The term “tsunami” comes from the Japanese language and means “big wave in the bay”. It occurs during an earthquake in the sea or ocean when conditions are created for the formation of powerful waves in the water column. These waves diverge in all directions from the epicenter onto the surface of the seabed (Mai, 2019). The focus is located in the Earth at a certain depth, most often within several tens of kilometers. The deformations of rocks arise in the area of the focus, which leads to ruptures and the release of accumulated energy (Abdolali et al.,2019). This paper aims to explain the phenomenon of a tsunami, and its speed, and describe how it is generated, how it travels, and reaches the coast.
Not every earthquake in the ocean can cause a tsunami. A giant wave is formed when there is a sudden, sharp displacement of the ocean floor, and especially often with an instantaneous vertical rise of one of the tectonic rupture wings (Röbke and Vött, 2017). A detailed analysis of the conditions for tsunami excitation showed that the maximum amplitude of tsunami waves occurs when rocks are displaced at a depth of about 10 km (Chagué-Goff et al., 2017). In case the hypocenter is located deeper, the amplitude gradually decreases. While in the ocean on a ship or yacht, a long surface wave may not be noticed, the situation changes when it approaches a shallow shore with a wide and gentle underwater slope.
Thus, a sharp and instantaneous displacement of the bottom causes a simultaneous rise of the ocean water column and waves on the surface, diverging sideways at a speed of up to 600-800 km / h (Attary et al., 2017). The deeper the ocean, the higher the speed of the waves, which is roughly proportional to the square root of the depth (Minowa et al., 2019). The wave speed is determined only by the local ocean depth and is equal to c = (g • h) 1/2, where g ≈ 10 m / s2 is the acceleration of gravity. For an average depth of the World Ocean, is about 4 km, c ≈ (10 m / s2 4000 m) 1/2 ≈ 200 m / s, which is 720 km / h (Attary et al., 2017). Consideration of details, the exact shape of the earthquake source, water compressibility, and corrections for the Earth’s rotation, significantly complicates the solution, but the main result remains practically unchanged.
The energy of the wave is redistributed since the friction of the water on the bottom slows down the movement of its lower part, while its upper part moves at a higher speed. This process begins to develop when the depth reaches about half the wavelength (Sassa and Takagawa, 2019). When approaching the coast, both the speed of the wave and its length decrease. For example, at depths of about 1 km, the wave speed is 350-360 km / h, and at a depth of 50 m, less than 100 km / h (Minowa et al., 2019). When the lower part of the wave begins to slow down, the wave grows, increasing its height, and all of its energy is concentrated on a relatively narrow front. A white breaker appears on the crest of a growing wave, and it acquires an asymmetric shape: the inner side is concave and steep, and the outer, facing the ocean, is flatter.
At the tsunami wave, the crest is crowned with a giant breaker, and the wave itself, 5, 10, or 30 m high, falls onto the shore with the entire mass of a giant water wall. The seething water rushes forward rapidly, sweeping away everything in its path (Tappin, 2018). If a wave enters a narrow bay, then its height increases several times, forming a water wall, the impact of which on the shore is like a salvo of hundreds of guns. Gradually, the force of the wave dries up, and the water returns to the ocean, carrying all floating objects, such as cars, animals, and people.
The day and hour of an earthquake cannot be predicted since this is a non-linear process. However, it is possible to establish areas where the risk of an earthquake is high, and to determine its probable strength, that is, to carry out seismic zoning of the territories of varying detail (Takayama et al., 2018). The main places of occurrence of tsunamis are the Pacific Ocean, the periphery of which accounts for more than 80% of tsunamis (Gusiakov, 2020). The famous Ring of Fire in the Pacific Ocean is characterized not only by a large number of active volcanoes but also by frequent strong earthquakes and a chain of deep-sea trenches.
A tsunami is a gravity-driven wave arising from large-scale short disturbances, such as underwater earthquakes, volcanic eruptions, landslides, falling rock fragments, or underwater explosions. In places called active continental margins, heavy, cold oceanic plates sink beneath the lighter and higher continental plates. The processes of interaction between the plates lead to earthquakes, volcanic eruptions, and the occurrence of tsunamis in the ocean. In the future, there are intentions to make a global network of tsunami warnings and equip it with the latest seismographs and special sensors, on which the recording equipment will be placed.
Reference List
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