The flat events are multiple reflections from the seafloor. [Applet source] [Application source] [Java documentation]
This HTML page imitates a miniature SEP reproducible research document.
The underlying applet implements the four buttons underneath
The image above shows the result of a seismic processing step: Normal Move Out maps hyperbolic events in time t to horizontal events in zero offset time:
where h is the horizontal offset and V the subsurface velocity.