In this paper we report on the motion of individual Sr adatoms within the limits of Si(111)-7×7 half unit cells (HUCs). The fast movement of the atom at the Si surface produces sharp signal fluctuations in scanning tunneling microscopy (STM) images resulting into noiselike patches. It is found that the length of the observed image streaks is a function of the scanning tip velocity. A Monte Carlo simulation implementing a model of independently moving Sr adatom and scanning tip, accounts for the observed STM image features quantitatively. Thus, by colleting STM images at various scanning speeds and matching simulated image features to the experimental observations, allows the average speed of Sr adatom within the limits of Si(111)-7×7 HUC to be estimated: 300 nm/s.