On 27 August 1883, a strong eruption almost destroyed the island of Karakoa. From this powerful explosion huge waves arose from the Ocean. The waves reached a height of 30 metres above sea level and managed to wipe out hundreds of villages with speeds of 1000 km per hour. People felt the after effects thousands of miles away in Australia and Canada.
In 1946, a big earthquake occurred at the bottom of the Ocean near the Aleutian Islands. One gigantic wave travelled over more than 3500 kilometres in only 4.5 hours and hit Hawaii. It threw houses and bridges in the air, and carried them hundreds of metres away. At that time 170 people drowned.
A tsunami like the ones described above is ranked high on the scale of natural disasters. Tsunami's waves are waves prompted by earthquakes. They don't look like common ocean waves which we can see near the coast, and they have nothing in common with wind and tides.
Scientists give a special name to these waves. They call them by the Japanese name tsunami. Tsunami means harbour waves. An earthquake wave or tsunami is triggered by some disturbance of the sea bottom, usually because of an earthquake at the bottom of the ocean. However, volcanic eruptions, some underwater explosions, landslides, mass movements above and under the water can lead to a tsunami.
After a disturbance, the ocean floor can suddenly rise or sink; the mass of water above it also rises or sinks. This strange movement of the water makes a series of strong waves. The first wave travels through the water the same as sound travels through the air. It goes really fast without losing energy. In the open ocean a tsunami can reach speeds of up to 950 kilometers per hour. When it approaches land, speed reduces and the height of the waves goes up. It increases in size and reaches places that normal high tides could never reach.
A tsunami can last for several hours or several days depending of its location. A tsunami is so powerful that it can strip off sand from beaches, pull off roofs and crush buildings. People and boats can do nothing about a tsunami. A tsunami cannot be prevented, but the impact can be reduced with community preparedness, timely warnings and effective emergency response. For example in Aonae, Japan 10 minutes after the earthquake tsunami started, many casualties were avoided as they were educated about tsunamis, they had developed evacuation plans, and above all, a warning was issued.
