How are tsunamis generated?
A tsunami can be generated by any disturbance that displaces a bulk amount of water from its equilibrium position. Such disturbances may be caused by submarine earthquakes, landslides, collapses of volcanic edifices, eruptions of submarine volcanoes, or even meteorite impacts.
By submarine earthquakes
Tsunamis can be generated when the sea floor abruptly deforms due to tectonic earthquake. Tectonic earthquake is a particular kind of earthquake that is associated with the earth's crust deformation. When these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass attempts to regain its equilibrium under the influence of gravity. When large areas of the sea floor elevate or subside, a tsunami can be created.
Large seafloor displacements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis. The earthquake generating 2004 Giant Tsunami occurred where Indian ocean plate subducting Sumatra plate.
The figure on the top shows continental crust is slowly bent due to the subducting of oceanic plate, while the one on the bottom illustrates how continental crust suddenly unbends and displaces large amount of water mass, which becomes the source of a tsunami (figures from Nature online).
By submarine landslides, volcanic or meteorite impacts
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 they 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. On the other hand, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water being displaced. Generally speaking, tsunamis generated from these mechanisms, unlike the tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.