TSUNAMI

TSUNAMI

  • Tsunami is a Japanese word for “Harbour wave”. A tsunami is a series of very long-wavelength waves in large water bodies like seas or large lakes caused by a major disturbance above or below the water surface or due to the displacement of a large volume of water.
  • They are sometimes referred to as tidal waves because of long wavelengths, although the attractions of the Moon and Sun play no role in their formation.
  • Earthquakes (e.g. 2004 Indian Ocean Tsunami), volcanic eruptions (e.g. tsunami caused by the violent eruption of Krakatoa in 1883), landslides (tsunami caused by the collapse of a section of Anak Krakatoa in 2018), underwater explosions, meteorite impacts, etc. have the potential to generate a tsunami.
  • Subduction zones off Chile, Nicaragua, Mexico and Indonesia have created killer tsunamis.
  • The Pacific among the oceans has witnessed the most number of tsunamis (over 790 since 1990).

 

MECHANISM OF TSUNAMI WAVES

 

Disturbance

  • Megathrust earthquakes cause a sudden displacement in a seabed sufficient to cause the sudden raising of a large body of water.
  • As the subducting plate plunges beneath the less dense plate, stresses build up, the locked zone between the plates give way abruptly, and the parts of the oceanic crust is then upthrust resulting in the displacement of a large column of water vertically.
  • The tsunami on December 26, 2004, was caused after an earthquake displaced the seabed off the coast of Sumatra, Indonesia.
  • A marine volcanic eruption can generate an impulsive force that displaces the water column and gives birth to a tsunami.
  • During a submarine landslide, the equilibrium sea-level is altered by sediment moving along the floor of the sea. Gravitational forces then propagate a tsunami.
  • Most destructive tsunamis can be caused due to the fall of extra-terrestrial objects on to the earth.

Propagation of the waves

  • Gravity acts to return the sea surface to its original shape.
  • The ripples then race outward, and a tsunami is caused.
  • As a tsunami leaves deep waters and propagates into the shallow waters, it transforms as the depth of the water decreases, the speed of the tsunami reduces. But the total energy of the tsunami remains constant.
  • With decreasing speed, the height of the tsunami wave grows. A tsunami which was imperceptible in deep water may grow to many metres high, and this is called the ‘shoaling’ effect.
  • Sometimes, the sea seems to at first draw a breath, but then this withdrawal is followed by the arrival of the crest of a tsunami wave. Tsunamis have been known to occur suddenly without warning.
  • In some cases, there are several great waves separated by intervals of several minutes or more.
  • The first of these waves is often preceded by an extraordinary recession of water from the shore, which may commence several minutes or even half an hour beforehand.

 

 

Tsunami is not mass of flowing water; it is in fact transfer of energy in which water molecules hits the next and so on.

 

PROPERTIES OF TSUNAMI WAVES

I) Basics

  • Wave crest and trough: The highest and lowest points of a wave are called the crest and trough respectively.
  • Wave height: It is the vertical distance from the bottom of a trough to the top of a crest of a wave.
  • Wave amplitude: It is one-half of the wave height.
  • Wave period: It is the time interval between two successive wave crests or troughs.
  • Wavelength: It is the horizontal distance between two successive crests.
  • Wave frequency: It is the number of waves passing a given point during a one second time interval.

 

 

II) Normal waves

  • The horizontal and vertical motions are common in ocean water bodies.
  • The horizontal motion refers to the ocean currents and waves. The vertical motion refers to tides.
  • Water moves ahead from one place to another through ocean currents while the water in the normal wind-generated waves do not move, but the wave trains move ahead.
  • The motion of normal waves seldom affects the stagnant deep bottom water of the oceans.
  • The actual motion of the water beneath the waves is circular. It indicates that things are carried up and forward as the wave approaches, and down and back as it passes.
  • As a wave approaches the beach, it slows down.And, when the depth of water is less than half the wavelength of the wave, the wave breaks (dies).

III) Normal waves vs Tsunami waves

 

 

  • Tsunamis are a series of waves of very, very long wavelengths and period.
  • Tsunamis are different from the wind-generated waves (period of five to twenty seconds).
  • Tsunamis behave as shallow-water waves because of their long wavelengths. They have a period in the range of ten minutes to two hours and a wavelength exceeding 500 km..
  • The rate of energy loss of a wave is inversely related to its wavelength. So, tsunamis lose little energy as they propagate because of their very large wavelength.
  • They travel at high speeds in deep waters, and their speed falls when they hit shallow waters.
  • A tsunami that occurs 1000 metres deep in water has a speed of more about 350 km per hour. At 6000 m, it can travel at speeds about 850 km per hour.
  • Tsunami waves are not noticed by ships far out at sea. Ships in the deep ocean doesn’t get affected from tsunami waves and crosses the same without even knowing that a tsunami waves has passed through them just like a wind wave.
  • Their amplitude is negligible when compared with their wavelength, and hence the waves go unnoticed in deep oceans.
  • When tsunamis approach shallow water, however, the wave amplitude increases (conservation of energy).
  • The waves may occasionally reach a height of 20 to 30 metres above mean sea level in closed harbours and inlets (funnelling effect).

 

 

 

GLOBAL DISTRIBUTION OF TSUNAMIS

  • 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.

 

 

 

CAUSES OF TSUNAMIS

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.

I) 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.

II) 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.

III) 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.

 

IV) 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.

V) 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.

VI) 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.

 


Important Facts to remember !

