Tsunami and Its Causes

Tsunami and Its Causes

Tsunami meaning "harbour wave" in literal translation comes from the Japanese characters for harbour ( tsu ) and wave ( name ). A tsunami also called seismic sea waves, is one of the most powerful and destructive natural forces. It is a series of extremely long waves caused by a large and sudden displacement of the ocean due to earthquake, volcanic eruptions etc. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that can last for several hours or days.

Global distribution of Tsunami

Globally, 70% of the confirmed tsunami sources have been in the Pacific Ocean, 9% in the Caribbean Sea, 15% in the Mediterranean Sea and the Atlantic Ocean and 6% Indian Ocean. Most of these Tsunamis were generated by earthquakes.

Tsunamis are frequently observed along the Pacific ring of fire, particularly along the coast of Alaska, Philippines, Japan and other islands of South Asia and Southeast Asia including Malaysia, Indonesia, Myanmar, Sri Lanka and India etc.

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Characteristics of Tsunami

  • Tsunamis are among Earth's most infrequent hazards and most of them are small and nondestructive
  • Tsunamis generally consist of a series of waves, with periods ranging from minutes to hours
  • Tsunamis radiate in all directions from the point of origin and they can cover entire ocean basins.
  • There is no season for tsunamis. We cannot predict where, when or how destructive the next tsunami will be.
  • Not all tsunamis act the same. And, an individual tsunami may impact coasts differently. A small tsunami in one place may be very large a few miles away.
  • Most tsunamis are caused by large earthquakes. Though, not all earthquakes cause tsunamis.
  • Tsunamis are waves generated by the tremors and not by an earthquake itself.
  • The effect of Tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high.
  • A tsunami can strike any ocean coast at any time. They pose a major threat to coastal communities.
  • The speed of the wave in the ocean depends upon the depth of water. It is more in the shallow water than in the ocean deep. As a result of this, the impact of a tsunami is more near the coast and less over the ocean
  • Over deep water, the tsunami has very long wavelengths (often hundreds of kilometres long) when a tsunami enters shallow water, its wave-length gets reduced and the period remains unchanged, which increases the wave height.
  • Tsunamis have a small amplitude (wave height) offshore. This can range from few centimetres to over 30 m height. However, most tsunamis have less than 3 m wave height.

How is a tsunami different from a wind-generated wave?

Most ocean waves are generated by wind. Tsunamis are not the same as wind waves. First of all, they have different sources. Also while wind waves only affect the ocean surface, tsunamis move through the entire water column, from the ocean surface to the ocean floor. Waves can also be described based on their wavelength (horizontal distance between wave crests), period (time between wave crests), and speed. These characteristics highlight the differences between wind waves and tsunamis.

Tsunami Wind Wave
Source Earthquakes, landslides, volcanic activity, certain types of weather, near earth objects Winds that blow across the surface of the ocean
Location of energy Entire water column, from the ocean surface to the ocean floor Ocean surface
Wavelength 100-500 Km 20-30 meters
Wave Period 5 minutes 2 hours 5-20 seconds
Wave Height 10-30 meters from centimeters to few meters
Wave Speed 800-900 Kmph (in deep water), 30-50 Kmph (near shore) 10-100 Kmph

Causes of Tsunami

A tsunami is caused by a large and sudden displacement of the ocean. Large earthquakes below or near the ocean floor are the most common cause. But landslides, volcanic activity, near earth objects (e.g., asteroids, comets), certain meteorological conditions and nuclear tests can also cause tsunamis.

Earthquake - Tsunami can be generated when the sea floor ruptures abruptly due to tectonic earthquakes, causing vertical displacement of the overlying water. Most of the earthquakes which generate tsunamis occur on thrust faults. These earthquakes occur mainly in the areas where tectonic plates move toward each other in subduction zones.

As per data, ten to fifteen percent of the most damaging tsunamis are generated by strike-slip earthquakes involving a horizontal movement of the earth.

