Tsunami: Mechanism & Properties, 2004 Indian Ocean Tsunami

PMF IAS Environment
  • 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 greatest number of tsunamis (over 790 since 1990).

Mechanism of Tsunami Waves

  • 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. This is because as the depth of the water decreases, the speed of the tsunami reduces. But the change of total energy of the tsunami remains constant.
  • With the decrease in 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.

Properties of Tsunami Waves

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.

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.

Wind generated wave motion

  • 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).

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.
  • 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).

2004 Indian Ocean Tsunami

  • 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.

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 direction.
  • 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.

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.

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.

Tsunami Warning Systems

  • 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 data.
  • Based on the data, warnings are issued.

India’s preparedness

  • 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 6 magnitude 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.
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