How does tsunami form and why is it so destructive?

On August 26, 1883, the people of Jakarta, the capital city of Indonesia, heard ear-splitting sounds and experienced mild tremors. Being used to witnessing eruption of volcanoes frequently, the people of Jakarta thought that the loud sound was the outcome of yet another volcanic eruption. They guessed it right, but none of them had the slightest idea about the extent of devastation this volcanic eruption was going to cause. The hereto dormant volcano of Krakatau Island had suddenly become active after a long ‘rest’ of 200 years. The terrific upward pressure of magma and steam inside this volcano had suddenly burst the mouth of a mountain with a loud explosion and thrown up the ejecta (huge boulders, stones, pebbles and hot lava) 183 meter (600 feet) high into the sky. The plumes of dust, ashes and smoke reached 27 kilometers (90,000 feet) high in the atmosphere. Due to the shock waves the island of Krakatau was largely destroyed. The sound of the explosion was so loud that even after nearly 4 hours at a distance of about 4,750 kilometers away the people on Rodriguez Island near Africa in the Indian Ocean also heard it.
The volcanic eruption caused tremors in the floor of the Indian Ocean. Due to the intense pressure the water of the ocean was displaced in all directions and tsunami waves lashed all the nearby islands at a speed of 960 kilometers. These tsunami waves destroyed 7,500 small and big vessels that had cast anchor at the various ports of Indonesia, Malaya, Thailand, all the way to Darwin in Australia and Kolkata (Calcutta) and Chennai (Madras) in India. Tsunami waves completely wiped out 300 villages and cities of different countries and claimed the lives of more than 36,000 people.

The Ring of Fire

Why was Indonesia the epicenter of destruction that occurred once more in December, 2004 – after 125 years of catastrophic eruption of Krakatau volcano? The answer to this question can be well understood with the help of the map shown below. The 100 kilometer thick Earth’s crust is natural divided into many tectonic plates. None of the plates are completely steady as yet. As shown in the map, sometimes two tectonic plates move away from each other at the rate of 5-6 centimeters annually. The molten lava which is present in the inner recesses of the Earth rises up slowly to fill up the resultant gap and once it cools down takes the form of solid rock. Generally, such tectonic plates which move away from each other are situated beneath the oceans, and so it can be said that the bed of the ocean is increasing on a daily basis.

If expansion takes place at one point then obviously to accommodate the expanded portion contraction should happen at another point. But the tectonic plates made of solid rocks can not contract – instead they collide against each other. Usually a small plate slips under a bigger one. It goes deep down where the temperature of 1000° Celsius melts the rocks and turns them into molten lava. Suppose the bigger tectonic plate does have a large area but is not very thick and strong then the molten lava which is inside will try time and again to come out and with the passage of time volcanoes are formed on the surface of the tectonic plate. This very state exists in Indonesia. The tectonic plate known as the Indo-Australian plate is gradually going beneath the larger tectonic plate of Eurasia. Hence there are a total of 48 volcanic mountains on the surface of Indonesian islands. These mountains erupt from time to time and throw out molten lava from their bellies. There is a long range of such mountains in the northwest-to-southeast direction which the geologists have named as the ‘Ring of Fire’. Krakatau also falls in this ring.

Reasons for tsunami

As happened in case of Krakatau, when a volcano erupts with a huge explosive force its tremors reach the nearby water bodies thus resulting in a tsunami. Nevertheless, the only reason for tsunamis is not volcanic eruptions. Most of the tsunami waves arise due to the earthquake at the bottom of the sea, on the bed or floor and such earthquakes are quite frequent in the depths of the oceans. In most of the cases the ‘slow-motion’ dashing of the tectonic plates against each other is like an invitation to the earthquakes. It so happens that the smaller tectonic plate which moves towards the molten lava in the recesses of the Earth and slips under the bigger plate does not do so on its own volition. The internal friction among the plates and the pressure of the lava itself causes a lot of resistance. Consequently, just like the two stripes of Velcro belts stay attached to each other for years together the two tectonic plates also keep on with their different resistance to each other. In the meantime the rocks of both the plates undergo a huge amount of pressure, tension and friction. Therefore, it is not certain which plate will break first. However, to regain balance one of them has to break sooner or later. It is quite possible that both the plates retain the state of imbalance for years on end. The imbalance goes on increasing with the passage of time, and one of the plates is bound to break. When it breaks it releases tremendous amount of energy in the form of shockwaves. Such waves spread through the plates at great speed in all directions resulting in the whole plate shaking vertically as well as horizontally. In short, a devastating earthquake strikes. State of imbalance between two tectonic plates near Honshu Island of Japan had stayed to too long, and when one of the plates released its tension on March 11, 2011 an earthquake of 8.9 magnitude struck Japan followed by tsunami waves.

