Every tsunami is a tidal wave—but not every tidal wave is a tsunami. From what dimension can it be classified like this?
“It needs to be gigantic in wavelength”, explains physicist Fernando Lang da Silveira, a professor at the Federal University of Rio Grande do Sul (UFRGS). This means that it needs to have a wavelength of at least 10 kilometers, and can reach up to 500 kilometers offshore.
“Wavelength” is the physics term that measures the distance between two consecutive crests (the “top” of the wave) or two valleys (the bottom).
In classifying tsunamis, factors such as amplitude (distance between crest and trough), period (time of a complete oscillation, i.e. a crest and a trough) and frequency (number of oscillations in a certain amount of time) are also considered. time).
In comparative terms, while a normal sea wave has a period of up to a few tens of seconds, in a tsunami this time is a few minutes or even half an hour. And, despite having a small amplitude on the high seas, when it gets close to the coast the wave can become gigantic.
biggest ever recorded
In a work published together with mathematician Maria Cristina Varriale, also from UFRGS, Lang says that the largest tsunami ever recorded occurred in Alaska, in 1958, when 90 million tons of rock and ice collapsed into a bay, generating a wave of 50 meters high.
Another impressive example is the tsunami that shook Sanriku, Japan, in 1896, reaching a height of 30 meters. The incident killed 27,000 people and injured 9,000. “In the Pacific Ocean region alone, more than 1,100 tsunamis have been recorded in the last twenty centuries,” says Lang.
But the height of the waves is not always the main aspect to be considered. In the few available images of the Indonesian event in 2004, which killed nearly 300,000 people, the waves did not appear to be that big. “A tsunami doesn’t have to be giant in amplitude to produce devastating effects,” says the physicist.
“Tsunami” is a word in Japanese that designates one or more waves produced in tidal waves. In other words, tsunami and tidal wave are terms used to name basically the same phenomenon, caused not only by seismic movements, but also by volcano activity, submarine landslides, meteorite impacts or meteorological phenomena.
This Japanese term became more popular in Brazil after 2004, when the catastrophe in Indonesia occurred. As the word is used in several countries, it ended up being adopted as a standard by news agencies.
*With text by Tatiana Pronin