What is a Tsunami? So far how many Tsunamis have occurred in India?

Tsunami means harbor wave in Japanese. These are huge waves of water which are caused by Earthquakes, volcanic eruptions, and other underwater explosions, landslides, meteorite impacts etc. These mentioned activities displace large quantities of water, triggering destructive waves which can measure up to 10 meters tall. They are series of waves with an interval ranging from 5 minutes to few hours.

They are very destructive in nature and can wipe out coastal areas, small islands, and ocean basins too. The Indian Ocean tsunami in 2004 was one of the fatal disaster killing approximately 2,30,000 people. Tsunamis are not tidal waves as tidal waves are influenced by the gravity of the moon and sun whereas tsunamis occur due to water displacement.

Causes of Tsunami

The displacement of a substantial volume of water or perturbation of the sea is the basic mechanism which causes the tsunami. These displacements are generally caused by Earthquakes, volcanic eruptions, nuclear tests etc.

• Earthquakes: When Earth's crustal deformation takes place, the sea floor abruptly deforms and vertically displaces the overlying water. Tsunamis have smaller heights when compared to their wavelengths which are up to 100 km long. They grow taller when they reach shallow regions. The 2004 Indian Ocean earthquake whose magnitude was 9.1-9.2 is the recent example of a powerful earthquake that created a huge tsunami.

• Landslides: Tsunamis are also caused when submarine landslides occur. Huge masses of the land rush into the water body displacing large amounts of water causing tsunamis. These phenomena occur mainly in the shallower regions of the coastlines. These are called as megatsunamis.

• Man-made: This is the age of nuclear weapons. During the world war period, attempts were made to generate an artificial tsunami by using nuclear weapons near the enemy coastlines. Nuclear tests are conducted in isolated ocean areas which vaporize most of the water and creates huge vertical displacements of large volumes of water.

History

The first known Tsunami occurred around 1600 BCE which destroyed Minoan civilization on Crete. One of the oldest tsunamis occurred in 479 BCE in Greece. Japan has the longest records of tsunamis.

The major tsunamis occurred in India are:

• A local tsunami flooded the Great Rann of Kutch in 1819

• An earthquake with a moment magnitude of 8.1 occurred in British India at 5:26 PST on 28 November 1945. It resulted from a fault rupture near the Makran Trench.

• The 2004 Indian Ocean tsunami is one of the deadliest tsunamis. It was triggered by an earthquake of magnitude 9.2 in Indonesia. It affected many countries like Indonesia, Thailand, India, Sri Lanka, Maldives and even a part of Eastern Africa. With waves reaching a height of 33 meters, it killed approximately 230,210 people.