Does a earthquake have to happen in the sea to cause a tsunami?

Answer 1

To cause a tsunami a vast amount of water must be displaced suddenly. The sudden sinking or uplifting of large sections of the crust during an earthquake are one means of doing this. Another means is an impact event. If a large enough meteor or comet were to impact the ocean a tsunami could also form. Lastly, an underwater landslide could cause a tsunami - although these are usually triggered by earthquakes, either in the sea or in the land near the shoreline.

I believe your question is also asking if a tsunami might form from an earthquake on land perhaps near the sea? This could only work if the land moved laterally - side to side - instead of vertically. In other words, if the land moved out into the sea far enough over a significant length of shoreline then enough water might be displaced to cause a tsunami. This would require a significant ripping of the crust apart further inland, however, which is rare. The only example I can think of at present is the African rift valley system.

http://harowo.com/2006/03/15/africas-new-ocean/

http://geology.com/articles/east-Africa-rift.shtml

Yet even this has not yet - to my knowledge - produced tsunamis, or at least none of measurable force / height.