A tsunami is a series of water waves caused by the displacement of a large volume of a body of water. They are a frequent occurrence in Japan. The term tsunami comes from the Japanese, meaning "harbor".
Earthquakes, volcanic eruptions and other underwater explosions, landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position and give rise to a significant tsunami.
Tsunamis have a small amplitude offshore, and a very long wavelength, which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.
Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsuna-mis may reach a maximum vertical height onshore above sea level, often called a run-up height, of 10, 20, and even 30 meters.