Example - 2004 Indian Ocean Tsunami was an earthquake-induced Tsunami, caused by an earthquake (Mw 9.2) in the Indian Ocean.

Landslides - landslide is a general term that involves the ground movement of different types, including rock slide, block slide, debris flows, avalanches, and glacial calving (referring to the breaking off of large pieces of ice from a glacier).

Tsunamis can be generated when a landslide enters the water and displaces it. Such generation of Tsunami depends on the amount of rock material that displaces the water, the speed with which it is moving, and the depth it moves to.

Landslide-generated tsunamis may be larger than seismic tsunamis near their source, but they usually lose energy quickly and rarely affect distant coastlines. A landslide big enough to cause a transoceanic tsunami has not occurred in the recorded history.

Example - 1998 Papua New Guinea Tsunami was generated by a landslide cause by an earthquake.

Volcanoes - volcanoes generated Tsunamis are very infrequent, both above and below water. However, different types of volcanic activity can displace enough water to generate tsunamis e.g. submarine explosions, caldera formations etc.

Like other non-seismic tsunamis, such as those generated by landslides, volcanic tsunamis usually lose energy quickly and rarely affect distant coastlines.

Example - 1883 Indonesia Tsunami was caused by the explosion of Krakatau volcano.

Near Earth Objects It is very rare for a near earth object like an asteroid or comet to reach the earth and its potential to generate Tsunami is still uncertain, as there are no records of a Tsunami caused by near earth objects, in recent human history. However, scientists are of the opinion there are two ways near earth objects could generate a tsunami.

Large objects (more than 1,000 meters in diameter) that make it through Earth s atmosphere without burning up could hit the ocean, displacing water and generating an impact tsunami.

If this happens above the ocean, the explosion could release energy into the ocean and generate an airburst tsunami.

Meteotsunamis - Some meteorological conditions, for example, air pressure disturbances often associated with fast moving weather systems, can displace bodies of water enough to generate Tsunamis. These meteotsunamis are similar to tsunamis generated by earthquakes, but usually with lower energies.

The development of these Meteotsunamis depends largely on the direction, intensity of air pressure and ocean depth. Meteotsunamis are regional, and it is found in some part of the world frequently due to regional factors such as topography of earth's surface both below and above the ocean.

Example 2013 New Jersey, USA Tsunami caused by a high-speed windstorm associated with thunderstorms.

Nuclear Weapon or tests it is an example of man-made disaster. Massive explosions created by a nuclear weapon of nuclear tests have the potential to cause Tsunami. There have been dangers of using this as a tectonic weapon.

There has been considerable speculation on the possibility of using nuclear weapons to cause tsunamis near an enemy coastline. In fact, In World War II, the New Zealand Military Forces, in a failed attempt, tried to create small tsunamis with explosives.

Effects of Tsunami

After the tsunamis reach the coast, an enormous energy stored in them is released which causes colossal loss of lives as well as the infrastructure of the place. As the port cities are economic hubs and densely populated the damage caused by the tsunami is devastating.

The Tusanami of 2004 in the Indian Ocean is one of the devastating natural disasters in the modern time. It took a toll of nearly 230000 people leaving in the coasts of Indian Ocean.

