Tsunamis can reach considerable heights on coasts due to the nature of the waves. In open sea, tsunamis have low amplitude but very long wavelength, allowing energy to concentrate as they approach shallower areas, causing a dramatic increase in wave height.
When tsunamis move across the open sea, they go unnoticed, with waves only a few dozen centimeters high, but beneath the surface, they carry a lot of energy. As they approach the coast, the water becomes shallower, and this energy has to go somewhere, which quickly becomes problematic. As the seabed rises, the speed of the wave significantly decreases, and to compensate, the height of the waves increases dramatically. In short, all this energy spread over a large area in the open sea compresses suddenly into a much taller and more powerful wave near the shore. This rapid transfer of speed to height explains how a discreet tsunami in the open ocean can turn into a destructive wall of water near the coast.
When a tsunami wave approaches the coast, it encounters the features of the seabed: elevations, depressions, or reefs. These irregularities play a huge role in how the energy of the wave circulates. A steep underwater relief, such as a steep slope or a reef, causes the water to suddenly slow down, resulting in a piling up of water and a significant increase in wave height. It's somewhat similar to driving a car: a steep incline slows down the speed but increases the pressure on the front wheels. Conversely, a shallow seabed over a long distance can gradually compress the energy, producing very high waves as it approaches the shore. It’s really all about how the seabeds shape the path of wave energy.
Some places act a bit like a resonance chamber. When a wave reaches a bay or a harbor, the particular shape of the coastline can trigger a phenomenon called coastal resonance. Imagine it like a swing: if you push it at just the right rhythm, you'll go higher with little effort. Here, the wave "lands just right" on this natural resonance frequency of the location, and its amplitude increases dramatically. As a result, a small, innocuous wave from the open sea can quickly turn into a giant wall of water near the shore, surprising everyone with its magnitude. This phenomenon explains why some coastlines sometimes experience tsunamis that are so tall they seem unreal.
When the tsunami approaches the coast, it can encounter coastal currents or a rising tide that strengthen it. Imagine a powerful wave arriving just at high tide: the already elevated water facilitates a spectacular amplification of the phenomenon. Conversely, if the tide is low or if the coastal current is opposing, it can slightly slow down the tsunami and take away some of its energy. Local tidal differences also explain why some areas will experience a huge devastating wave, while others, just next to them, will be relatively spared. Similarly, some strong currents can laterally shift the energy of the tsunami, directing it towards coasts where the impact is then significantly more violent.
Wild animals sometimes have the astonishing ability to sense the arrival of a tsunami, thanks to their sensitivity to ground vibrations or changes in atmospheric pressure, which can allow them to flee in time to higher ground.
In 1958, a gigantic landslide in Lituya Bay, Alaska, generated a tsunami that reached the incredible height of over 500 meters — the highest wave ever recorded!
The sea level may first drop dramatically before the arrival of a tsunami, sometimes revealing areas that are usually underwater — this phenomenon is a clear warning sign that should prompt a quick evacuation of the coast.
The term 'tsunami' comes from the Japanese 津波 — 'tsu' meaning harbor and 'nami' meaning wave, literally a 'harbor wave', as these phenomena are first noticed due to their violent impact on the coasts.
The exact height of a tsunami as it approaches the coast is difficult to predict accurately because it heavily depends on the underwater topography, the shape of the coastline, and other local interactions such as coastal currents or tides. However, modern numerical models allow for reasonable estimates to alert at-risk populations.
Among the warning signs, it is common to observe a rapid and unusual drop in sea level, thus exposing the seabed. There may also be a series of unusual or fast waves arriving at the shore, signaling the imminent approach of a tsunami.
No, it is not automatic. Only sufficiently powerful underwater earthquakes that involve a significant displacement of the seabed can trigger major tsunamis. Many underwater earthquakes have no noticeable effect on the surface of the ocean.
A typical ocean wave, generated by the wind, carries a small amount of energy and only affects the surface of the water. In contrast, a tsunami generally results from disturbances in the seabed, has a significant depth of water mobilized, and carries immense energy over great distances.
Yes, a single tsunami can travel long distances across an entire ocean. Its radial propagation means that it can reach several distant coasts at different times, sometimes several hours after its origin.
In open water, tsunamis have a very long wavelength and a low amplitude, often less than one meter. This makes them almost imperceptible to ships sailing in deep waters, which explains why they often go unnoticed before approaching the shores.
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