Most large earthquakes originate in subduction zones where an oceanic plate slides under a continental plate or under a younger oceanic plate. When this occurs on the ocean floor it can lead to a tsunami like the one that hit Japan on March 11, 2011. The tsunami is formed by a series of extremely long ocean waves – with wavelengths of the order of hundreds of kilometers – that, in deep water, propagate at a speed that can exceed 800 kilometers per hour. As the tsunami reaches coastal waters the water column slows down, rapidly increasing the energy and height of the wave . The waters recede just before the tsunami erupts, sometimes exposing the reefs.
The tsunami attacks the coast, devastating everything both when entering the land and when leaving the sea again. In fact, the return wave can be as devastating or more devastating than the incoming wave , since when the body of water flows back to the sea it carries debris that increases its pushing force.
“Tsunami” is a Japanese word that means “wave in the bay”, but that does not say much about its nature, since tsunamis are not limited to ports or bays. A more precise term would be “seismic waves of the sea”, and it would describe them more precisely. However, the word tsunami is what we all use today. Some people refer to them as “tidal waves”, but that term is technically incorrect and should be avoided in this context.
Tsunamis are waves, but unlike waves caused by the wind, they have a much longer wavelength. In the context of physics, not in the context of ocean waves, a defining characteristic of each wave is its wavelength. Wind waves have short wavelengths that can be seen clearly on any coast. They come every few seconds, but a tsunami has a great wavelength, often over a hundred kilometers and that is why they are so dangerous.
The destruction of a tsunami comes from all the energy stored in that water. The wave hits the land and due to the amount of energy that the water pushes, it just flows towards the land. It does not stop, because there is more water behind with so much energy pushing. This is what causes destruction. It is not a massive wave that swallows entire buildings. Instead, it is the energy behind the wave that does all the damage.
In rare cases, tsunamis can also be caused by landslides, volcano eruptions, and meteorites. In all cases, the main principle is the same: a body of water moves and when it approaches the coast it begins to grow in height. However, the displacement mechanism differs in each of the scenarios.
Landslide
Underwater, landslides are often similar to volcanoes that create avalanches in the sea. This process occurs as a result of an earthquake, so in a way, the main source is still an earthquake. However, earthquakes can also simply loosen the land mass that begins to fall at some later point.
Volcanoes can form tsunamis through two mechanisms: either they collapse or they eject matter with such force that they lift the water. In the first case, terrestrial volcanoes can also cause tsunamis, if they are very close to the sea.
By now, you may have realized why tsunamis are so dangerous. They can be very long (100 kilometers is a reasonable length), very tall (the 2011 Japan tsunami measured more than 10 meters), and they can travel extremely fast without losing much of their energy. An earthquake in the ocean can send several devastating tsunamis hundreds or even thousands of miles away.
