In most cases, the surface of the ocean is falling because the trough of tsunami waves arrive the coast first. Trough is the lowest point of wave. If it lands first, the sea will definitely fall. At the same time, the tsunami waves are different from general waves because the wave length is very large. So when troughes land, it will be a long time for crests to arrive.
In addition, if this happens near the epicenter, it could be another cause: when the earthquake occures, there is a large area of land at the bottom of the sea up and down. At this time, the sea water near the epicenter also rises and falls consequently, forming a tsunami.
In addition, the submarine volcano, landslide, artificial nuclear explosion as well as meteorite impacts can also cause tsunami. What's more, the height of “water wall” caused by meteorite impacts can reach up hundreds of feet and occur at any waters, not necessarily in eartuquake zones. But tsunami caused by meteorite impacts happenes only once in a thousand years.
Tsunami is a very destructive and devastating wave. Although its arrival is somewhat prophetic, it will not be noticed due to its depth in the outer sea and small fluctuations of the waves. So when it comes, people are always unprepared and pay a heavy price.
A tsunami, usually caused by underwater earthquake, is devastating waves. A tsunami occurs when there is an earthquake of more than magnitude 6.5 underwater within 50 km. Besides, underwater or coastal landslide or volcanic eruptions are also major causes of tsunamis.
After a shaking, the shock wave will form widening circles on the surface of sea, which can spread far away. The wavelength is larger than the maximum depth of the ocean and its motion can produce huge waves of waters, which can reach to tens of meters. Faced with this tremendous energy, any human intervention is meaningless, that is to say, people have no way to avoid.
Every tsunami will cause huge loss of lives and properties. However, tsunami belongs to natural disasters and people can’t avoid it. If tsunami disaters can be predicted in advance, people will be able to escape before it. but under today’s undeveloped predictions, people can only accept it.
Tsunamis in the world always occurs in Hawaii Islands, Alsaka, the Kamchatka - Thousand Island, islands, Japan and surrounding areas, China and its adjacent area, Philippines, Indonesia, New Guinea - Solomon islands, New Zealand - Australia and the south of Pacific area, Columbia – Ecuador and Chile coast, Central America and the USA, the west coast of the Canada and northeastern Mediterranean coastal areas, etc.
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