterich modification to describe step changes in normal stress, provides a good fit to our experimental results for all A and V. At short periods, however, theory predicts more dynamic weakening than we observed experimentally, suggesting that existing rate and state friction laws do not account for the full physics of our laboratory experiments. Our data show that normal-force vibrations can weaken and potentially destabilize steadily creeping fault zones.