"Imagine a flood of water swirling up the street,
lifting cars and flinging them through flimsy buildings
as it goes. Then imagine the buildings themselves being
washed away, leaving behind little more than scattered rubble."
This is a tsunami.
( Natural Disasters, Readers Digest, 1996.)
A tsunami is often misnamed a tidal wave, but in fact a tsunami is not just one wave but usually a series of seven or eight, that have nothing to do with the tide. In the open ocean, tsunamis are only about one metre high, but as they approach shallower waters and the shore, they grow to heights as high as eighty-five meters.
(Natural Disasters Readers Digest 1996)
|The most common causes of tsunamis are volcanoes,
earthquakes and earthslides - mostly undersea.|
Volcanoes that have been erupting continuously for a long time have empty magma chambers. The roof then collapses forming a crater somtimes upto one kilometre in diameter. Water gushes into this crater in a very short amount of time, causing a tsunami.
Earthquake originated tsunamis occur when portions of the
Earth's crust on either side of a fault jolt past each other. For a
tsunami to occur however, there must be some kind of vertical
movement along the fault. This vertical movement must be
capable of displacing huge amounts of water, thus causing
Most tsunamis occur in the Pacific ocean as it is located on a plate mostly made of water. The Pacific ocean is also surrounded by the Ring of Fire, a highly active volcano and earthquake zone. The Ring of Fire circles the ocean from Alaska down to the west coasts of North and South America and up along the east coast of Asia, taking in parts of China, Japan and Russia. Tsunamis can only occur in coastal regions; islands are the main targets.
|Puerto Rico Trench, Caribbean||Port Royal, Jamaica permanently submerged||2 000|
|November 1, 1755||Atlantic Ocean||Lisbon destroyed||60 000|
|February 20, 1835||Peru-Chile Trench||Concepción, Chile destroyed||Not Known|
|August 8, 1868||Peru-Chile Trench||Ships washed several miles inland, Town of Africa Destroyed||10 000 - 15 000|
|August 27, 1883||Krakatoa||Devastation in East Indies||36 000|
|June 15, 1896||Japan Trench||Swept the east coast of Japan, with waves of 100 ft (30.5 m) at Yoshihimama||27 122|
|December 28, 1908||Sicily||East coast of Sicily, including Messina, and toe of Italy badly damaged||58 000 (including quake victims)|
|March 3, 1933||Japan Trench||9 000 houses and 8 000 ships destroyed in Sanriku district, Honshu||3 000|
|April 1, 1946||Aleutian Trench||Damage to Alaska and Hawaii||159|
|May 22, 1960||South-central Chile||Coinciding with a week of earthquakes. Damage to Chile and Hawaii||1 500 (61 in Hawaii)|
|March 27, 1964||Anchorage, Alaska||Severe damage to south coast of Alaska||115|
|August 23, 1976||Celebes Sea||South-west Philippines struck, devastating Alicia, Pagadian, Cotabato and Davao||8 000|
In the open ocean tsunamis are almost undetectable. However a tsunami warning now operates in the Pacific. This system monitors sea movements and can be used to map the path of tsunamis and estimate the rate of approach. The authorities then notify the public and evacuations are undertaken. One such warning saved many lives in Honolulu in 1952 when an approaching tsunami was detected.