Understand the phenomenon that makes the sea “shrink” before the giant wave

The sudden retreat of the sea occurs before the arrival of the tsunami due to the formation of the “valley” of the wave, when the front water is withdrawn to form the summit that will come after that. In tsunami, this movement includes the entire water column from the bottom of the ocean to the surface and can display large areas of the coast. This phenomenon was recently recorded before the waves reached Russia, Japan and Hawaii, and it has already occurred in tragedies such as Thailand in 2004 and Indonesia in 2018.

Experts explain that retreat occurs only when it strikes the lower part of the coast wave before the top, which does not occur in each tsunami. Marcelo Dottori, from the USP Oceanic Science Institute, warns that “rapid and unfamiliar” shrinkage “can be a sign of an imminent danger, especially in areas such as the Pacific Ocean. Unlike the low and slow and predictable tide, the pre -trimmami decline is surprising and important.

The interval may differ between decline and the wave arrival from 10 to 15 minutes, which is not enough for frequency. The recommendation is to search for a shelter immediately in high places and away from the coast, where a tsunami usually occurs in sequence, with subsequent waves and may be stronger than the first.

In the open sea, a tsunami can travel at an altitude of 720 km/h. As you approach the coast, the depth decrease reduces speed and increases the height of the wave, which accumulates a large volume of water advanced on the coast. Therefore, recognition and response with the sudden decline of the sea can be decisive in saving lives.