Out at sea, a tsunami is a swell only about 30 centimetres high
A tsunami can cross an ocean almost unnoticed, raising the surface by roughly 300 millimetres, because its wavelength stretches hundreds of kilometres. Only in shallow coastal water does it rear up, often looking less like a breaker than a tide that refuses to stop rising. Its Japanese name simply means harbour wave.
Tsunamis are trains of waves set off when a large volume of water is suddenly shoved aside. Undersea earthquakes are the usual trigger, especially when thrust faults at converging plates jolt the seabed upward, but landslides, volcanic eruptions, calving glaciers, explosions and rapid pressure changes in the atmosphere can do it too; whether meteorites can is debated. Ordinary waves have wavelengths of 30 or 40 metres; a tsunami's can run to hundreds of kilometres, with gaps of minutes to hours between crests, and it can flood a coast even at low tide.
Scientists discourage the old label tidal wave, since tides come from the pull of the Moon and Sun and have nothing to do with tsunamis. Seismic sea wave is also imperfect, as not every tsunami comes from an earthquake. Most English speakers drop the initial t sound, which English words rarely begin with. A wave that cannot be matched to any known quake is called an orphan tsunami, such as one reaching Japan from a distant earthquake in the Americas before global communications existed. Japan has the longest recorded tsunami history.
People have puzzled over them for millennia. The oldest human record dates to 479 BC at Potidaea, a Greek colony, where a wave may have spared the town from a Persian attack. Around 426 BC Thucydides became the first to argue that sea earthquakes cause them. Ammianus Marcellinus described the classic sequence of quake, retreating sea and enormous wave after the tsunami that struck Alexandria in 365 AD.
Warning signs can save lives. In 1946 an Aleutian earthquake sent a 14-metre surge into Hilo, Hawaii, but on Molokai villagers who saw the ocean pull back rang bells and fled uphill, and nobody there died. Europe is at risk too: the 1908 Messina event killed more than 123,000 people in Sicily and Calabria. The 2004 Indian Ocean tsunami left around 230,000 dead or missing across 14 countries, and researchers are still trying to learn why some big quakes produce tsunamis while smaller ones sometimes do.
Source: Tsunami
