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What you need to know about tsunami waves

What you need to know about tsunami waves

The word “tsunami” originates from Japanese and refers to a giant wave caused by underwater earthquakes, landslides or volcanic eruptions. Tsunami waves are almost imperceptible in the open ocean, but they reach immense height near the coast, causing widespread destruction. Tsunamis can spread for thousands of kilometers and affect several continents, even in areas where an earthquake or volcanic eruption that caused them was not felt.

What types of tsunamis are the most dangerous

Scientists identify three types of tsunamis, which may differ in nature, origin and destructive power:

  • local tsunamis occur within a range of up to 100 kilometers and are caused by earthquakes (magnitude 6.5 – 7.5), landslides or volcanic eruptions;
  • regional tsunamis extend from 100 to 1000 kilometers and are almost always the result of underwater earthquakes in subduction zones;
  • megatsunamis can devastate coastlines thousands of kilometers from the source.

Which types of tsunamis are considered the most dangerous? Scientists say that the destruction of the coastline and the number of tsunami victims depend not only on the wave height, strength and propagation speed. The topography of the coastline is also very important. While steep slopes deflect tsunami waves, gentler shores allow the water to penetrate further inland. Although mainland coastlines can be severely damaged by a devastating tsunami, atoll islands are often protected by coral reefs that break the waves.

Magnitude and intensity scale

How do scientists identify tsunamis? Several magnitude and intensity scales are used to measure the destructive waves.

Magnitude scale

Tsunamis are often classified by their magnitude, which is a measure of the total amount of energy they release. There are several methods for determining the magnitudes, the most used is the Imamura-Iida scale. It was introduced in 1942 and improved in 1956.

The scale includes 6 grades of magnitude (denoted by m):

  • m -1 – minor tsunami;
  • m 0 – the highest wave reaches 1 m high, not causing much damage;
  • m 1 – waves reach 2 m high, damaging houses and ships;
  • m 2 – 4–6 m waves destroy ships, port infrastructure, leading to casualties;
  • m 3 – waves reach 10–20 m high, destruction spreads along coastlines more than 200 m wide;
  • m 4 – the highest waves reach more than 30 m high and catastrophic destruction is observed along coastlines up to 500 m wide.

The magnitude scale is suitable for the analysis of tsunamis, regardless of their power and origin.

Intensity scales

In 1927, a German geophysicist August Sieberg proposed a scale for measuring intensity based on determining the damage caused by a tsunami. The Sieberg scale includes 6 intensity levels:

  • I – wave is weak and can only be registered by measuring instruments (mareographs);
  • II – effects of the tsunami are noticed only on the beaches;
  • III – tsunami causes damage to small vessels in harbors and light structures situated near the coasts;
  • IV – tsunami leads to the destruction of small boats, as well as coastal structures and beach infrastructure, vegetation and green spaces. The coast is flooded, debris is floating everywhere.
  • V – powerful waves several meters high causes flooding, destruction of buildings and casualties;
  • VI – catastrophic tsunami with giant waves moving at high speed and violently crashing against the coastline for kilometers. The number of casualties is in the hundreds or even thousands.

Measuring tsunami intensity is not internationally standardized. There are other quite comparable intensity scales. Among them are the scales proposed by Ambraseys and Soloviev.

Characteristics of a tsunami

What are the characteristics of a tsunami? The level of danger from tsunami waves moving toward the coast is determined by their main characteristics:

  • wavelength – the distance between two points with the same oscillation phases, most often the crest of a sine wave;
  • height – the difference in levels between the crests and troughs;
  • amplitude – the difference in height between the crest of a wave and the sea level;
  • speed of a wave traveling across the ocean;
  • run-up – the height to which the tsunami wave rises above sea level when it reaches the coastline.

By considering and analyzing these indicators, scientists can predict the arrival time of tsunami waves and estimate the potential level of destruction. This makes it possible to warn the population of an approaching tsunami and know exactly how long coastal communities will be in danger before the disaster recedes.