Natural Disasters- Structure and Actions of Tsunamis

A Tsunami is a series of waves caused by a displacement of a very large amount of water. These usually happen in oceans or, sometimes, very large lakes. Events that could trigger a tsunami are earthquakes, volcanic eruptions, underwater landslides, glacier calvings or explosions.

Unlike normal waves, generated by the wind or the pull of gravity from the moon and sun a tsunami is caused by a large event displacing a large amount of water. This displacement is usually noticeable by the water receding from the shore very quickly.

Tsunamis cause damage by two mechanisms: the force of a wall of water travelling at high speed, and the destructive power of a large volume of water draining off the land and carrying a large amount of debris with it, even with objectively small waves.

“Normal” waves have a wavelength of around 100 meters and a height of about 2 meters. A Tsunami has a wavelength of up to 22 kilometers in the deep ocean and can travel at the high speed of around 800 kilometers per hour.

Owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 meter - making tsunamis difficult to detect over deep water, where ships are unable to feel their age.

The speed of a tsunami can be calculated using the square root of the depth of the water multiplied by the acceleration due to gravity.

All waves have a positive and negative peak. For sea-waves these are called ridge and trough. In the case of a tsunami, either could be the first to arrive.

If the ridge arrives first, the first noticeable effect on land will be a massive breaking wave or sudden flooding. If the first part to arrive is a trough, the shoreline will recede dramatically. The drawback can exceed a couple hundred meters exposing normally submerged areas.

The typical wave period for a tsunami is around twelve minutes. The drawback phase is around three minutes. The next six minutes, the ridge builds that may flood the coast and the destruction ensues, the following minutes the ridge changes into a trough and sweeps debris – sometimes also people - into the ocean. The process then repeats as a series of succeeding waves.