What causes tsunami ?
A tsunami can come up when there is a large, impulsive water displacement.
The first reason of the water displacement that deserve consideration is an earthquake. Tectonic earthquake is the kind of earthquake that is associated with earth's deformation. The sea floor deformation causes the water displacement, which can generate tsunami. The basic mechanism is shown on the pictures underneath.
Subduction earthquakes, that occur at plate boundaries, are paticularly effective in generating tsunamis. Subduction is a process in which, for example around the margins of the Pacific Ocean denser oceanic plates slip under continental plates.
Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.
The good ilustration of a meteorite-caused tsunami is throwing a pebble into a small, still pond. The pebble represents a meteorite, and the pond represents the ocean. The ripples that travel out in all directions from the focus, or the point where the pebble hit the water, represent the energy that creates a sea wave. Notice how the waves become larger as they reach shore, where the water is shallower.