Relation between earthquake and tsunami

Let us now understand the relation between an oceanic earthquake and tsunami in minute detail. The pressurized tectonic plate seeks for a better and more comfortable position, but many a time it finds this position only after breaking down itself or causing an adjoining plate to break. No matter which plate breaks, the result is that one plate goes up pushing the other further into the bowels of the Earth. Only those earthquakes which strike below the ocean bed are responsible for creating tsunami waves. The exact repercussions of whatever has occurred at the bottom of the sea are seen on the surface of the water. In other words, the sea water rises at the area where the plate had gone up and recedes at the area where the other plate had gone down. Within a short time the sea becomes extremely violent. The huge volume of water which was raised by the tectonic plate below tries to get to its original level, and therefore comes down with great speed thus displacing the surrounding water in all directions. There are no empty spaces for the displaced volume of water and hence its level rises alarmingly after getting displaced. This raised level causes high waves and they spread all over the surrounding area. Just as ripples form concentric circles when they spread on the calm surface of a lake, the tsunami waves also form concentric circles and travel in all directions. When such waves are formed in the middle of the ocean they are deceptive and sometimes are not recognized as tsunami waves. Their height is not more than 3 meters while the distance between two waves is about 150 kilometers. It is quite possible that seafarers sailing in the deep seas thousands of kilometers away from land encounter such waves but think nothing of them and do not realize their true significance. Such tsunami waves are easily camouflaged among the naturally occurring 2 meter high waves that are produced by strong winds. However, tsunami waves have nothing in common with the naturally occurring waves created by strong winds.

A normal wave rise only superficially while a tsunami wave is spread right from topmost surface of the ocean all the way to its bottom – and it does not even ebb instantly like a normal wave does. On the contrary it can travel thousands of kilometers if no land form obstructs its journey. (On may 22, 1960, when an oceanic earthquake struck Chile, South America, an area of 1,60,000 square kilometers of the bed of the ocean was raised by nearly 9.15 meters and the resultant tsunami wave started off from Chile and traveled approximately 10,600 kilometers all the way to Hawaii Island and claimed the lives of about 1,000 people residing in and around the costal areas of this belt). The other salient feature of tsunami waves is their speed which is never less than 800 kilometers. Though the volume of water, which has a wavelength of 150 kilometers, does not proceeds so fast on its own, it attains this tremendous speed by creating a new wave in the water which is in front of it by displacing it. In short, the pressure is transferred towards the front as the wave moves ahead. To create such tremendous pressure again and again a tsunami must have a lot of energy. The tsunami receives only 1% of the total energy released, the obvious source of which is the earthquake itself, but still its destruction is as horrific and potent as 125 nuclear bombs, like the one which was dropped on Hiroshima. As the tsunami proceeds further in the deep sea it loses about 2/3rd of its energy every 12 hours. But it traverses about 10,000 kilometers in that much time and eventually releases its energy at the coastal areas. The reason why energy is retained for long time is that there is no obstruction to the wave which rises in the middle of the ocean.

The situation changes as soon as the waves approach the cost. The ocean bed near the coast is shallower – and so it creates obstruction to the waves. Along with the speed the wavelength of the wave also decreases. As a result there is less distance between two waves and each wave rises above the other. How high the final wave will go depends upon the formation of the bed of the ocean. If there are rocky ledges on the bed then the waves will rise accordingly and reach higher altitudes. Tough the speed does not decrease suddenly, the force and volume of water is so great that a person standing on the beach can not outrun and escape it even if he attains a speed of 40 kilometers.

