Megatsunami, also known as iminami, 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 megatsunamis have originated from large scale landslides or impact events.
A megatsunami is meant to refer to a tsunami with an initial wave amplitude (wave height) measured in several tens, hundreds or possibly thousands of metres.
Normal tsunamis generated at sea have a small wave height offshore, and a very long wavelength (often hundreds of kilometers long). They 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.
Megatsunamis can be caused by giant landslides, and asteroid impacts. Underwater earthquakes do not normally generate such large tsunamis, but landslides next to bodies of water resulting from earthquakes do, since they cause a massive amount of displacement.