Many americans believe that shark attacks happen more frequently than they actually do. assuming that this is due to the way these rare events are covered by the media, this would be considered:

I believe the answer is: an unintentional, long-term effect of the media

when a certain things is reported by media over and over again, people become much more likely to perceive those things as something that keep reoccurring. This very phenomenon is used by media company to help them control the narratives of that perceived by their audiences.