Megatsunamis are caused by a very large impact or landslide into a body of water when the water cannot disperse in all directions. For this reason, they are usually a highly localized effect, either occurring when the origin of a tsunami is extremely close to the shore, or in deep, narrow inlets, lakes or other water passages.

The principal source of these tsunami-like ocean waves are travelling air pressure disturbances, including those associated with atmospheric gravity waves, roll clouds, pressure jumps, frontal passages, and squalls, which normally generate barotropic ocean waves in the open ocean and amplify them near the coast through specific resonance mechanisms. In contrast to ‘ordinary’ impulse-type tsunami sources, a travelling atmospheric disturbance normally interacts with the ocean over a limited period of time (from several minutes to several hours).

These types of waves are common all over the world and are better known by their local names: Rissaga (Spain), Milghuba (Malta), Marrubio (Italy), Abiki (Japan).