MEGATSUNAMI

Megatsunami (often hyphenated as mega-tsunami, also known as iminami or "wave of purification") is an informal term used by popular media and popular science to describe a very large tsunami-like wave significantly beyond the size reached by tsunamis (typically around 10 meters). For this reason, there is no scientific definition of a megatsunami. Informally, the term generally refers to waves beyond the norm for tsunamis, ranging from over 40 metres (131 feet) to giants over 100 metres (328 feet) tall. Note that megatsunamis often reach higher than their wave height when they meet land, as the water often floods upwards from the force of impact.
Megatsunamis are related to tsunamis in name only: they do not usually have the same cause or appearance, the only connection being that both are very large scale water movements caused by point events. They are also not the same as freak waves which appear in ocean waters and are often up to 30 meters tall.Unlike tsunamis, which are primarily due to seismic activity in the earth's crust raising or lowering the sea bed by a few meters over a large area, 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.