Characteristics of the tsunami
Tsunami wave has a long wavelength, huge energy and fast propagation speed.
One of the characteristics of tsunami is its speed. The deeper the sea water is, the faster the tsunami is. The deeper the water is, the more water is surging due to the fluctuation of the seabed, and the faster it moves on the surface after the tsunami. If the water depth of the earthquake is 5000m, the tsunami and the jet plane can fly at a speed of about 800km/h, if they move to a depth of 10m, the speed will slow down to 40km/h. As the front waves decelerate and the back waves push over and overlap, the waves from the tsunami to the shore rise sharply, if the coastal seabed is shaped like a V, the waves from the tsunami will be even higher.
Manifestation of the tsunami
First, the coastal waters, islands or bays have abnormal ebb tide or rivers have no water, and then the sea suddenly sweeps over and rushes to land; secondly, the sea water rises sharply and suddenly forms a water wall with a height of tens of meters, which surges to the coastal land with a loud rumble, and then the sea water suddenly recedes.
Tsunami wave belongs to the ocean long wave, once generated in the source area, in the absence of island groups or large shoals, shallow water shelf obstruction, generally can spread for thousands of kilometers with little energy attenuation, therefore, tsunami disasters can also occur thousands of kilometres away. For example, the great tsunamis off the coast of Japan in 1896 and 1933, which crossed the Pacific Ocean, also affected Hawaii, San Francisco and Chile. The mainland of China is surrounded by a wide continental shelf and island chain, by the time the tsunami wave passes over them and reaches the coast, most of the energy has disappeared, therefore, tsunami generally does not cause serious harm to China.
Classification of the tsunami
Compared with the scene of the disaster, tsunamis can be divided into two categories: remote tsunami and local tsunami.
A remote tsunami is a tsunami that travels across the ocean or travels far away, also called a transoceanic tsunami. Tsunami wave belongs to the ocean long wave, once generated in the source area, in the absence of island groups or large shoals, shallow water shelf obstruction, generally can spread for thousands of kilometers with little energy attenuation, so can spread far away, tsunami disasters can also occur thousands of kilometres away probably. For example, the tsunami in Indonesia at the end of 2004 affected Sri Lanka thousands of kilometers away, and the tsunami in Chile in 1960 also caused severe disasters in Hawaii and Japan thousands of kilometers away. Due to the long arrival time of the transoceanic tsunami, there is still time to take measures to mitigate disaster losses. The tsunami is not easy to attract attention because of its low water depth and small wave fluctuation, but when it reaches the shallow water area near the shore, the huge energy makes the waves rise suddenly, forming a "water wall" with great energy, which is as high as ten meters or even tens of meters, sweeping over the land and often causing serious damage to life and property.
In 365 AD, Roman Empire historian Amianos Marcelus marveled at a tsunami in Alexandria Harbor:“ the sea was pushed back, the sea retreated, and a large area of the seabed was exposed, leaving behind many marine life…...unexpectedly, a great deal of sea water poured back, swallowed up and killed thousands of people... big waves rolled over some big ships and even pushed some ships to land 3 km off the coast.
Local tsunami
Local tsunamis are relatively close from their origins to the affected coastal areas, and the time for waves to reach the coast is relatively short, only a few minutes. Tsunami early warning time is short or there is no early warning time at all, which makes it difficult to defend, and often causes extremely serious disasters.
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