Tsunami: Causes, Effects
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Tsunami: Causes, Effects

Tsunami: its causes and disastrous effects.

Tsunami comes from the Japanese word tsu which means harbor and name which means wave. It is a series of waves generated when water in a lake or sea is rapidly displaced on a massive scale.

What causes these waves and how are they generated?

  • Earthquakes, landslides, volcanic eruptions and large meteorite impacts all have the potential to generate a tsunami
  • Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the earth’s crust can occur at plate boundaries. Subduction earthquakes are particularly effective in generating tsunamis and occur where denser oceanic plates slip under continental plates in a process known as subduction.
  • Submarine landslides, collapses of volcanic edifices may also disturb the overlying water column as sediment and rocks slide down slope and are redistributed across the sea floor. A violent submarine eruption can uplift the water column and form a tsunami.
  • Tsunami move the entire depth of the ocean rather than just the surface, so they contain immense energy, propagate at high speeds and can travel great trans-oceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometers from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the seismic wave generated by the originating event arrives.

Tsunami has been, at various times, associated with the following:

  • An earthquake may be felt.
  • Large quantities of gas may bubble to the water surface and make the sea look as if it is boiling.
  • The water in the waves may be unusually hot.
  • The water may smell of rotten eggs or of petrol or oil.
  • The water may sting the skin.
  • A thunderous boom may be heard followed by a roaring noise as of a jet plane or a noise akin to periodic whop-whop of a helicopter or a whistling sound.
  • The sea may recede to a considerable distance.
  • A flash of red light might be seen near the horizon. As the wave approaches, the top of the wave may glow red.

In instances where the leading edge of the tsunami wave is a trough, the sea will recede the coast half of the wave’s period before the wave’s arrival. If the slope is shallow, this recession can exceed many hundreds of meters.

Regions with a high risk of tsunamis can warn the general population before the wave reaches land. Computer models can roughly predict tsunami arrival and impact based on information about the event that triggered it and the shape of the seafloor and coastal land. One of the early warnings comes from nearby animals. Many animals sense danger and flee to higher ground before the water arrives.

The above adapted from:

    ECOLOGY by Rhodora S. Agote et. al. (no date of publication and publisher’s name)

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One of the worst tsunami disasters in modern history occurred on December 26, 2004 after a 9.1 magnitude earthquake hit the Indian Ocean, off the coast of Sumatra, Indonesia, the third strongest earthquake in recorded history. Behind the 9.5 magnitude earthquake in 1960 that hit Chile and the 1964 Good Friday earthquake that hit Alaska with a magnitude of 9.2.

The 2004 Indian Ocean earthquake was so powerful, that it is estimated that the entire Earth vibrated 1 cm.

The 2004 earthquake set off one of the most deadly and destructive tsunamis in known history. The earthquake lasted about 10 minutes and scientists estimate that sea floor was raised by several meters, almost 10 feet.

The tsunami hit Sumatra ½ hour after the earthquake, Thailand about 1.5 hours after the quake and the tsunami hit Sri Lanka approximately 2.5 hours after the earthquake. Maximum wave heights in Sumatra were estimated to be 10 to 15 meters (33 to 49 feet).

The death toll was terrible with 168,000 dead in Indonesia, 35,000 in Sri Lanka, 18,000 in India, and 8,000 were killed in Thailand. All in all, more than 230,000 people were killed during this disaster.

Because the earthquake happened during the Christmas holiday, thousands of foreign tourists were also killed during the tsunami. In Thailand, 26 Australians were killed while on vacation. 

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