All you need to know about tsunami

What is a tsunami?

The tsunami sometimes does not appear on the shore as massive breaking waves but instead resembles a quickly surging tide that inundates coastal areas.

Characteristics of a tsunami:

When a wave enters shallow water, it slows down and its amplitude and height increases before it hits the land. Only the largest of such waves crest.

tsunamis cause damage with the smashing force of a wall of water travelling at high speed and the destructive power of a large volume of water draining off the land, carrying a large amount of debris with it.

While everyday wind waves have a wavelength (from crest to crest) of about 100 meters and a height of roughly 2 meters a tsunami in the deep ocean has a much larger wavelength of up to 200 kilometers. Such a wave travels at over 800 kilometers per hour, but owing to its enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has amplitude of only about 1 meter. This makes tsunamis difficult to detect in deep waters, where ships cannot feel their passage.

As the tsunami approaches the coast and the waters become shallow, the wave shoaling compresses the wave and its speed decreases below 80 kilometers per hour Its wavelength diminishes to less than 20 kilometers and its amplitude grows enormously. Since the wave still has the same long period, the tsunami may take minutes to reach its maximum height. Except in the case of largest tsunamis, the approaching waves do not break, but rather appear as fast-moving tidal bores. Open bays and coastlines adjacent to deep waters may shape the tsunami further into a step-like wave with a steep-breaking front.

When the tsunami's wave peak reaches the shore, the resulting temporary rise in the sea level is termed the “run up”. Run up is measured in meters above a reference sea level. A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run up.

About 80 per cent of tsunamis occur in the Pacific Ocean alone. Other water bodies like lakes, rivers do experience tsunamis and the reason behind this is earthquakes, landslides, volcanic explosions, glacier calvings, and bolides.

Causes of tsunami:

tsunami is the displacement of a substantial volume of water or perturbation of the sea. This displacement of water is usually attributed to earthquakes, landslides, volcanic eruptions, and glacier calvings or more rarely by meteorites and nuclear tests. The waves thus formed are sustained by gravity. Tides do not play any part in the generation of tsunamis.

Seismicity

tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquakes that are associated with the Earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of the movement involved. Movements on normal faults also cause displacement of the seabed.

Change in the tectonic plate boundary before an earth quake leads to the overriding of the plate, which bulges the under strain resulting in the upliftment of the tectonic plate which slips, releasing the energy into water producing tsunami waves.

tsunamis have small amplitudes offshore, and very long wavelengths (often hundreds of kilometers long, whereas normal ocean waves have a wavelength of only 30 or 40 meters), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimeters above the normal sea surface. They grow in height when they reach shallower waters, in a wave shoaling process. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.

An earthquake of magnitude 7.8 occurred near Aleutian Islands, Alaska, where the Pacific Ocean floor sub ducted in 1946 was enough to generate a 14 meter high tsunami which inundated Hilo on the Hawai Island.

Storegga tsunami which occurred 8,000 years ago and Grand Banks in 1929, Papua New Guinea in 1988 are the examples of tsunami caused due to converging boundaries. While the Grand Banks and Papua New Guinea tsunamis followed after the earthquake which destabilised sediments, causing them to flow into the ocean and generate a tsunami. But their intensity was not that great to travel transoceanic distances.

The reaons for the destabilasation of the ocean floor are unknown, but scientists opinioned that overloading of the sediments, an earthquake or a release of gas hydrates methane etc. might be responsible.

Megathrust earthquakes release an intense energy, tsunamis generated because of these travel transoceanic distances. Valdivia earthquake in 1960, Alaska earthquake in 1964, 2004 Indian Ocean earthquake and 2011 Tōhoku earthquake (Mw9.0) are best examples of powerful megathrust earthquakes that generated tsunamis that can cross entire oceans. There are smaller earthquakes which occur due to the tectonic plate movement in Pacific Ocean can devastate nearby coasts of Japan in few minutes.

Landslides

Landslides can be another potential reason behind the tsunami, scientists in 1950’s has discovered that larger number of tsunamis hit the coasts in the time were due to the giant submarine landslides. These rapidly displace large water volumes, as energy transfers to the water at a rate faster than the water can absorb. Their existence was confirmed in 1958, when a giant landslide in Lituya Bay, Alaska, caused the highest wave ever recorded, which had a height of 524 meters.

The year 1963 witnessed another landslide tsunami when the Monte Toc went into the Vajont Dam in Italy. The wave generated due to the landslide rose up to 262 meters overtopping the dam and destroyed several towns. While scientists call these phenomenons as mega-tsunami and these are caused due to extremely large landslides from volcanic island collapses which can cross oceans.

In general, landslides generate displacements mainly in the shallower parts of the coastline, and there is conjecture about the nature of truly large landslides that end in water. This is proven to lead to huge effect in closed bays and lakes, but an open oceanic landslide big enough to cause a tsunami across an ocean has not yet happened since before seismology has been a major area of scientific study, and only very rarely in human history. Susceptible areas focus for now on the islands of Hawaii and Las Palmas in the Canary Islands, where large masses of relatively unconsolidated volcanic shield on slopes occur. Considerable doubt exists about how loosely linked these slopes actually are.

Meteotsunamis

Meteotsunami is a tsunami caused due to the meteorological conditions. Conditions especially deep depressions such as tropical cyclones can generate a type of storm surge called a meteotsunami which raises water heights above normal levels, often suddenly at the shoreline.

Meteotsunamic storm surge over the Bolivar Peninsula in 2008 due to Hurricane Ike's is a best example.

In the case of deep tropical cyclones, this is due to very low atmospheric pressure and inward swirling winds causing an uplifted dome of water to form under and travel in tandem with the storm. When these water domes reach shore, they rear up in shallows and surge laterally like earthquake-generated tsunamis, typically arriving shortly after landfall of the storm'seye.

Man-made or triggered tsunamis

There have been many attempts by the scientists to generate an artificial tsunami.

In World War II, the New Zealand Military Forces initiated Project Seal, which attempted to create small tsunamis with explosives in the area of today's Shakespeare Regional Park, but the attempts failed.

There is also a significant conjecture on the possibility of using nuclear weapons to cause tsunamis near to an enemy coastline. Even during World War II consideration of the idea using conventional explosives was explored. Nuclear testing in the Pacific Proving ground by the United States seemed to generate poor results. Analysis of the effects of shallow and deep underwater explosions indicate that the energy of the explosions doesn't easily generate the kind of deep, all-ocean waveforms which are tsunamis; most of the energy creates steam, causes vertical fountains above the water, and creates compressional waveforms. tsunamis are hallmarked by permanent large vertical displacements of very large volumes of water which don't occur in explosions.