  • Unfortunately escaping a tsunami is nearly impossible. Hundreds and thousands of people are killed by tsunamis, most commonly by drowning, electrocution, explosions from gas and collapsing of buildings etc.
  • Flooding and contamination of drinking water can cause disease such as Malaria to spread in the tsunami-hit areas.
  • Tsunamis not only destroy human life, but also have a devastating effect on animal and plant life and other natural resources. A tsunami changes the landscape. It uproots trees and plants and destroys animal habitats.
  • Contamination of soil and water is the second key environmental impact of a tsunami.
  • There may be nuclear pollution due to radiation resulting from damaged nuclear plants, as it happened in Fukushima, Japan in March 2011.
  • Tsunamis are extremely dangerous to coastal life and coastal property. They produce unusually strong currents, rapidly flooding the land and causing great damage to coastal property and life.
  • The flow and force of the water and the debris it carries can destroy boats, vehicles, and buildings and other structures as the tsunami moves across the land.
  • Victims of tsunami events often suffer psychological problems such as PTSD (Post Traumatic Stress Disorder) which can last for days, years or an entire lifetime.
  • Massive economic costs hit communities and nations when a tsunami happens., severely affecting the economy of the nation.
  • Poor nations are more prone to large-scale destruction as the infrastructure are not well developed, and warning systems are not robust or unavailable. Also, their ability to cope with such massive disaster remains inadequate.
  • The water can be just as threatening (if not more so) as it returns to the sea, taking debris and people with it. Flooding and dangerous currents can last for days.

Tsunami Early Warning System

Tsunami is the most unpredictable natural disaster in the world and to prevent the Tsunami is next to impossible. Hence, the only way to effectively mitigate the impact of a tsunami is through an early warning system.

Tsunamis are detected in advance using a tsunami warning system (TWS) and early warnings are issued to safeguard the life of people. It is made up of two equally important components: a network of sensors to detect tsunamis and a communications infrastructure to issue timely alarms to permit evacuation of the coastal areas.

There are many regional and international early warning systems installed all across the globe. National governments warn citizens through a variety of means, including SMS messages, radio and television broadcasts, and sirens from dedicated platforms, mosque loudspeakers and police vehicles with loudspeakers.

India had volunteered to join the International Tsunami Warning System after the December 2004 tsunami disaster. The Indian Tsunami Early Warning Centre (ITEWC) embedded with specific systems called Deep Ocean Assessment and Reporting of Tsunamis (DART), established in 2007 at Indian National Centre for Ocean Information Sciences, (INCOIS - ESSO) Hyderabad, autonomous body under Ministry of Earth Sciences, is up and running to provide tsunami advisories for the events occurring in the global oceans.

It has been recognized as one of the best systems in the world. The ITEWC includes a real-time seismic monitoring network of seventeen broadband seismic stations to detect tsunamigenic earthquakes and to provide timely warnings to the vulnerable community. It also receives earthquake data from all other global networks to detect earthquakes (of M6.5).

Since its inception in October 2007, so far ITEWC has monitored 339 earthquakes of M 6.5. ITEWC also acts as one of the Regional Tsunami advisory Service Provider (RTSP) along with Australia & Indonesia for the Indian Ocean region.

Way forward and recommendation

2004 Indian Ocean Tsunami was a brutal reminder of disaster preparedness in India. While the current early warning system in India is state of the art, it is still inadequate in terms of preparedness for Tsunami. Following suggestions can be observed for enhancing India s preparedness for future Tsunami events:-

  • Adopting integrated multi-hazard approach with emphasis on cyclone and tsunami risk mitigation in coastal areas
  • Strict implementation of the coastal zone regulations
  • Plantation of mangroves and coastal forests along the coastline
  • Identification of vulnerable structures and appropriate retrofitting for tsunami resistance of all such buildings as well as appropriate planning, designing, construction of new facilities
  • Capacity building programmes and public awareness campaigns should be held at Tsunami prone areas
  • Streamlining the relief distribution system and evacuation plans in Tsunami prone areas
  • Component of planning for reconstruction and rehabilitation should be added to disaster management plans at all levels
  • Emphasis on mental health and to socio-psychological issues during post-disaster period should be accorded in every plan

Conclusion

Tsunami is one of the most hazardous and unpredictable natural force. Tsunamis have no seasons and they can occur at anytime and anywhere. We certainly cannot prevent Tsunami. But what we can do is take necessary steps to minimize the damage caused by it. Tsunami is a global and transnational event. Hence, it is important that all countries across the world should join hands to evolve new scientific ways to predict Tsunamis and to design mitigation strategies to cope with this disastrous force.

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