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Post 2004 Indian Ocean Tsunami, the word “Tsunami” took a center stage among all natural calamities. Though their number is small compared to other calamities, the havoc they cause is considerable. The latest is the Japan Earthquake & Tsunami of 2011 which caused a death of more than 15,000 individuals. The tsunami caused nuclear accidents, primarily the meltdowns at three reactors in the Fukushima Daiichi Nuclear Power Plant complex, raising questions about nuclear safety in the events of natural calamities.
- Tsunami is a Japanese word for “Harbour wave”. They are also known as seismic sea waves.
- They are very long-wavelength water waves in oceans or seas. They are commonly referred to as tidal waves because of long wavelengths, although the attractions of the Moon and Sun play no role in their formation.
- They sometimes come ashore to great heights – tens of metres above mean tide level – and may be extremely destructive.
Similar Post on Earthquakes, another major natural calamity.
What causes Tsunami?
- A tsunami can be caused by any disturbance that displaces a large water mass from its equilibrium position.
- The usual immediate cause of a tsunami is sudden displacement in a seabed due to submarine earthquakes sufficient to cause the sudden raising or lowering of a large body of water. The tsunami on December 26, 2004 was caused after an earthquake displaced the seabed off the coast of Sumatra, Indonesia.
- Large volcanic eruptions along shorelines, such as Krakatoa (1883 CE), have also produced notable tsunamis.
- 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.
- Landslides along the coast, high intensity explosions can also cause tsunami.
- Most destructive tsunamis can be caused due to the fall of extra-terrestrial objects on to the earth.
Mechanism in Earthquake induced Tsunami’s
- An undersea earthquake causes buckling of the sea floor, something that occurs at subduction zones, places where drifting plates that constitute the outer shell of the earth converge and the heavier oceanic plate subducts below the lighter continents.
- As a plate plunges into the interior of the earth it gets stuck against the edge of a continental plate for a while, when stresses build up, then the locked zone gives way. Parts of the ocean floor then snap upward and other areas sink downward.
- In the instant after the quake, the sea surface shape resembles the contours of the seafloor. But then gravity acts to return the sea surface to its original shape. The ripples then race outward and a tsunami is caused.
Propagation of tsunami waves
- The long gravity tsunami waves are caused by two interacting processes.
- There is the slope of the sea surface which creates a horizontal pressure force.
- Then there is the piling up or lowering of sea surface as water moves in varying speeds in the direction that the wave form is moving. These processes together create propagating waves.
- 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 decrease in speed, 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 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
- Tsunamis are a series of waves of very, very long wavelengths and period created in oceans by an impulsive disturbance.
- Tsunamis are different from the wind-generated waves which usually have a 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.
- So they will travel at high speeds in deep waters and travel great distances as well losing little energy. A tsunami that occurs 1000 metres deep in water has a speed of 356 km per hour.
- At 6000 m, it travels at 873 km per hour.
- It travels at different speeds in water: it travels slow in water that is shallow and fast in deep water.
- 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 waves does not move, but the wave trains move ahead.
- Waves are actually the energy, not the water as such, which moves across the ocean surface. Water particles only travel in a small circle as a wave passes.
- Wind provides energy to the waves. Wind causes waves to travel in the ocean and the energy is released on shorelines.
- The motion of the surface water seldom affects the stagnant deep bottom water of the oceans.
- As a wave approaches the beach, it slows down. This is due to the friction occurring between the dynamic water and the sea floor.
And, when the depth of water is less than half the wavelength of the wave, the wave breaks (dies).
- The largest waves are found in the open oceans. Waves continue to grow larger as they move and absorb energy from the wind.
- When a breeze of two knots or less blows over calm water, small ripples form and grow as the wind speed increases until white caps appear in the breaking waves.
- Waves may travel thousands of km before rolling ashore, breaking and dissolving as surf. A wave’s size and shape reveal its origin.
Steep waves are fairly young ones and are probably formed by local wind. Slow and steady waves originate from faraway places, possibly from another hemisphere.
- Waves travel because wind pushes the water body in its course while gravity pulls the crests of the waves downward.
- The falling water pushes the former troughs upward, and the wave moves to a new position.
- 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.
Characteristics of Waves
- 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 merely the time interval between two successive wave crests or troughs as they pass a fixed point.
- Wavelength: It is the horizontal distance between two successive crests.
- Wave speed: It is the rate at which the wave moves through the water, and is measured in knots.
- Wave frequency: It is the number of waves passing a given point during a one second time interval.
Normal waves vs Tsunami waves
Tsunami waves are not noticed by ships far out at sea
- As tsunami waves are long wavelength waves, they cannot be perceived in deep oceans. 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 U- and V-shaped harbours and inlets (funneling 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 earthquake that had its epicenter near western boundary of Sumatra.
- The magnitude of the earthquake was 9.0 on the Richter scale.
- 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 downwardly 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 tsunami.
- Tsunami traveled 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 epicenter from Sumatra towards the Andaman Islands and Sri Lanka the wave length 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 traveled 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.
- Subduction zones off Chile, Nicaragua, Mexico and Indonesia have created killer tsunamis.
- The Pacific among the oceans has witnessed most number of tsunamis (over 790 since 1990).
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.
- 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 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 monitors data.
- Based on the data, warnings are issued.
- The Deep Ocean Assessment and Reporting System (DOARS) was set up in the Indian Ocean post 2014.
- The Indian government plans to set up a network with Indonesia, Myanmar and Thailand etc..
- A National Tsunami Early Warning Centre, which has the capability to 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 analyze the seismic data following an earthquake.