Voltage pulses applied between the tip of a scanning tunneling microscope (STM) and a crystal surface often result in extraction or deposition of single atoms or clusters of atoms on the surface, which are detected by imaging the affected area with the STM after the pulse. Here, atomic-scale displacements of the STM tip during such surface modifications, due to structural changes occurring at the tunnel junction are studied. Using this technique, it is found that for the Si(111)7X7 surface and a W tip, modification during the pulse is field induced and not current induced, but that a considerable fraction of modification events occur shortly after a pulse. The ability to predict the type of modification from the direction of the tip displacement is investigated, and the applicability of this approach to high-speed nanofabrication is discussed.