Tsunami

SOMA, Japan -- Dangerous levels of radiation leaking from a crippled nuclear plant forced Japan to order 140,000 people to seal themselves indoors Tuesday after an explosion and a fire dramatically escalated the crisis spawned by a deadly tsunami.

Tsunami is a series of water waves (also called a tsunami wave train) caused by the displacement of a large volume of a body of water, usually an ocean, though it can occur in large lakes. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded. Owing to the immense volumes of water and the high energy involved, tsunamis can devastate coastal regions.

Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), 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.

The Greek historian Thucydides was the first to relate tsunami to submarine earthquakes,[5][6] but the understanding of a tsunami's nature remained slim until the 20th century and is the subject of ongoing research. Many early geological, geographical, and oceanographic texts refer to tsunamis as "seismic sea waves."

Some meteorological conditions, such as deep depressions that cause tropical cyclones, can generate a storm surge, called a meteotsunami, which can raise tides several metres above normal levels. The displacement comes from low atmospheric pressure within the centre of the depression. As these storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land.

Tsunamis cause damage by two mechanisms: the smashing force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying all with it, even if the wave did not look large.

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft).[19] This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously. Since the wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like a fast moving tidal bore.[20] Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.

When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed run up. Run up is measured in metres above a reference sea level.[20] A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run up.

About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, and bolides

In a nationally televised statement, Prime Minister Naoto Kan said radiation has spread from four reactors of the Fukushima Dai-ichi nuclear plant in Fukushima state, one of the hardest-hit in Friday's 9.0-magnitude earthquake and the ensuing tsunami that has killed more than 10,000 people, plunged millions into misery and pummeled the world's third-largest economy.

Though Kan and other officials urged calm, Tuesday's developments fueled a growing panic in Japan and around the world amid widespread uncertainty over what would happen next. In the worst case scenario, the reactor's core would completely melt down, a disaster that could spew large amounts of radioactity into the atmosphere.

The radiation fears added to the catastrophe that has been unfolding in Japan, where at least 10,000 people are believed to have been killed and mllions of people have spent four nights with little food, water or heating in near-freezing temperatures as they dealt with the loss of homes and loved ones.

Asia's richest country hasn't seen such hardship since World War II. The stock market plunged for a second day and a spate of panic buying saw stores running out of necessities, raising government fears that hoarding may hurt the delivery of emergency food aid to those who really need it.