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Formation and damage processes of tsunami

2019-01-10  |   Editor : houxue2018  

Formation process of tsunami

The formation of tsunami can be roughly divided into four stages: generation, propagation, amplification and ascension.

The main cause of tsunami is earthquake, once the crust has vertical disturbance, the sea surface follows it, and then affected by the gravity field, waves are then transmitted from the source to the surrounding areas by a gravitational field. In the process of transmission, because the energy attenuation is very small, the tsunami can transmitted the energy generated by the earthquake through the tsunami wave, from deep water to shallow water, across the ocean, to the other side. When the tsunami wave approaches the shore, the effect of the bottom bed will cause the tsunami wave to be lifted along with the terrain due to the shallow water depth, in addition, the velocity of the tsunami wave will also be slowed down due to the shallow water depth, therefore, the tsunami will be slower as it gets closer to the shore, and the tsunami behind will accumulate, resulting in the amplification of the whole tsunami wave heights. This is the third process: tsunami amplification. The destructive power of tsunami with amplified wave heights to coastal areas increases with the increase of wave heights. Next, the tsunami will reach land and begin to destroy it. A tsunami can reach the highest point on land, called ascent. In the process of ascending, it was the tsunami that caused the disaster. The inland tsunami will move forward in a flood-like manner, even thousands of meters away. It can take up to an hour for a tsunami to reach inland before it subsides. Unlike floods, when a tsunami moves forward, the front edge of the tsunami will first destroy buildings or structures by impact, or weaken the strength of structures. The following tsunami itself contains a strong turbulence mechanism, which will erode the surface coverage and cause serious loss of road or housing roadbed. It is more destructive than a flood. In severe cases, the tsunami will travel upstream and then return to the sea, which creates a second degree of erosion.

Damage process of tsunami

Energy from the interior of the water pushes the water upward, forcing energy to propagate horizontally along the water surface under the action of gravity, and further away from the original crustal movement site. In the case of earthquakes, the tremendous force of the earthquake gives the tsunami incredible speed.

The ability of a tsunami to maintain speed is directly affected by the depth of the water. The deeper the water, the faster the tsunami moves; conversely, the slower it moves. Unlike normal waves, the driving energy of a tsunami travels through water rather than moving on the surface. So when a tsunami moves through deep water at hundreds of kilometers per hour, it is almost imperceptible above the water line. Tsunamis usually do not reach a height of 1 m until they are close to the coast.

Usually, a tsunami comes to shore as a series of powerful and rapid tides, rather than as a single giant wave. When a tsunami reaches land, it hits shallower bodies of water. Shallow water and coastal water act as a compression of energy passing through the water body. As the wave velocity decreases, the wave height increases significantly (compressed energy pushes water upward). As a typical tsunami approaches land, its speed drops to about 50km/h and its wave height may reach 30m above sea level. In this process, the wavelength decreases significantly with the increase of wave height. Then there may be a raging tide-a large vertical wave with a sharp roll in the front. Fast floods usually follow rage tides, making them particularly destructive. After the initial shock, other waves will follow within 5 to 90 minutes - after traveling a long distance as a series of waves, the tsunami waves begin to drain all their energy onto land.

During the tsunami impact, the most dangerous places are within 1.6km of the coastline (due to floods and scattered debris) and less than 15m above sea level (due to the height of the waves causing the impact).

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