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What are tsunamis?

Historically, seismic ocean waves have been incorrectly called "tidal waves." Actually, these events are not related to tides in any way. Seismic ocean waves are now commonly referred to as tsunamis (Japanese for "harbour waves"). A tsunami is traditionally defined as a series of ocean waves with very long wavelengths that can travel great distances. However, tsunamis can also occur in large lakes; in deep oceans, tsunamis can reach speeds over 800 kilometres per hour. Tsunami wave heights near a shore average 9 meters, but have been recorded over 30 meters.

Why do tsunamis occur? Which are the main tsunamigenic forces?

Tsunamis principally result from tectonically generated earthquakes of magnitude greater than 6.5 and focal depth shallower than 50 km. However, not all undersea earthquakes are tsunamigenic (create tsunamis), as this depends on the nature and degree of displacement of the sea-water column.

The main tsunamigenic forces are the following:
Vertical displacements of a sea water caused by undersea earthquakes which involve abrupt movement of fault blocks on the sea bed;
Horizontal displacements of the water column as a result of earthquake movements (those which occur on land may generate a tsunami if coasts are impacted with sufficient violence);
Volcanic eruptions at sea (which must displace the surrounding water with considerable violence if a tsunami is to be generated);
Rapid mass movements (landslides) on the sea floor (these, however, are seldom responsible for tsunamis of any great size).

Where do tsunamis occur?

Of damaging tsunamis, 90 per cent occur in the Pacific Basin at an average of more than two each year. In the past 190 years the Hawaiian Islands have experienced more than 150 such events, and destruction has often been widespread. The USA suffers more than $26 million in property damage each year, and 500,00 people live in areas of the country at risk from tsunami waves 15m high, while 1.2 million are at risk from waves reaching 30m (White & Haas 1975). The west coast and Hawaiian Islands run the greatest risk. The Atlantic and Mediterranean Basins are by no means immune from tsunamis, but these are less common there and hence records are incomplete and return periods are largely unknown. Nevertheless, the eruption of Santorini (Greece) in 1500 BC resulted in tsunamis which travelled all over the eastern Mediterranean Sea, and the earthquake which occurred in 1775 near Lisbon, Portugal, is estimated to have caused 25.000 deaths by tsunami in that city and to have involved waves that arrived in the Caribbean.

What is the impact of tsunamis?

Tsunamis usually become dangerous as they approach a coast. When they do, they can carry the largest ocean vessels kilometres inland, inundate coasts, and drag entire communities out to sea as they recede. Tsunamis can be generated by any event that displaces a large volume of ocean water, such as an earthquake, volcanic eruption, or landslide.

How do they predict Tsunamis i.e.? What type of equipment is it used?

The predictions of tsunamis are taking place with two ways. The first is to use computers to predict how high tsunami waves will be along the coast for different kinds of earthquakes. When put on maps, these predictions give an idea of the best roads for people to take away from the beach if a tsunami happens.

The second type of prediction is made by the tsunami warning centres right after an earthquake occurs that might create a dangerous tsunami. They use seismographs to estimate the strength and location of the earthquake. They use tide gages near the earthquake to see how large the tsunami waves are. Past experience also tells them which areas along the coast tend to have the largest risk. Using computers, they also tell how long it will take the tsunami waves to reach these areas.

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