  1. Not all earthquakes generate Tsunamis. An intensity of 7.5 or above on richter scale can produce a destructive Tsunami
  2. If wavelength generated is twice the depth of sea , only then Tsunami occurs
  3. A Tsunami can travel 10,000 Km & it can generate a travelling speed of 600-800 Km/hr
  4. Tsunami is not a single wave but series of waves
  5. A small Tsunami a mile away can turn into a giant wave near shoreline
  6. If an earthquake is having Richter Scale 7.5 reading or more but is an outcome of strike slip fault , Tsunami will never come.

 

 

So a strike slip fault earthquake never cause a Tsunami . Animation of strike slip fault is as below:

 

 


 

 

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.

 

INDIAN OCEAN TSUNAMI 2004

  • Tsunami or the Harbour wave struck havoc in the Indian Ocean on the 26th of December 2004.
  • The wave was the result of an earthquake that had its epicentre near the western boundary of Sumatra.
  • The magnitude of the earthquake was 9.0 on the Richter scale.

 

 

I) Plate tectonics

  • Indian plate went under the Burma plate, there was a sudden movement of the sea floor, causing the earthquake.
  • The ocean floor was displaced by about 10 – 20m and tilted in a downward
  • A huge mass of ocean water flowed to fill in the gap that was being created by the displacement.
  • This marked the withdrawal of the water mass from the coastlines of the landmasses in the south and Southeast Asia.
  • After thrusting of the Indian plate below the Burma plate, the water mass rushed back towards the coastline as a tsunami.
  • Animation of earthquake and subsequently the Tsunami in 2004 is as below :

 

 

 

II) Tsunami waves

  • Tsunami travelled at a speed of about 800 km. per hour, comparable to speed of commercial aircraft and completely washed away some of the islands in the Indian ocean.
  • The Indira point in the Andaman and Nicobar Islands that marked the southernmost point of India got completely submerged.
  • As the wave moved from earthquake epicentre from Sumatra towards the Andaman Islands and Sri Lanka, the wavelength decreased with decreasing depth of water.
  • The travel speed also declined from 700-900 km per hour to less than 70 km per hour.
  • Tsunami waves travelled up to a depth of 3 km from the coast killing more than 10,000 people and affected more than lakh of houses.
  • In India, the worst affected were the coastal areas of Andhra Pradesh, Tamil Nadu, Kerala, Pondicherry and the Andaman and Nicobar Islands.

III) Shifts in Geography

  • Tsunamis and earthquakes can cause changes in geography.
  • The December 26 earthquake and tsunami shifted the North Pole by 2.5 cm in the direction of 145 degrees East longitude and reduced the length of the day by 2.68 microseconds.
  • This, in turn, affected the velocity of earth’s rotation and the Coriolis force which plays a strong role in weather patterns.
  • The Andaman and Nicobar Islands may have (moved by about 1.25 m owing to the impact of the colossal earthquake and the tsunami.

 

WARNING SYSTEM

 

  • While the earthquake cannot be predicted in advance, it is possible to give a three-hour notice of a potential tsunami.
  • Such early warning systems are in place across the Pacific Ocean. Post-2004, they were installed in the Indian Ocean as well.
  • In 1965, early warning system was started by the National Oceanic and Atmospheric Administration (NOAA). The member states of the NOAA include the major Pacific Rim countries.
  • NOAA has developed the ‘Deep Ocean Assessment and Reporting of Tsunamis’ (DART) gauge.
  • Each gauge has a very sensitive pressure recorder on the sea floor. Data is generated whenever changes in water pressure occur.
  • The data is transmitted to a surface buoy which then relays it over satellite.
  • Computer systems at the Pacific Tsunami Warning Centre (PTWC) in Hawaii monitor
  • Based on the data, warnings are issued.

 

INDIA’S PREPARDNESS

  • The Deep Ocean Assessment and Reporting System (DOARS) was set up in the Indian Ocean post-2004.
  • The Indian government plans to set up a network with Indonesia, Myanmar and Thailand etc.
  • A National Tsunami Early Warning Centre, which can detect earthquakes of more than 6magnitude in the Indian Ocean, was inaugurated in 2007 in India.
  • Set up by the Ministry of Earth Sciences in the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, the tsunami warning system would take 10-30 minutes to analyse the seismic data following an earthquake.

 

WAY FORWARD AND RECOMMENDATIONS

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

 

DECESION MAKING AT TIME OF TSUNAMI !

  1. Be prepared before Tsunami strikes
  2. Know the Tsunami history and flooding , elevation of area where you live and work
  3. Know the location of nearest Tsunami centre and safe area
  4. Learn safe route to shelter
  5. Have a portable supply kit with atleast 3 days food and water
  6. Have a family emergency plan

 

 

A CASE STUDY FROM ABORIGINALS OF ANDAMAN AND NICOBAR:

  • The Indian Ocean tsunami in 2004 it was feared that the aboriginal people tucked away in the forests of the Andaman and Nicobar Islands may get worst hit.
  • The tribals never had an access to weather satellite information. Neither did they have emergency radios or the latest hi-tech communication devices.
  • “Not a single tribal, Sentinelese not Jarwa, not Onge faced death while mainland India lost 7000 people”
  • The tribals and their ways which are “primitive”  but the tsunami show us vividly the power of their knowledge base. Their ancient knowledge about nature, hardwired into their brains, had helped them ride that tsunami.
  • The tsunami in a way laid bare a sophisticated knowledge base of languages that date back perhaps 20,000 years or more. Great Andamanes tribes are the last survivors of a pre-Neolithic population, descended from a migration from Africa that took place over 70,000 years ago.
  • Their “primitive knowledge” shall be included in the modern disaster management systems that our Civilization rely upon.

 

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