Another noteworthy feature of the tsunami is that, just before the destroyer waves lash the coast, there is a sudden ebbing of the sea so much so that the shallow bottom of the seacoast is visible. Logically, there should be an abnormal tide rising but contrary to this the sea recedes dramatically. Just like a wheel moves ahead in a circular motion, the waves also proceed in a similar manner. The water on the surface goes down and the water underneath get churned in an upward direction. To give this momentum enough supply of water the water near the coast recedes into the sea of it own accord. It is as if the tsunami wave itself pulls the volume of water towards it so much so that sometimes the sea near the coast recedes about 10 to 15 kilometers. This situation, however, does not last long. Within minutes the wall-like huge tsunami wave rears its head over the coastal areas and washes over everything in the surrounding areas. The people that collected on the shore to watch the scenes of the sudden ebbing of the sea have lost their lives quite a few times in the past. When an oceanic earthquake struck the coastal city of Lisbon, Portugal on November 1, 1775, the sea started receding due to which a lot of small and big vessels anchored to the posts on the wharfs broke loose and were pulled into the raging waters by the huge and forceful receding volume of water. The port, having breadth of nearly 12 kilometers, suddenly ran dry. Many of residents of Lisbon city gathered on the shores to watch this amazing sight. When after a short while a 7 meter high wall-like wave rushed into Lisbon all the people present on the shore lost their life.

The tsunami waves which start in the center of the sea finally expend all their energies on the coastal areas, and so their destructive force is evidently quite astounding. One example is of the oceanic earthquake that struck near the American State of Alaska on March 27, 1964. The epicenter of the earthquake was about 100 kilometers from the mainland. After the earthquake the first catastrophic tsunami wave that hit the port named Seward not only demolished the goods train along the railway line but also lifted the huge, heavy railway engines and hurled them 36 meters away! The most amazing example of the effect of the boundless energy of a tsunami has been recorded in Japan on April 24, 1771, when the coastal wave which rushed toward the coastal coral reefs, broke them, sliced off a large piece of coral weighing about 827 tons, raised it high on to its tip and flung it nearly 2 kilometers away on the plateau region of the island.

Such huge levels of energy of tsunamis are due to the insufficient space available to it along the nearby coastal areas. The wave which forms a long wall and rises amidst ocean flows freely without obstruction. But when it encounters a shallow areas and broken coastlines it experiences hurdles in the form of rocks underneath and to its right and left sides. Consequently, it gets compressed from three sides and so rises even higher. Thus, the total energy of wave gets concentrated within smaller area generating more energy than it otherwise would have within lesser volume of water. This is the greatest difference between speedster tsunami waves and the comparatively slow moving regular tidal waves. The second distinguishing factor is that a tsunami wave does not recede immediately after causing havoc. The energetic waves which follow it keep on pushing it towards the shore again and again and this ritual goes on for a long time.

Measures to prevent tsunami

The key to halt the destruction cause by tsunami has not yet been found by mankind and looking at the level of energy generated by such waves – equivalent to 100-125 nuclear bombs – it does not seem possible that man will find a solution to combat this destruction in the near future too. Nevertheless, with a timely warning of a tsunami at least loss of life, if not loss of property, can definitely be controlled to a certain extent. The biggest network for such warnings has been set up in the coastal regions surrounding the Pacific Ocean, since the highest number (almost 85%) of tsunamis occur here. America, Chile, Japan Canada, Taiwan, Philippines, New Zealand etc are some of the places around the Pacific Ocean where early warning buoys are kept afloat. The detector machine which lies on the floor of the ocean near the anchorage of a buoy measures the pressure of water with its spring. If an earthquake disturbs the ocean floor, the pressure of water increases and the spring gets compressed suddenly. Immediately the detector sends a sound signal to the hydrophone of the buoy. This message is conveyed by the buoy to the satellite which in turn relays the message immediately to all the member countries of the network thus alerting them well in advance.

The mechanism is simple but advance warning is justified only when all the town and villages lying on and around the coastal areas of all the countries receive the warning in time. That the warning is taken seriously is also equally important. In 1964, when an earthquake struck the Pacific Ocean on the coastal areas of Chile, Japan and all the other coastal regions had been warned well in time of the catastrophe but even then many people foolishly believed that the upsurge of the tsunami waves would not reach Japan which was 16,000 kilometers away. Exactly 24 hours later the tsunami struck the coastal areas of Japan killing 200 people. If the distance between the epicenter of the Pacific Ocean and Japan had been 1,600 kilometers instead of 16,000 kilometers, imagine what would have happened!

Last month (March 2011) an earthquake measuring 9.0 on the Richter scale shook the tectonic plates that lie 24 kilometers underneath on the floor of the Pacific Ocean near Japan. Thereafter 10 meter high waves rushed into the areas around this coast and damaged many towns and villages. The sole reason for this destruction is that due to the small distance between the epicenter and the shores most of the energy of the tsunami stayed concentrated in the waves and got spent on the ill fated coastal regions of Japan.

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