Mega-Tsunami destroying New York City in the film Deep Impact
Mega-tsunami' is an informal term to describe a tsunami that has initial wave heights that are much larger than normal tsunamis. Unlike usual tsunamis – which originate from tectonic activity and the raising or lowering of the sea floor – known mega-tsunamis have originated from a large scale landslide, collision, or volcanic eruption event.
Concept[]
A megatsunami is meant to refer to a tsunami with an initial wave amplitude (height) measured in several tens, hundreds, or possibly thousands of meters.
Normal tsunamis generated at sea result from movement of the sea floor. They have a small wave height offshore, and a very long wavelength (often hundreds of kilometers long), and generally pass unnoticed at sea, forming only a slight swell usually of the order of 30 cm (12 in) above the normal sea surface. When they reach land the wave height increases dramatically as the base of the wave pushes the water column above it upwards.
By contrast, megatsunamis are caused by giant landslides and other impact events. This could also refer to a meteorite hitting an ocean. Underwater earthquakes or volcanic eruptions do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes can, since they cause a massive amount of displacement. If the landslide or impact occurs in a limited body of water, as happened at the Vajont Dam (1963) and Lituya Bay (1958) then the water may be unable to disperse and one or more exceedingly large waves may result.
Two heights are sometimes quoted for megatsunamis – the height of the wave itself (in water), and the height to which it surges when it reaches land, which depending upon the locale, can be several times larger.
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