The Physics of Tsunamis
Humans have known about the destructive force of tsunamis for a long time, however, until the devastating tsunami incident in Indian Ocean in 2004 not much international effort was put into the research and development of methods and tools to effectively predict, warn and mitigate the upcoming disaster. The topic of creating an effective early warning system for the areas threatened by tsunamis is very important, but this paper focuses on the physics behind the processes of the formation and propagation of the tsunami waves which, in fact, is the basic knowledge required for developing the warning systems mentioned above.
When people are talking about a tsunami they generally mean a single devastating wave which suddenly hits the coast. In reality the tsunami is a series of waves which are called the tsunami train (Helal & Mehanna, 2008, p. 788). As any other wave, the tsunami train has positive and negative peaks and the effect of it depends on what part of the wave reaches the coastal area first. In case the negative peak reaches the shore first, the shoreline will recede, exposing the areas of the seabed which normally are submerged at all times, even during the low tide. The recede usually is not long, ranging from seconds to a few minutes (in very rare cases hours) in time. Then the peak of the wave arrives and the shore and its inhabitants face the devastating force of it. In case the peak reaches the shore first, the coastal areas have little to no time in order to undertake any actions to evacuate. Right before the peak hits, the water level increases rapidly resembling a very quick high tide.
Tsunami waves carry an enormous amount of energy which they get from the place of their origin. There are several natural causes for tsunamis which are further explained in more detail. The reason for generation of the tsunami waves is the sudden vertical displacement of a large volume of water. This displacement generally occurs as a result of the seismic motion of the seabed (Levin & Nosov, 2008, p. 3), which means that most of the tsunamis are caused by underwater earthquakes, volcano eruptions and landslides. However, these are not the only causes that generate tsunami waves. Waves similar to tsunamis can also be caused by sharp changes in the atmospheric pressure which are able to raise the masses of water in one area and lower the masses of water in another (Levin & Nosov, 2008, p. 3); such waves are referred to as meteotsunamis. There is a theory about the possibility to cause tsunamis by underwater explosions, however, the research conducted during the World War II did not show any positive results, probably, because the crucial factor in tsunami formation is the vertical displacement of a large volume of water, and bombs detonated underwater are not able to achieve this effect. There is yet another theory, according to which a tsunami can be caused by a meteorite impact. In any case, the key to tsunami formation is a large-scale sharp vertical displacement of water.
The event causing a large volume of water to shift, such as the earthquake, eruption or a landslide, by default implies a huge release of energy (Helal & Mehanna, 2008, p. 788). Such a sudden release of a big amount of energy is impossible for the water masses to absorb and, thus, creates a wave. Tsunami waves are characterized by their enormous wavelengths which often reach hundreds of kilometers (Margaritondoб 2005, p. 402). Another characteristic feature of such waves is that because of the enormous amount of energy they carry, a very large volume of water gets involved in the movement (Levin & Nosov, 2008, p. 3) and tsunami waves lose little energy while they travel from the place of their origin through the areas with significant depth because “the rate at which a wave loses its energy is inversely proportional to its wavelength” (Helal & Mehanna, 2008, p. 788).
As it travels through the deep waters, tsunami wave is imperceptible to an observer because it has small amplitude and large periods between the wave crests and presents no danger whatsoever. Translated from Japanese, tsunami means “harbor wave” which is exactly the observable and devastating part of it. There is a known case of a tsunami destroying everything along the almost 300 kilometers long coastline in Japan and not being noticed at all by the fishermen who were only 40 kilometers far from the coast (Levin & Nosov, 2008, p. 3).
The tsunami wave retains its velocity while it travels through deep water and, depending on the depth, can reach speeds at which it can cross the Pacific Ocean in less than a day (Helal & Mehanna, 2008, p. 788). As it approaches the shallow coastal areas, the wave begins to transform, losing much of its velocity due to the decrease in depth, however, its energy remains the same (Helal & Mehanna, 2008, p. 788), and for this very reason the amplitude of the wave increases dramatically, along with the decrease of its length. In this state a tsunami wave presents the most danger to the population and the structures located on the coast.
So, tsunamis behave as any other wave does and have the same qualities, such as wavelength, peaks and peaks altitude. They are generally caused by the destructive events that occur in the Earth’s crust, such as earthquakes, landslides or volcano eruptions; in order for such an event to generate a tsunami, it has to free up a massive amount of energy in a manner that would rapidly shift and elevate the water column in relation to other water masses. Due to the high amount of energy tsunamis have, they propagate rapidly through deep waters while having very small wave peaks altitude, long distance between wave crests and extremely long wavelengths compared to regular waves generated by wind. A tsunami reaches its most devastating power near the shore where the water is shallow enough to cause tsunami wave to lose its high velocity and convert the energy into a wave height. Since tsunamis are waves, their peaks are followed by the troughs; when it happens the shore seems to be safe enough to return and many people have lost their lives trying to get to their homes thinking that the tsunami has passed, while in reality it has been a trough before another upcoming wave.
References:
Helal, M. & Mehanna, M. (2008). Tsunamis from nature to physics. Chaos, Solitons & Fractals, 36(4), 787-796. http://dx.doi.org/10.1016/j.chaos.2007.08.044
Levin, B. & Nosov, M. (2008). Physics of tsunamis. Dordrecht: Springer.
Margaritondo, G. (2005). Explaining the physics of tsunamis to undergraduate and non-physics students. European Journal Of Physics, 26(3), 401-407. http://dx.doi.org/10.1088/0143-0807/26/3/007
