The term Tsunami comes from the Japanese language meaning harbour wave. Tsunamis are seismic waves that are caused by earthquakes that travel through water. Resultantly, high sea waves are generated which cause great damage to life and property. It is a series of waves generated when water in a lake or the sea is rapidly displaced on a massive scale. Earthquakes, landslides, volcanic eruptions etc all can generate a tsunami. Tsunamis have been historically referred to as tidal waves because as they approach land they take on the characteristics of a violent onrushing tide, which is usually a wrongly used term as tsunamis are not tidal waves in any sense. Hence, the use of the term is conceptually wrong.
The most common cause is an undersea earthquake. An earthquake that is too small to create a tsunami by itself may trigger an undersea landslide quite capable of generating a tsunami. Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the earth’s crust can occur at plate boundaries. Sub-marine landslides; which are sometimes triggered by large earthquakes; as well as collapses of volcanic edifices, may also disturb the overlying water column as sediments and rocks slide downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift the water column and form a tsunami. Although often referred to as ‘tidal waves’, a tsunami does not look like the popular impression of “a normal wave only much bigger”.
Instead, it looks rather like an endlessly onrushing tide which forces its way around and through any obstacle. Most of the damage is caused by the huge mass of water behind the initial wave front, as the height of the sea keeps rising fast and floods powerfully into the coastal areas. The sheer weight of water is enough to pulverize objects-in its path, often reducing buildings to their foundations and scouring exposed ground to the bedrock. Large objects such as ships and boulders can be carried several miles inland before a tsunami subsides.
Recently it has been discovered that larger tsunamis than previously believed possible could be caused by landslides, explosive volcanic actions and earth-scouring impact events. These phenomena rapidly displace large volumes of water, as energy from falling debris or expansion is transferred to the water into which the debris falls. Tsunamis caused by these mechanisms, unlike the ocean-wide tsunamis caused by some earthquakes, generally dissipate quickly and rarely affect coastlines distant from the source due to the small area of the sea affected.
Tsunamis move the entire depth of the ocean (often several kilometres deep) rather than just the surface, so they contain immense energy, propagate at high speeds and can travel great trans-oceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometres from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the seismic wave generated by the originating event arrives.
Although the total or overall loss of energy is small, the total energy is spread over a larger and larger circumference as the wave travels, so the energy per linear metre in the wave decreases as the inverse power of the distance from the source. This is the two-dimensional equivalent of the inverse square law in three dimensions. A single tsunami event may involve a series of waves of varying heights; the set of waves is called a train. In open water, tsunamis have extremely long periods from minutes to hours, and long wavelengths of up to several hundred kilometres. This is very different from typical wind-generated swells on the ocean, which might have a period of about 10 seconds and a wavelength of 150 metres.
A few signs may be triggered by nature to warn a huge tsunami wave. An earthquake may be felt. Large quantities of gas may bubble to the water surface and make the sea look as if it is boiling. The water in the waves may be unusually hot. The water may sometimes smell of rotten eggs due to the presence of hydrogen sulphide or of petrol or oil. The water may sting the skin. A thunderous boom may be heard followed by a roaring noise as of a jet plane or a noise-like the sound of a helicopter, or a whistling sound. The sea may recede to a considerable distance.
A flash of red light might be seen near the horizon and as the wave approaches, the top of the wave may glow red, These signals have been recorded from time to time over the ages before every tsunami tragedy. But tsunamis cannot be prevented or precisely predicted, but there are some warning signs of an impending tsunami, and oceanographers, scientists, geologists and environmentalists are working on making some kind of systems which can—if not prevent—at least signal the impending Tsunami.
Regions with a high risk of tsunamis may use tsunami warning systems now available to detect tsunamis and warn the general populace before the wave reaches the coasts. In some communities on the west coast of the United States, which is prone to Pacific Ocean tsunamis, warning signs advise people where to run in the event of an incoming tsunami. Computer models can roughly predict tsunami arrival and impact based on information about the event that triggered it and the shape of the sea floor and the coastal landmass. One of the early warnings comes from nearby animals. Many animals sense danger and flee to higher ground before the water arrives.
The Lisbon quake is the first documented case of such a phenomenon in Europe back in 1755 which had generated an almost 12 metre high sea Wave which in turn had destroyed most part of the city and had killed around 60,000 people. This phenomenon was also perceived in Sri Lanka in the 2004 Indian Ocean earthquake. Some scientists speculate that animals may have an ability to sense such natural disasters.
While it is not possible to prevent a tsunami, in particularly tsunami-prone countries some measures have been taken to reduce the damage caused on the shores Japan has implemented an extensive programme of building tsunami walls of up to 4.5m (13.5ft) high in front of populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunamis. However, their effectiveness has been questioned, as tsunamis are often higher than the barriers. For instance, the tsunami which hit the island of Hokkaido on July 12, 1993 created waves as much as 30m (100ft) tall – as high as a 10-storey building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area.
The wall may have succeeded in slowing down and moderating the height of the tsunami but it did not prevent major destruction and loss of life. Yet the effects of a tsunami can be mitigated by natural factors such as tree cover on the shoreline. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed as a result of the tsunami’s energy being sapped by a belt of trees such as coconut palms and mangroves. In one striking example, the village of Naluvedapathy in India’s Tamil Nadu region suffered minimal damages and few deaths as the wave broke up on a forest of 80,224 trees planted along the stretches of seacoasts that are prone to tsunami risks.
While it would take some years for the trees to grow to a useful size, such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than the costly and environmentally destructive method of erecting artificial barriers. The kutchchh earthquake of 1819 generated a huge tsunami which had submerged a whole lot of coastal areas and caused great damage to ships and boats. The Pacific Ocean is more prone to the tsunami because it is surrounded by the famous rig of volcanoes and earthquakes. Tsunamis are rare in the Atlantic Ocean where there are no such faults found.