The word "tsunami" comes from the Japanese words tsu(harbor) and nami(wave), the original meaning of "harbour wave", "tsuna-mi" has gradually become an international expression term.
Tsunami is a powerful destructive wave. When an earthquake occurs at the bottom of the sea, the seawater fluctuates sharply due to the dynamic force of the seismic waves, forming shock waves, advancing forward, and inundating the coastal areas one by one, which is called tsunami. Tsunamis are usually caused by submarine earthquakes of magnitude 6.5 or above, with the source within 50 km below the sea floor. The wave length of the tsunami is larger than the maximum depth of the ocean, and the wave propagation is not much impediment near the seabed, regardless of the depth of the ocean, the wave can travel through the ocean, the speed of the tsunami propagation in the ocean is about 500 ~ 1000km/h, and the distance between two adjacent waves may be as far as 500 ~ 650km. When a tsunami wave enters the continental shelf, the depth becomes shallow and the wave height suddenly increases, this wave motion can cause waves up to tens of meters high and form a "water wall". The whistling wave water wall repeats every few minutes or tens of minutes, destroying the embankment and submerging the land, endangering the safety of human life and property, with great destructive force.
In addition, underwater volcanic eruptions, landslides, mudslides and other sudden changes in the seabed topography and man-made underwater nuclear explosions can also cause tsunamis. But the most common are tsunamis caused by earthquakes.
The waves of a tsunami are different from those of the sea, generally, the wave only fluctuates in a certain depth of water, and the amplitude of the wave decays rapidly with the depth of water; the fluctuation of water caused by tsunami is the fluctuation of the whole water layer from the sea surface to the seabed. Of course, the initial increase in the water level near the epicenter of the mid-ocean is only 1 to 2m, such fluctuations can range in wavelength from tens to hundreds of kilometers in deep-water oceans, the period varies in the range of 2 to 200 minutes, the most common is 2 to 40 minutes, and the propagation speed can reach 1000 km/h, which is larger than the speed of large jet airliners, so tsunamis will not cause disasters in deep-sea oceans, and even sailing ships can hardly detect such fluctuations. When a tsunami occurs, it is safer to be out at sea. However, when the tsunami wave enters the continental shelf, its depth sharply becomes shallow, energy concentration and wave height sharply increases, when enters the narrow shallow waters, the tsunami wave carries tremendous energy directly into the bay and the shore, at this time, there may be waves of more than 10 to 20 meters, which has been impacted by the trend of overturning mountains and seas, especially at the top of funnel-shaped bay. If the tsunami wave is reflected repeatedly in the seaport and the bay, it will also induce the inherent vibration of the sea water in the bay to trigger the pseudo-tide, which will increase the wave height and cause greater harm. Such waves can cause devastating disasters, as shown in Fig. 5-1 and Table 5-1.
Figure 5-1 Tsunami Wave Propagation Time 7K Intention
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