  We derive an effective Hamiltonian for spin dynamics of fluctuating smectic
stripes from the t-J model in the weak coupling limit t >> J. Besides the
modulation of spin magnitude, the high energy hopping term would induce a
low-energy anti-ferromagnetic interaction between two neighboring ``blocks of
spins". Based on the effective Hamiltonian, we applied the linear spin-wave
theory and found that the spin-wave velocity is almost isotropic for
La_{2-x}Sr_x CuO_4 unless the structural effect is considered. The intensity of
the second harmonic mode is found to be about 10% to that of the fundamental
mode.
