Tsunami

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What is a Tsunami?

A tsunami (pronounced sue-nahm-ee) is a series of huge waves that can cause great devastation and loss of life when they strike a coast.

The word tsunami comes from the Japanese word meaning "harbor wave." Tsunamis are sometimes incorrectly called "tidal waves" -- tsunamis are not caused by the tides (tides are caused by the gravitational force of the moon on the sea). Regular waves are caused by the wind.

What causes Tsunamis?

Tsunamis are caused by an underwater earthquake, a volcanic eruption, an sub-marine rockslide, or, more rarely, by an asteroid or meteoroid crashing into in the water from space. Most tsunamis are caused by underwater earthquakes, but not all underwater earthquakes cause tsunamis - an earthquake has to be over about magnitude 6.75 on the Richter scale for it to cause a tsunami. About 90 percent of all tsunamis occur in the Pacific Ocean.

The Development of a Tsunami

A tsunami starts when a huge volume of water is quickly shifted. This rapid movement can happen as the result of an underwater earthquake (when the sea floor quickly moves up or down), a rock slide, a volcanic eruption, or another high-energy event.

After the huge volume of water has moved, the resulting wave is very long (the distance from crest to crest can be hundred of miles long) but not very tall (roughly 3 feet tall). The wave propagates (spreads) across the sea in all directions; it can travel great distances from the source at tremendous speeds.

The Size of a Tsunami

Tsunamis have an extremely long wavelength (wavelength is the distance between the crest (top) of one wave and the crest of the next wave) -- up to several hundred miles long. The period (the time between two successive waves) is also very long -- about an hour in deep water.

In the deep sea, a tsunami's height can be only about 1 m (3 feet) tall. Tsunamis are often barely visible when they are in the deep sea. This makes tsunami detection in the deep sea very difficult.

The Speed of a Tsunami

A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast as a jet flies. It can take only a few hours for a tsunami to travel across an entire ocean. A regular wave (generated by the wind) travels at up to about 90 km/hr.

What happens when a Tsunami hits the coast?

As a tsunami wave approaches the coast (where the sea becomes shallow), the trough (bottom) of a wave hits the beach floor, causing the wave to slow down, to increase in height (the amplitude is magnified many times) and to decrease in wavelength (the distance from crest to crest). At landfall, a tsunami wave can be hundreds of meters tall. Steeper shorelines produce higher tsunami waves.

In addition to large tsunami waves that crash onto shore, the waves push a large amount of water onto the shore above the regular sea level (this is called runup). The runup can cause tremendous damage inland and is much more common than huge, thundering tsunami waves.

Tsunami Warning Signs

If you see the water recede quickly and unexpectedly from a beach (this is called drawback), run toward higher ground or inland -- there may be a tsunami coming. Also, if you are on the coast and there is an earthquake, it may have caused a tsunami, so run toward higher ground or go inland. The first wave in a tsunami is often not the largest; if you experience one abnormally-huge wave, go inland quickly -- even bigger waves could be coming soon. Some beaches have tsunami warning sirens -- do not ignore them. However, it can be too late by the time you see the signs, even if you recognise them.

Detecting Tsunamis

Many tsunamis could be detected before they hit land, and the loss of life could be minimized, with the use of modern technology, including seismographs (which detect earthquakes), computerized offshore buoys that can measure changes in wave height, and a system of sirens on the beach to alert people of potential tsunami danger.

Early Detection: Tsunami Warning Systems

Tsunami warning systems exist in many places around the world. As scientists continuously monitor seismic activity (earthquakes), a series of buoys float off the coast and monitor changes in sea level. Unfortunately, since tsunamis are not very tall in height when they are out at sea, detection is not easy and there are many false alarms. Sirens at affected beaches may be activated -- do not ignore them!

Wind-Generated Waves vs. Tsunami Waves

Regular waves (caused by the wind) are very different from tsunami waves. Tsunami waves are much faster than wind-generated waves and they have a much longer wavelength (the distance from crest to crest). In the deep sea, tsunami waves are very small, but by the coast, they dwarf regular waves.

How Often do Tsunamis Occur?

Tsunamis are very rare. There are roughly six major tsunamis each century.

Where do Tsunamis Occur?

About four out of five tsunamis happen within the “Ring of Fire,” a zone of frequent earthquakes and volcanic eruptions roughly matching the borders of the Pacific Ocean.

Along the ring’s edges, giant slabs of the earth’s crust, called tectonic plates, grind together. Sometimes the plates get stuck, and pressure builds. Then, the plates can suddenly come apart and slam into a new position. The jolt causes an earthquake. If an earthquake lifts or drops part of the ocean floor, the water above it starts moving too. This triggers a tsunami.

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