This article: A tsunami is an unusual form of wave caused by an infrequent powerful event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption or a collapse of land into the sea. These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater is turned into kinetic energy, creating a shallow wave, a tsunami, radiating outwards at a velocity proportional to the square root of the depth of the water and which therefore travels much faster in the open ocean than on a continental shelf. In the deep open sea, tsunamis have wavelengths of around 80 to 300 miles (130 to 480 km), travel at speeds of over 600 miles per hour (970 km/hr) and usually have a height of less than three feet, so they often pass unnoticed at this stage. In contrast, ocean surface waves caused by winds have wavelengths of a few hundred feet, travel at up to 65 miles per hour (105 km/h) and are up to 45 feet (14 metres) high.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. The energy of the wave is dissipated only gradually, but is spread out over the wave front, so as the wave radiates away from the source, the front gets longer and the average energy reduces, so distant shores will, on average, be hit by weaker waves. However, as the speed of the wave is controlled by the water depth, it does not travel at the same speed in all directions, and this affects the direction of the wave front - an effect known as refraction - which can focus the strength of the advancing tsunami on some areas and weaken it in others according to undersea topography. As a tsunami moves into shallower water its speed decreases, its wavelength shortens and its amplitude increases enormously, behaving in the same way as a wind-generated wave in shallow water, but on a vastly greater scale.  Either the trough or the crest of a tsunami can arrive at the coast first.... contains an answer for the question: What is the name used when the sea draws back and leaves subtidal areas close to the shore exposed?, what is it ?
trough