Synoptic Description of Tsunami

Tsunami is a Japanese word used as the scientific term for seismic sea wave, or a series of large ocean wave with extremely long wavelengths and long period. A tsunami may travel hundreds of kilometers across the deep ocean, reaching speeds of about 725 to 800 km/hr (about 450 to 500 mph). Upon entering shallow coastal waters, the wave, which may have been only about half a meter (a foot or two) high out at sea, suddenly begins growing rapidly. By the time it reaches the shore, it may become a towering wall of water 15m (50 ft) high or more, capable of destroying entire coastal settlements. They are typify by shallow-water waves that can have a period on the range of ten minutes to two hours and a wavelength in excess of 300 miles. Moreover, since it has a very large wave length, it loses only a small amount of energy as it moves through the waters at times crossing great transoceanic distances at high speeds. Scale of Magnitude by Professor Imamura rates tsunamis from 0(less destructive) to 4(most destructive).

Causes of a Tsunami

A tsunami can be generated by ANY disturbance that displaces a large water mass from its equilibrium position. There are two categories for the trigger mechanism of a tsunami:
  • Terrestrial trigger mechanism are most often earthquakes, however, undersea landslides, volcanic eruptions, and explosions can also cause a tsunami.
  • Extraterrestrial trigger mechanism are impacts of cosmic bodies, such as meteorites in the ocean. Of course, these are rare occurences but is theorized that the disappearance of the dinosaurs can be linked to impacts of extraterrestrial material in the ocean which caused various large tsunami and destroyed everything in it's path.
In the case of earthquake generated tsunamis, the water is disturbed by the movement of the sea floor.

Tsunami generated by Earthquake of May 26, 1983 in Japan. Inundation at maximum level at Oga Aquarium in Akita Prefecture,Japan. Photograph from Tsunami Slide Set 648-A11-001.)

Submarine landslides, which usually accompany large earthquakes, as well as collapses of volcanic edifices, can also distort the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an sudden force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts also disturb the water from above.

Occurrences of a Tsunami

Most tsunamis originate along the so-called Ring of Fire, a zone of volcanoes and seismic activity 32,500 km long, that encircles the Pacific Ocean. Since 1819, about forty tsunamis have struck the Hawaiian Islands. Prediction of tsunami are difficult because they are linked to other natural disasters, such as earthquakes and volcanoes, which on their own are hard to predict. Studies have shown that there is an average of two destructive tsunamis per year in the Pacific Basin. However, Pacific wide tsunami are rare and can occur on average, every 10 to 12 years.Tsunamis can savagely attack coastlines. According to The National Oceanic and Atmosphere Administration, U.S. coastal communities are threatened by tsunamis that are generated by both local earthquakes and distant earthquakes. Local tsunamis give residents only a few minutes to seek safety. Tsunamis of distant origins give residents more time to evacuate threatened coastal areas but increase the need for timely and accurate assessment of the tsunami hazard to avoid costly false alarms. Thus, U.S. residents in Alaska can experience a local earthquake and tsunami while residents of Hawaii and the west coast may experience this disaster as a distant tsunami. Similarly, west coast residents can experience a local tsunami that may also have an impact on the distant states of Alaska and Hawaii. Of the two, local tsunamis are more devastating.

Social and Economical Impact of Tsunami

Tsunamis can cause very destructive effects. These effect are seen in three categories:
  • Hydrostatic is the lifting and carrying of wooden houses and objects, such as cars.
  • Hydrodynamic is the tearing of buildings and structures.
  • Hydroshock is where the crashing and piling of all debris is seen.
The large earthquakes in Hawaii's active rift zones of Mauna Loa and Kilauea have triggered tsunami in 1898 and 1975 which has devastated low-lying coastal areas. Landslides in Alaskan volcanoes have generated tsunamis that destroyed coastal villages.

Photographs from the 27 March 1964 Alaska Event from http://www.usc.edu/dept/tsunamis/alaska/

In the west coast of Nicaragua, 116 people were killed, and 4,000 people where left homeless when a tsunami crashed on the coast. It left 25 million dollars of damages and destroyed the infrastructure of the towns and the ecology of the area, such as the vegetation.

Tsunami!: The WWW Tsunami Information Resource
Related WWW sites
Tsunamis Database
Tsunami Publications and Data
A list of Tsunamis Events 1992-Present
OKUSHIRI Computer Simulation