We study the coherent dynamics of G-band phonons in single-wall carbon nanotubes through impulsive stimulated Stokes and anti-Stokes Raman scattering. The probe energy dependence of the phonon amplitude as well as the preferential occurrence between Stokes and anti-Stokes components in response to chirpedpulse excitation are well explained within our model. The temperature dependence of the observed dephasing rate clearly exhibits a thermally activated component, with an activation energy that coincides with the frequency of the radial breathing mode (RBM). This fact provides a clear picture for the dephasing of G-band phonons by random frequency modulation via interaction with the RBM through anharmonicity.