About Tsunami

About Tsunami

Tsunamis are a series of waves usually generated by movement of the seafloor. These movements are caused by different types of geophysical phenomena such as earthquakes, landslides and volcanic eruptions.The word tsunami is a Japanese word, represented by two characters: tsu, meaning, "harbor", and nami meaning, "wave".

Tsunamis are move at a speed equals to the square root of the product of gravity and the depth of the water. The tsunami waves behave very differently in deep water than in shallow water as their speed is related to the water depth. Tsunami waves form only a small hump, barely noticeable and harmless, which generally travels at a very high speed of 500 to 1,000 km/h. The tsunami's speed diminishes as it travels into shallower water to only tens of kilometers an hour, consequently increasing the wave height. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast forming the large destructive waves.

Tsunamis are most commonly generated by earthquakes in marine and coastal regions. Major tsunamis are produced by large (greater than 7 on the Richter scale), shallow focus (< 30km depth in the earth) earthquakes associated with the movement of oceanic and continental plates. They frequently occur in the Pacific, where dense oceanic plates slide under the lighter continental plates. When these plates fracture they provide a vertical movement of the seafloor that allows a quick and efficient transfer of energy from the solid earth to the ocean. When a powerful earthquake (magnitude 9.3) struck the coastal region of Indonesia in 2004, the movement of the seafloor produced a tsunami in excess of 30 meters (100 feet) along the adjacent coastline killing more than 240,000 people. From this source the tsunami radiated outward and within 2 hours had claimed 58,000 lives in Thailand, Sri Lanka, and India.

Generation of Tsunami

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Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

Wave Propagation

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Sumatra Animation
As a tsunami leaves the deep water of the open Ocean and travels into the shallower water near the coast, it transforms. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.

Run-up and Inundation

During the tsunami, maximum vertical height to which the water is observed with reference to sea level (spring tide or Mean sea level) is referred to as run-up. The maximum horizontal distance that is reached by a tsunami is referred to as inundation

The run-up and horizontal inundation during a tsunami can be highly variable in a local area depending on the underwater topography, orientation to the oncoming wave, tide level, and the magnitude of the tsunami. Because direct physical measurement of a tsunami wave would be a life threatening event, the most common method for determining tsunami wave height is by measuring the run-up, the highest vertical point reached by the wave. Run-up heights are measured by looking at the distance and extent of salt-killed vegetation, and the debris left once the wave has receded. This distance is referenced to a datum level, usually being the mean sea level. A measurement of run-up level is useful in determining extent of vulnerability of an area to tsunami waves.

Historic Tsunamis in India

The Indian coastal belt has not recorded many Tsunamis in the past. Waves accompanying earthquake activity have been reported over the North Bay of Bengal. During an earthquake in 1881 which had its epicenter near the centre of the Bay of Bengal, Tsunamis were reported. The earthquake of 1941 in Bay of Bengal caused some damage in Andaman region. This was unusual because most Tsunamis are generated by shocks which occur at or near the flanks of continental slopes. During the earthquakes of 1819 and 1845 near the Rann of Kutch, there were rapid movements of water into the sea. There is no mention of waves resulting from these earthquakes along the coast adjacent to the Arabian sea, and it is unlikely that Tsunamis were generated. Further west, in the Persian Gulf, the 1945 Makran earthquake (magnitude 8.1) generated Tsunami of 12 to 15 meters height. This caused a huge deluge, with considerable loss of life and property at Ormara and Pasi. The estimated height of Tsunami at Gulf of Cambay was 15m but no report of damage is available. The estimated height of waves was about 2 meters at Mumbai, where boats were taken away from their moorings and casualties occurred. A list showing the Tsunami that affected Indian coast prior to Sumatra Earthquake of December 26, 2004, is given in the below table.

Date Cause Impact
12th April, 1762 Earthquake in Bay of Bengal. Tsunami wave of 1.8m at Bangladesh coast
31st December, 1881 Magnitude 7.8 earthquake beneath the Car Nicobar Entire East coast of India including Andaman & Nicobar coast was affected by tsunami
27th August, 1883 Eruption of karkatoa volcano (Sunda Strait) Indonesia East coast of India was affected and 2 m Tsunami was reported at Chennai.
26th June, 1941 A 8.1 Magnitude earthquake in Andaman East Coast of India was affected by tsunami.
27th November, 1945 27th November, 1945 (21 Hour 56 Min. 50 Sec. UTC) in the Makran subduction zone (Baluchistan, Pakistan) (24.5N & 63.0E) with Mw of 8.1 West coast of India was affected by Tsunami.

Tsunamigenic Source Identification

The Pacific Rim countries are affected frequently by tsunamis (about 900 events in the 20th century) due to the high seismic activity (75 per cent of global activity) in the circum-Pacific belt. The Indian Ocean rim countries are likely to be affected by tsunamis generated in the two known tsunamigenic zones, viz., the Andaman-Nicobar-Sumatra island arc and the Makran subduction zone north of Arabian Sea (about 5 per cent or less). These zones have been identified by considering the historical tsunamis and earthquakes, their magnitudes, location of the area relative to a fault, and also by tsunami modelling. The east and west coasts of India and the island regions are likely to be affected by Tsunamis generated mainly by subduction zone related earthquakes from the two potential source regions, viz., the Andaman-Nicobar-Sumatra Island Arc and the Makran subduction zone north of Arabian Sea. These two possible locations with major plate boundaries are shown in the blue ellipses in the figure below. However, in spite of infrequent occurrence of tsunamis (about 6 events reported in the 20th century) in the Indian Ocean, they could occur at any time and could be very devastating.