Tsunami - a seismic sea wave - means in Japanese "harbor-wave". It is also misleadingly called "tidal wave". It is an ocean wave caused by an earthquake of magnitude 6.5 on Richter scale (or greater) that occurs less than 50 kilometers beneath seafloor. Tsunamis can also be caused by volcanic eruptions and by landslides.
Tsunami waves are followed by three to five oscillations of continental shelf waters. These convulsions may last up to a week. If initial wave reaches shore at its trough phase, water recede and expose seafloor. This happened in Lisbon Port on November 1, 1755. A few minutes later, displaced waters return with energetic vengeance.
In ocean, tsunami waves are merely 0.5-2 meters high with a wavelength of up to 200 kilometers. Consequently, they are virtually impalpable though they move at speeds of up to 700 kilometers per hour. As waves near shoreline, friction with shallow bottom reduces their velocity, shortens their wavelength, increases their amplitude and their height.
The tsunami wave that swept across coasts of Indonesia, Sri Lanka, Thailand, India, and Africa on December 26, 2004 was 10-12 meters high. It traveled almost 6000 kilometers. It killed almost 150,000 people. An earthquake in fjord-like Lituya Bay, Alaska, on July 9, 1958, generated a tsunami wave 524 meters (1719 feet) high, moving at a speed of 160 kilometers per hour. Luckily, area was largely uninhabited.
Other notable tsunamis:
In 1703 at Awa, Japan with more than 100,000 people dead.
On April 24, 1771, a tsunami caused by an underwater earthquake struck Japanese island of Ishigaki (in Ryuku chain). It was 85 meters high. It was so powerful that it hurled a 750 ton piece of coral to a distance of 2.5 kilometers inland.
Again in Japan, 27,000 people drowned in 1896, in a giant tsunami.
In wake of underwater volcanic eruptions that obliterated island of Krakatau (Krakatoa) on August 26-27, 1883, a wave 35 meters high swept across East Indies killing in excess of 36,000 people.
Triggered by a submarine landslide, a tsunami at least 375 meters high struck island of Lanai in Hawaii about 105,000 years ago.
The 1960 earhquake in Chile created tsunami waves that traveled more than 10,000 kilometers to Hilo, Hawaii. The 12 meters high water wall killed 61 people and destroyed many buildings.
The Seismic Sea Wave Warning System (SSWWS), based in Honolulu, is an early warning system covering entire, tsunami-prone, Pacific Ocean.
Little known facts about temblors:
The epicenter of an earthquake is not same as its hypocenter (focus, point of origin within a fault-line). The epicenter is point on surface of Earth directly above focus. Dangerous, shallow-focus quakes originate 0-70 kilometers below surface. Less damaging deep-focus tremors occur between 70-700 kilometers down. Subduction zone earthquakes (like one that gave rise to lethal tsunami on December 26, 2004) occur when one tectonic plate moves under another (subducts). There are interplate and intraplate quakes, which take place along plate boundaries or within fracturing crust of a single plate, respectively.
Earthquakes are not rare at all - several hundred earthquakes occur every day. There are about 1 million of them annually - of which 50,000 can be felt without aid of instruments. Tremors of magnitude of Kobe in 1995 (which caused an estimated damage of $100 billion ) are measured 20 times in an average year.
The Encyclopedia Britannica (2005 edition) describes a "swarm" of such events thus:
"In Matsushiro region of Japan, for instance, there occurred between August 1965 and 1967 a series of hundreds of thousands of earthquakes, some sufficiently strong (up to local magnitude 5) to cause property damage but no casualties. The maximum frequency was 6,780 small earthquakes on April 17, 1966."
The Pacific ocean is unhappy recipient of well over 80 percent of all energy released by earthquakes worldwide. Japan alone suffers from 1500 tremors annually (of which two thirds are greater than 3.5 in magnitude). Fault lines abound and new ones are discovered frequently. One fault line runs under 125th street in Manhattan, New-York.
Still, in last 5 centuries, all earthquakes combined killed less than one tenth victims of World War II - and this includes 240,000 who died in 1976 Tang-Shan, China event.
Earthquakes are composites of:
I. Primary (or compression) and secondary (or shearing) body waves (that travel in rocks under surface of Earth at speeds of up to 7 kilometers per second and frequencies of between 20 Hertz and one vibration per 54 minutes)
II. Two types of surface waves, named after British physicist Lord Rayleigh and British geophysicist A. E. H. Love (with frequencies of 1-0.005 Hertz).
Some earthquakes are caused by human activities (such as filling of water reservoirs behind dams, injecting water into deep wells, and underground nuclear tests). More than 600 tremors were recorded in decade following filling of Lake Mead behind Hoover Dam on Nevada-Arizona state border.