Wind generated waves
Speed, height and wavelength of tsunamis depending on the depth of the sea
Tsunami (or seaquake) waves have completely different causes, dynamics and characteristics from those caused by wind. Seaquake waves differ from common sea waves in some substantial characteristics: understanding these differences is essential to protect oneself from risk and can save many lives.
Tsunami waves are generated, in fact, by the sudden movement of a large mass of water due to earthquakes, volcanic eruptions or landslides. Much more rarely, from the fall of meteorites into the sea.
In the Mediterranean Sea there are several geological structures capable of generating tsunamis, including large ones. The most important one is the subduction area between the eastern Mediterranean and the Aegean Sea, called the Hellenic Arch, where the European and African plates meet. This area is the most active in the Mediterranean. Many scholars believe that the earthquake of 21 July 365 AD, with a magnitude of more than eight, was generated by the movement of a segment of this structure, located in a stretch of sea south west of Crete.
Tsunamis occurring in the Mediterranean Sea can be generated either by earthquakes linked to the phenomenon of subduction, as described above, or by events generated by crustal faults. Generally, the latter are relatively less strong than the subduction ones, but can give rise to very dangerous tsunamis at local level, as occurred with the one that hit the coasts of Calabria and Sicily immediately after the Messina earthquake in 1908.
Common sea waves produced by the wind move only the most superficial layers of the water column and do not cause any movement in depth, while tsunami waves move the entire water column, from the bottom to the surface, carrying huge amounts of energy.
Even the "shape" of tsunami waves is very different from that of sea waves: From a physical point of view, seaquake waves are characterised by very long wavelengths (distance between two ridges), in the order of tens or hundreds of kilometres.
In addition, tsunami waves are much faster than any wave produced by the wind: in open sea they can, in fact, reach even 700-800 km/hour. Tsunami waves in the open sea often go unnoticed due to their low height, from a few centimetres to a few dozen centimetres: a ship in the open sea can be hit by a tsunami without passengers noticing anything.
However, as the coast approaches, the waves change shape, shortening in length and increasing in height: their speed is reduced (as it is directly proportional to the depth of the water) and consequently the height of the wave increases, creating wave fronts which can reach even a few dozen metres.
Tsunami waves are able to propagate for thousands of kilometres: since they maintain their energy almost unchanged, they are therefore able to strike with exceptional violence even on coasts very far from their point of origin.
This phenomenon, known as teletsunami, is very common with major tsunamis such as the 2004 Sumatran tsunami, which caused casualties in some African coastal nations, many thousands of miles away. In Somalia alone, almost three hundred people were killed, and many other deaths were recorded in Kenya, Tanzania and South Africa.
This explains why tsunami waves, even low ones, have great energy and are able to penetrate the hinterland for several hundred metres or kilometres, crushing everything they encounter and causing serious damage.
Sometimes the tidal wave manifests itself with a phenomenon of initial withdrawal of the waters (marine regression) which leaves ports and ships dry. In some films shot in Thailand on the occasion of the 2004 tsunami, you can see large fish floating on the just uncovered seabed.
This phenomenon usually depends on the orientation of the fault which generated the earthquake in relation to the coast: if the fault block closest to the coast drops, it draws water back to the source area. In reality, this withdrawal is nothing more than the wave trough (negative wave) and, therefore, heralds the arrival of the next crest and the consequent flooding (ingression).
When the tsunami reaches the coast it can appear similar to a tide which grows very rapidly, raising the water level even by many metres; or it can appear as a series of waves, of which the first is not necessarily the largest; or it appears as an actual wall of water and, in these cases, the impact of tsunami waves on the coast is devastating.
However, even "small" tsunamis arriving on the coast at heights of only thirty or forty centimetres can be very dangerous for people: when they arrive in fact, their speed can reach up to forty kilometres per hour, enough to drop any adult to the ground and to drag him to sea. In some surveillance videos shot during the tsunami of Kos/Bodrum on 21 July 2017 you can see how these small tsunami waves managed to move heavy cars tens of metres inland from the coast.