As a tsunami approaches the coast and the water becomes shallower

A tsunami is a very unusual form of wave and is caused by a geological event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea. This trigger event temporarily lifts the surface of the water, usually by a few feet (one meter). The potential energy of the raised sea above the site is turned into kinetic energy, creating a shallow wave, known as a tsunami, radiating outwards at a speed proportional to the square of the sea depth. A trigger event on the continental shelf may cause a local tsunami on the land side and a distant tsunami that travels out across the ocean.[26] Normal surface waves are up to 45 feet (14 metres) high, have a wavelength of a few hundred feet (one hundred meters) and travel at up to 65 miles per hour (105 km/h). Tsunami have a wavelength of 80 to 300 miles (130 to 480 km) and travel about ten times faster. In the open sea they may pass unnoticed as their height at this stage is usually less than three feet; it is as they enter shallower water that their dimensions change.[26]

As a tsunami approaches the coast and the water becomes shallower, the wave is compressed and its speed decreases to below 80 miles per hour (130 km/h). Its wavelength diminishes to less than 12 miles (19 km), and its amplitude increases enormously. Its behaviour is similar to a wind-generated wave, but the scale is vastly different and involves not just the surface layers of the sea but the whole water column. The water in front of the wave may be sucked back and added into the crest, leaving the seabed close to the shore exposed. The wave begins to tower in the same way as a normal wave but on a vastly greater scale. It does not usually break but instead rushes inland, engulfing all in its path. Much of the destruction wreaked may be caused by the water draining back into the sea after the wave has struck, dragging debris and people with it. Often several tsunami are caused by a single geological event and arrive at intervals of somewhere between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive