We report laboratory results on sound velocities of granite, either sheared to rupture or undergoing stick-slip sliding. We found that the variations in velocity premonitory to these two types of instability are dramatically different. Velocities of both compressional and shear waves decreased significantly prior to the occurrence of rupture, while no change was found during the stick-slip motion. We suggest that for shallow earthquakes appreciable variation in the faulting processes is likely: Some faulting may be associated with pervasive dilation and fracturing of surrounding rocks, while others may involve little such processes. These variations may be the cause for the conficting findings in the variations of seismic velocities premonitory to earthquakes.