A Tsunami is a wave or series of waves produced by sudden vertical displacement of the column of water. The displacement may be caused by seismic activity, volcanic eruption and a landslide above or below water.
Tsunami waves are sometimes also referred to as tidal waves due to its long wavelength. However, it is not related to the attraction of the sun and moon. Tsunami waves are generated in oceans, bays and other water bodies.
The word Tsunami comes from Japanese Tsu (harbour) and Nami (waves) because it mainly affects coastal areas and harbour.
In the 1990s, around 14 Tsunami events occurred throughout the World, it did not cause much death and destruction, but the Tsunami of 26, December 2004, perturbed the entire world. It struck due to the largest underwater earthquake ever recorded off the coast of Northern Indonesia. It generated a devastating tsunami that swept the northern Indian Ocean and killed thousands of people who had never anticipated such an event.
Causes of Tsunami
Earthquakes:
It is the most common cause behind the origin of a tsunami. Over 80 per cent of all tsunamis that occurred in the Pacific Oceans were generated due to seismic activity. When the earth’s crust is displaced by several metres during an underwater earthquake, it covers thousands of square km of area and induces tremendous potential energy to the water body.
A tsunami can only be triggered by earthquakes that originate mainly in the upper 100 km of the oceanic crust. It has been found that earthquake-induced tsunami is associated with the MS Magnitude of 7.0 or greater on the Richter scale.
Most tsunami-forming earthquakes are shallow foci and occur at a depth of 0- 70 km. The greater the vertical displacement, the greater the amplitude of the tsunami, therefore, thrust faults associated with subduction zones are the preferred mechanism for the generation of tsunamis.
Landslides:
The topography along the continental shelf and margins are often very steep, particularly near ocean trenches. Sediments lying over the continental shelf move under gravity down the slope, generating marine landslides. It can form small to mega-tsunamis, whose magnitude may even surpass those generated by earthquakes. The most notable example is Grand Bank Tsunami (1929).
Volcanic Eruptions:
The contribution of a volcanic eruption in the generation of tsunamis is relatively lesser (4.6%) than seismic events and underwater landslides. Explosive volcanism with caldera formation can cause tsunamis. It is mainly limited to a few areas, such as the Japanese-Kuril Islands and the Philippine and Indonesian archipelagos.
Comets and Asteroids:
Any asteroid and comet entering into the atmosphere at a shallow angle is more likely to reach the ocean without breaking and can create a cavity that would be ten times greater than the diameter of the objects. It can generate waves in a different direction that may result in tsunamis.
Mechanism and Propagation of Tsunami
When an earthquake strikes undersea, the vertical displacement of the seafloor also displaces the overlying water. All the water rushes towards the point of displacement to fill the depression created by the downthrown region; as a result, water recedes from the shore.
Once the surface rocks are adjusted, all water rapidly moves towards the shore, forming tidal waves. If the seafloor is lifted, then it will create a hill that would collapse eventually and sends waves travelling towards shore, leading to a tsunami.
In the open sea, tsunami waves propagate fast, low and long wavelengths, although they resemble similar to other waves of the sea. Tsunami, travel across the open sea in a series of long waves with low crests (1-2 m high) (people on a ship would not be able to detect the deadly tsunami passing below them).
A tsunami does not lose energy during its travel to the shore. When a tsunami reaches close to the shore, it enters shallow waters, it slows down, a portion of the wave closest to the coast or beach slows down, but its back maintains fast speed forming higher and steeper waves. The tsunami produced by the Krakatoa explosion caused a tsunami with a wave height of 98 feet (30 metres).
Tsunami Hazard
The regions where tsunami occurs frequently are the Pacific Ring of Fire, the Mediterranean Sea, the Caribbean Region and the Indian Ocean. The following are the major hazards associated with tsunamis.
Inundation: tsunami waves can push a lot of water onto the shore, leading to a flood-like situation.
Destruction and Damage to Property: It destroys anything in its paths such as boats, buildings, houses, telephone lines and other infrastructure.
Death and Fatalities: One of the worst impacts of tsunamis is the loss of human life. December 2004 Indian Ocean Tsunami caused death to 30, 974 people in Sri Lanka, 122, 232 in Indonesia, 6400 in India and 5395 in Thailand.
Fires and Explosions: It causes damage to oil and natural gas storage and pipelines resulting in intense fires and explosions. In Japan, due to the tsunami of 2011, various oil tankers at ports and gas cylinders at industrial complexes were damaged and caused massive fires and explosions. Fukushima Daiichi nuclear power plant, 150 miles northeast of Tokyo, was severely damaged by the earthquake and tsunami, impairing its cooling systems and resulting in a series of explosions, meltdowns – and the world’s worst nuclear accident in 25 years.
Monetary Loss, Diseases and Psychological Problems: It also causes a lot of monetary loss to individuals, families and the government. The victims may also suffer from various diseases due to stagnant water and decomposing dead bodies of humans and animals. Many people also tend to develop psychological problems after the event.
Risk Reduction
The most important aspect of tsunami preparedness is its detection and early warning. Indian Ocean tsunami compelled world scientists to develop a widespread tsunami warning system. The international community undertook the following activities to develop an early warning system:
- Seismic stations started collecting data on undersea earthquakes and transmitting it to the monitoring centres such as Pacific Tsunami Warning System (PTWS) situated in Hawaii.
- A tsunami watch is released for regions which will be affected later by the tsunami event.
- Tide gauges were monitored to detect the changes in the waves.
- NOAA (US National Oceanic and Atmospheric Administration) developed bottom pressure sensors to measure wave characteristics and pressure changes.
- Through DART (Deep Ocean Assessment and Reporting of Tsunami) using sea surface Buoys, NOAA transmits signals to satellites, transferring data to shore-based Regional Warning Systems in the Pacific and Indian Oceans.
Other measures for risk reduction include:
- Site planning and management – designating or zoning tsunami-prone areas and changing the land use accordingly.
- Constructions of structures and coastal homes at higher elevations.
- Water breakers to minimize the velocity of waves.
- Construction of community halls and shelters.
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