A submerged water jet impinging upon a free air-water surface has been investigated experimentally. The present work focuses on the dynamic deformation of the surface bump generated in the jet impingement region. The surface bump has a quake motion comprising two basic oscillations: fluttering motion (transversely) and swelling motion (vertically). The velocity measurement was made beneath the bump with hot-wire anemometry. The motion of the geometrical stagnation point was analyzed with the aid of an image processing. It has been ascertained that a helical instability of the approaching jet is the origin of the fluttering motion of the surface bump and that the swelling motion is controlled by the large-scale vortical/turbulent structure of the approaching jet convected with phase velocity.