What is a Tsunami? Definition, Facts and Figures.

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The word “tsunami” originated in Japan. It means “harbor wave”. The term originated from the impressions of fishermen who had returned to their home port and found it devastated. The destruction hit them completely unprepared because they hadn’t noticed any waves at sea. The word “tsunami” was therefore used to describe the destructive effects of the wave, particularly on the coast. The term “tidal wave” is often used to refer to tsunamis. According to NOAA, shallow water wave caused by effects of gravity are unrelated to tsunamis.

Main causes of tsunamis

90% of the tsunamis are caused by violent earthquakes that take place on the ocean floor. The remaining 10% are caused by volcanic eruptions, underground landslides or, in rare cases, by meteorite strikes.

A tsunami arises as a result of sudden, large-scale elevation or subsidence of parts of the ocean floor in the event of an earthquake or the sliding of large masses. The movement of the ocean floor causes water masses to vibrate. The displacement of water leads to peak-valley wave development on the surface of the sea. These waves spread in all directions.

Thus, a tsunami forms by the sudden displacement of large amounts of water. This wave is flat on the open sea and remains almost unnoticed there. For example, at water depth of 5,000 meters, the wave on the high seas can be a few centimeters to one meter high. It therefore remains almost unnoticed there and poses no danger to ships.

However, as soon as the water becomes shallow, it builds up and can pile up to a wave that is tens of meters high. A significant drop in water level often precedes  a tsunami. This drop is a clear indication of a possible future tsunami. However, this connection is often not known to residents of regions at risk or tourists.

When the tidal wave reaches the coast, water can penetrate a few hundred meters to several kilometers inland, and cause severe flooding there. In addition to earthquakes, landslides, submarine landslides, meteorite strikes or volcanic eruptions in which large amounts of earth slide into the water can trigger tsunami waves.

Wave-propagation and speed

The rate of propagation of a tsunami wave strongly depends on the depth of the water. In the deep area, the tsunami wave remains very flat, but is extremely fast at a speed of around 800 km/h. This roughly corresponds to the speed of a passenger plane. In the shallow water area, the speed is reduced considerably, but this causes the wave to rise and can reach a height of more than 30 meters.

The speed of a tsunami depends on the depth of the sea. The deeper the sea, the faster the tsunami moves. The flatter it becomes, the slower it moves .

Speed of the tsunami (meters/second), can be calculated using the following formula:

  • Tsunami wave speed = root of wave’s acceleration due to gravity (a constant 9.81 meters/second) times water depth at which the quake originated.

The speed at ocean (at a depth of +/- 5000 m) is +/- 800 km per hour. This is extremely fast and roughly corresponds to the speed of an airplane. Tsunami waves can cross entire oceans in a few hours and spread over 20,000 km.

However, when the tsunami hits shore, the speed of the wave can still exceed 30 kilometers an hour. This would usually be far too quick for running away. Often, in these extremely dangerous situations, the only available option is to escape to a higher point as quickly as possible.

Other characteristics of tsunamis

A typical wavelength of a tsunami is between 100 and 500 km. Waves formed by the wind, for example, only have a wavelength of 100-200m. This explains why the Japanese fishermen did not notice any waves at sea.

Often, the tsunami is also underestimated because those affected on site assume that the danger is over with the first wave. But the arrival of a tsunami can occur in several waves and it may well be that later waves are even higher than the first one. Since these subsequent tsunami waves can only arrive after several hours, it regularly happens that residents assume that the danger has already been averted. They return to their homes and then are surprised by the following wave.

Tsunami effects

What happens when a tsunami hits a coast? The total energy of a wave is composed of the kinetic energy (depending on the speed of the wave) and the altitude energy (depending on the height of the wave). The depth of the sea decreases near the coast. If we look at the formula for calculating the wave speed, it follows that the wave slows down and the kinetic energy of the wave decreases.

In simple terms, the total energy of the wave remains constant. As a result, the altitude energy increases and the wave becomes higher. This explains why the village of Japanese fishermen could be destroyed by such a huge wave.

Typical wave heights of tsunamis when hitting the coast are +/- 10 meters. On April 24, 1971, a record height of 85 meters was measured on flat ground on the Japanese island of Ishigaki. If a tsunami hits a fjord, the wave can build up to well over 100 meters.

Regions at risk of tsunami

Tsunamis occur particularly along tectonic plate boundaries, the so-called tectonic subduction zones. There the oceanic plate meets the continental plate and is pushed under it. This creates tensions in the earth’s crust, which are discharged by earthquakes. A particularly vulnerable region is the so-called Pacific Ring of Fire, which covers almost the entire peripheral area of ​​the Pacific.

The two most devastating tsunami disasters in recent years were triggered by very strong earthquakes in 2004 in the Indian Ocean off the coast of Indonesia. And in 2011 in the Pacific off the coast of Japan along subduction zones. But not only the Pacific is an endangered region. In the past, tsunamis were also triggered in the Atlantic and Mediterranean by earthquakes and volcanic eruptions.

For example, in 1775 the Portuguese capital Lisbon was almost completely destroyed by a tsunami up to 20 meters high and the subsequent major fire. It is now believed that this natural disaster cost the lives of more than 50,000 people. To improve the protection of the population, tsunami early warning systems now exist in many hazard regions around the world.

Major tsunami events in the last 100 years

Year Location Earthquake magnitude * Water level Fatalities
2011 Japan 9 38.9 15,844
2004 Indonesia 9.3 50.9 227,898
1998 Papua New Guinea 7 15.03 2,205
1976 Philippines 8.1 8.5 4,376
1952 Russia 9 18 4,000
1946 Dominican Republic 7.8 5 1,790
1945 Pakistan 8 15.24 4,000
1941 India 7.6 1.5 5,000
1933 Japan 8.4 29 3,022
1923 Japan 7.9 13 2,144

* The magnitude of the earthquake that caused the tsunami

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