Tsunamis can be divided into four types. That is, storm surges caused by meteorological changes, volcanic tsunamis caused by volcanic eruption, underwater landslides and tsnamis caused by underwater earthquakes.
Earthquake tsunami refers to the strong vibration caused by submarine topography sharp fluctuation. There are two forms of the mechanism: the “falling” tsunami and the “uplift” tsunami.
“Fall” type tsunami Some structural earthquake causes a sharp decline of a large area of underwater crust. The water first floods suddenly to rupture sunken space and form water accumulation on the top of sea. When the sea encounters bottom resistance, it will go back to the surface to produce compression wave, forming long waves and spreading out. The “fall” type tsunami caused by the motion of underwater crust shows the abnormal low tide phenomenon first. Earthquake in 1960 in Chillie tsunami belongs to this type of tsunami: some structural earthquakes cause the underwater crust increase sharply and consequently, the sea water rises together and forms a large scale of water accumulation on the top of uplift zone. Under the effect of gravity, the water must be kept in an equipotential surface to achieve a relative balance. So water wave area spread out and form choppy waves.
The tsunami waves caused by the motions of underwater crust in “uplift” tsunami show abnormal high tides first. On May 26th, 1983, the tsunami caused by an earthquake of magnitude 7.7 in Japan belongs to this type.
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