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| Sunday, 2 January 2005 |
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Junior Observer |  |
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Tsunamis : Devastating tidal waves Tsunamis (tidal waves) like those that wreaked havoc across Sri Lanka as well as many other countries in the Asian region on Sunday, December 26, are massive waves that are usually caused by earthquakes deep under the ocean floor. These waves can travel vast distances, as we saw last week.
However, despite the terror they can unleash on coastal areas, they are sometimes barely noticeable out at sea. According to some, a tsunami could hardly be felt if you are in the sea on a boat. It becomes powerful only when it is near the shore and reaches shallow water, which then can push waves over 10 times higher than the sea water level. While tsunamis can also be caused by landslides and volcanic eruptions, the most common cause is an undersea earthquake, especially in areas such as the Pacific where there is significant movement of the Earth's tectonic plates.
The speed of a tsunami is linked to the depth of the water. It can travel at speeds of several hundred kilometres per hour. Alert to the destructive capacity of tsunamis, Pacific Rim countries coordinate and share their observations of the ocean. A tsunami alert centre in Hawaii collects information about possible tidal waves, although they cannot be easily predicted.
Last week's earthquake, the world's most powerful in more than 50 years, triggered massive tidal waves that slammed into villages and seaside resorts across Asia, killing thousands across five countries. The initial quake, measuring 8.9 in magnitude on the Richter Scale, struck about 100 miles (160 km) off the coast of Indonesia's Sumatra island (at a depth of 6.2 miles below sea level) around 7.00am local time, according to the U.S. Geological Survey's National Earthquake Information Center. The Richter Scale is used to express the magnitude of earthquakes. On this scale, moderate earthquakes usually register a value above 5.3 and a magnitude of 6.3; anything above that represents a strong earthquake. These earthquakes lead to Tsunami waves or sudden sea waves which spread from the earthquake centre when a sudden upward movement of large area of water occurs during an undersea earthquake (submarine earthquake).The earthquake, which caused these massive tidal waves, was one of the most powerful since the start of the 20th century, according to the U.S. Geological Survey. It was the fifth largest since 1900, said geophysicists for the U.S. Geological Survey's Earthquake Hazards Program in Golden, Colorado. There had been no signs of the impending quake. They are very difficult to predict as these big earthquakes occur with no warning, no foreshock activity or any other signs. As the Earth moves and its plates hit each other, it breaks in one place and pressure builds up in a different area. When that pressure increases, another earthquake occurs. In this instance, about 1,000 km of the Andaman thrust (or faultline) broke. The magnitude of the quake increased the likelihood of aftershocks - the tremors that follow as minor readjustments occur along the fault after the main shock. An aftershock of magnitude 7.3 had been seen while there were several aftershocks in the magnitude of 5-6. The tidal waves, about 10 metres tall, which began in Sri Lanka's Eastern Coastal Belt, also affected the south, devastating Hambantota, Matara, Galle, Kalutara and Beruwela before moving westwards. Over 4,000 persons were killed while millions of rupees worth of property was damaged. It left a massive trail of death and destruction, rendering millions homeless. Sri Lanka was very heavily affected by the tsunami which also battered the coastline of India, Indonesia, Thailand, Malaysia, Bangladesh and the Maldives. |
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Caused by strong seismic shocks, tsunamis can reach huge heights and
speeds, picking up strength as they cross the ocean. They often culminate
with disastrous results, thousands of kilometres from where they
originated.
These are not common occurrences as only an earthquake that's over 7.7
on the Richter Scale is usually capable of causing tidal waves. They
travel outward in all directions from the epicentre of an earthquake and
can wreak havoc in coastlines, washing people and animals out to the sea,
causing flooding and devastating property.
Weather phenomena, especially extreme thermal changes which can lead to
depressions that cause strong winds, can also cause smaller tidal waves.
