Tsunami come about when the sea floor deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation such as moving of the plates at fault line; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its new position. More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in huge amount of water displacement, owing to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami. Many countries in the word are near fault lines such as Japan and US. Singapore is lucky that there are not fault lines near it and is protected all round.
Tsunami caused by volcanic eruptions:
There are two different ways that volcanoes can cause Tsunami waves.One of them is that when there is an eruption, huge amounts of ashes and debris and being deposited in the sea. This causes the displacement of water which results in the tsunami.
Tsunamis can also be caused by submarine volcanoes. These underwater volcanoes collapse downwards or spew forth lava heating the surrounding water quickly during eruption. Imagine this as a sort of implosion. A vacuum will be created and the movement of water which result in tsunamis
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