Tsunamis only occur in the ocean
Tsunamis - the danger of seaquakes
At the latest since Christmas Day 2004, when huge waves ravaged the coasts of the Indian Ocean, tsunamis have also been a household name in this country. They are sometimes mistakenly called tidal waves - but they have nothing to do with the tides. The cause of a tsunami is usually a seaquake.
Not all tremors in the earth's crust under the sea have catastrophic consequences on the coasts. Because for a tsunami to occur, the seaquake must meet two conditions: firstly, the center of the quake must not be too deep, and secondly, the earthquake must be strong enough to displace large masses of water by moving the sea floor.
These conditions are often met in subduction zones where one continental plate is pushed under another, such as the so-called ring of fire that surrounds the Pacific and on which Sumatra is also located. If the tension between the plates is suddenly released, the earth will shake.
The ring of fire surrounds the Pacific Ocean.
An earthquake that is not too deep for a tsunami, i.e., from the point of view of geophysicists, is close to the sea floor, means here: The epicenter is no deeper than 70 kilometers. Because the energy of the quake must be able to be transferred to the water. A strong but deep seaquake hardly shakes the sea floor and therefore cannot trigger tsunamis.
The shifts on the sea floor are the actual trigger for the tsunamis. In the 2004 Sumatran seaquake, the quake zone stretched more than 1200 kilometers, and the sea floor rose and fell in some places by several decimeters. The seaquake does not necessarily have to be that huge, it can also be only 100 kilometers long. But the sea floor should move vertically by a few decimeters to meters quickly, because that is the decisive criterion for whether a tsunami occurs.
How a seaquake in a subduction zone triggers a tsunami
A seaquake that sets off a tsunami usually releases enormous amounts of energy - hundreds to ten thousand times that of the atomic bomb explosion in Hiroshima. On the moment magnitude scale, which is related to energy, tsunami quakes usually have a value greater than 6.5.
When the sea floor rises or falls, the entire column of water above it shifts. An imbalance arises: In the balance, waves are formed, which are called gravity waves, because gravity is the restoring force of this oscillation. Tsunamis are not pressure waves. They have both a transverse and a longitudinal wave character. In contrast to the wind-driven waves on the surface, in a tsunami the entire mass of water is in motion from top to bottom - but the individual water droplets only move a few centimeters to decimeters on an elliptical orbit. Together they transport the energy with almost no loss because the molecular friction is low. All waves are equally fast and therefore, unlike many other wave phenomena, do not diverge. Physicists say that the speed of propagation shows no dispersion. For these reasons, tsunamis can cause great damage even at a distance of several thousand kilometers, such as the Sumatra tsunami on the East African coast.
A tsunami quickly reaches even distant coasts
Tsunamis travel across the open sea at almost unbelievable speed - they are faster than a commercial airplane. At a depth of 7,500 meters, the waves rush towards the coast at 950 kilometers per hour. In the case of linear waves, the speed does not depend on the amplitude. But tsunamis are non-linear wave phenomena - they change their speed and shape depending on the depth of the sea: the deeper the ocean, the faster the tsunami, the greater the distance between the wave crests and the smaller the amplitude.
As a result, the sea level only rises and falls by a few decimeters above the abyss of the oceans. This explains the name of the giant waves: Japanese fishermen gave the phenomenon its name. After they returned to their port from fishing, they found it completely devastated by large waves. But they hadn't noticed the slightest swell in the open sea. That is why they called the mysterious waves Tsu-nami, which means "wave in the harbor".
The physics of tsunamis is now well understood. The energy of the waves, which is still widely distributed in the free ocean, is concentrated by non-linear mechanisms when the tsunamis come close to the coasts. Then the waves are braked, compressed and steepen.
The speed of the tsunamis slows down on the coast. The waves then still have a speed like a fast cyclist. While the crests of the waves are several hundred kilometers apart in the open sea, they crowd close together on the shore, like an accordion, and follow at intervals of a few thousand meters. Then the waves arrive every 10 to 60 minutes.
It is not necessarily the case that a crest of waves rushes up on the coast - a wave trough hurrying ahead often lures ignorant beach visitors into a deadly trap. The first wave doesn't have to be the biggest either.
Depending on the shape of the bank, the waves pile up at different heights. On steep coasts, the crests of the waves form particularly suddenly and become very high - ten to thirty meters. This is often the case in Japan, for example. Small differences in the shape of the coast can cause large differences in the shape of the waves: In fjord-like bays, the wave crests can reach a height of more than 100 meters in individual cases. However, tsunamis do not develop significant wave crests on other parts of the coast. On flat coasts, for example, the tsunamis are often not particularly high. But there are strong currents that penetrate far inland.
A tsunami quickly reaches even distant coasts
Not only seaquakes cause tsunamis - submarine volcanic eruptions and landslides on coasts are also possible causes. In this case, the water masses are not suddenly displaced by an earthquake, but by a mass movement on an undersea slope. It can be triggered by a volcanic eruption. For example, it is feared that in the event of a volcano erupting on the Canary Island of La Palma, a mountain flank could slide into the sea and trigger a megatsunami that devastated the American east coast - as was done 120,000 years ago by Hierro, another island in the Canary Islands happened once. At that time, huge masses of rock fell on the coast. Also, if a meteor falls into the sea and displaces large amounts of ocean water, it can cause a tsunami.
By far the most common trigger are the seaquakes, especially in the Pacific. Around two thirds of all tsunamis worldwide occur there - the Hawaiian archipelago is hit on average once a year. But also in the Indian Ocean, the Atlantic Ocean and the Mediterranean Sea, these waves crash against the banks. On May 21, 2003, a violent earthquake off the coast of Algeria caused a weak tsunami on the coast of Mallorca. It only took the tsunami about an hour to travel from the North African coast to the Balearic Islands. The one meter high “wave in the harbor” destroyed around a hundred boats.
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