What are the general characteristics of earthquakes
The most dangerous side effects of an earthquake for humans are giant waves, the so-called tsunamis. They are mainly caused by submarine earthquakes, can reproduce for many thousands of kilometers in the oceans and ultimately destroy distant coastal areas.
How do tsunamis develop?
For a tsunami to actually develop from a seaquake, three conditions must be met:
- A very strong earthquake must take place at the bottom of an ocean. The measured magnitude must be at least 7.
- The center of the seaquake must be near the sea floor. “Hypocentres” located deeper in the earth's crust do not trigger tsunamis.
- During the seaquake, the sea floor must rise or fall suddenly. A horizontal displacement of tectonic plates does not create a giant wave.
Tsunamis have special properties that clearly distinguish them from wind waves. In particular, a tsunami is not a phenomenon on the surface of the water, but rather a movement of water that extends to the bottom of the sea: When the sea floor rises or falls during an earthquake, it causes the entire water column above to vibrate. From this point of origin, the tsunami spreads through the oceans at a very high speed. 800 kilometers per hour are not uncommon.
A tsunami can hardly be seen with the naked eye on the open sea, as the wave crests are very low. The phenomenon only becomes a destructive giant wave when the tsunami approaches shallower waters. Then the wavelength shortens and wave crests build up, which in extreme cases over 100 meters high to reach.
This peculiar behavior - inconspicuous on the open sea, destructive on the coast - contributed to the naming “tsunami”: When Japanese fishermen returned from their catch, they found their home port devastated, even though they had not noticed a giant wave at sea. Therefore the fishermen coined the term "Tsu-nami", which translates "Wave in the harbor" means.
Worldwide spread of tsunamis
Tsunamis can basically occur anywhere where the conditions mentioned above (strong seaquakes with vertical displacement of the sea floor) occur. In almost 80 percent of cases, tsunamis occur on the edges of the Pacific tectonic plate, the so-called Pacific ring of fire. Especially those tsunamis that occur in great depths of the sea can spread very far without losing any of their destructive energy. In about half a day, for example, they can cross the entire Pacific Ocean from Japan and reach the North American coast.
Tsunamis are also possible in the Mediterranean. If there are several earthquakes in ancient greece tsunamis have been shown to have occurred and destroyed coastal cities - long before the term “tsunami” was even invented. In Greece, the special course of the coastline promotes the wave height and destructive power: in narrow bays and straits, the water can accumulate particularly quickly and high.
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