tsunami


(soo-NAH-mee)

Tsunami wave

Terminology:

Tsunami is the name given a wave
caused by seismic disturbances.

The name is a Japanese word that is a combination of two words: `ami' – wave, and `tsu' – a particular point at the waterline. So, a tsunami is a wave that approaches the shoreline. The term `tidal wave' is incorrect and is no longer used. Tides are predictable, rhythmic variations of the sea surface induced by the attraction of the moon and the sun.

How fast does a tsunami move?

The speed of the tsunami wave depends on water depth. If an earthquake on the sea bottom occurred offshore at a depth of 1000m, the speed of a tsunami is about 360 km/h. A tsunami is a very long wave of the order of kilometres that often remains undetected offshore. However, when a tsunami reaches shallow water, the speed decreases. At a water depth of 10m, its speed is about 35 km/h.

How big can a tsunami be?

The height of a tsunami depends on the intensity of the earthquake. Assuming that at the initial offshore point (at a water depth of 1000m) the initial height is 2m, at the coastline (at 1m water depth) a tsunami wave will be as high as 11.2m.

Can we protect the coast against tsunamis?

Because of the enormous energy of tsunami waves, it is not possible to protect coastal regions. The only solution is to establish a warning system and evacuate all coastal populations on time.

Tsunamis occur mostly in the Pacific Ocean region as this ocean experiences frequent seismic activity. Catastrophic tsunamis have been recorded in many historical Japanese, Chinese and American documents.

In 1495 , a large tsunami wave struck the southern coast of Honshu Island, Japan. A huge hall, containing an 11.3m tall bronze statue of Buddha weighting 800 tonnes, was completely washed away. The statue was eventually found 50 m above sea level, 1.5 km from its original location.

Coastal communities in eastern Russia, Japan, Alaska and northern California are particularly threatened by tsunamis generated by local earthquakes. When a large earthquake occurs, the first tsunami waves may reach nearby coastal communities within 10 minutes of the event.

On October 4, 1994 , an earthquake of magnitude Ms 8.0 (Mercalli Intensity Scale) struck the southern region of the Kuril Islands, Russia. A tsunami run-up of approximately 1.8m was reported in Nemuro, Japan, about 90 minutes after the earthquake. In the South Kuril Islands, 11 people were killed and 242 were injured. In Hokkaido, Japan, one person was killed and 140 injured. None of the casualties were due directly to the tsunami, in spite of its significant run-up of approximately 10 m on Shikotan Island. Perhaps this is because most of the residential houses are made of wood which tends to be invulnerable to shaking. On the other hand, inadequately reinforced masonry structures could not withstand the strong shaking and were destroyed.

In order to minimise the number of casualties and reduce damage due to tsunamis, three types of tsunami warning systems exist:

  1. the Pacific Tsunami Warning Centre,
  2. five regional systems (two in the United States and one each in Japan, Russia, and French Polynesia), and
  3. local systems in Chile and Japan.

The Pacific-wide System can issue a warning about 1 hour prior to arrival of the tsunami (useful for populations located more than about 750 km from the source).

Regional systems warn about 10 minutes prior to arrival of the tsunami (useful for 100--750 km from the source).

Local systems warn in about 5 minutes prior to arrival of the tsunami (useful for areas situated less than 100 km from the source).

References
Dr. S. Massel, Fluid Mechanics for Marine Ecology, published by Springer-Verlag (Germany).