tsunami

tsunami, a Japanese term (tsu, harbour, nami, wave) for waves triggered by earthquakes, landslides, volcanic eruptions, or large meteorites splashing down in the ocean. They are sometimes mistakenly called tidal waves but they are nothing to do with tides. They are very large scale versions of the ripples that radiate out when a stone is dropped into water. They occur most frequently in the Pacific Ocean where major earthquakes are common occurrences along the ocean's active margins where the oceanic plates are slipping down below the continents. So the disaster in the Indian Ocean on Boxing Day 2004 was unexpected. Associated with the major plate boundary between the Indian and Eurasian tectonic plates, it was triggered by a major earthquake over more than 100 kilometres (63 mls.) of seabed off the north-west coast of Sumatra. Its epicentre was in deep water at 3.32° N., 95.85° E. and measured 9.0 on the Richter Scale, the fourth largest earthquake recorded for a century. The displacement of the seabed represented about 130 years of seafloor spreading (see geological oceanography), hence the violence of the shock. The fault line is orientated north/south so the waves spread more east/west; fortunately for vulnerable countries like Bangladesh it did not travel far in a northerly direction, otherwise the subsequent loss of life would have been even more shocking. Another factor was that the earthquake occurred only just below the seabed; if it had been deeper within the Earth's crust the shock waves would have stayed within the rocks and would not have radiated out in the ocean.

A tsunami is not one but a series of waves. Over deep water each wave is only about a metre high, and the distance between wave crests is over 100 kilometres (63 mls.). They travel at speeds of up to 700 kph (437 mph), about as fast as a passenger jet flies, but are almost impossible to detect out in the open ocean. The waves from the 2004 tsunami took just fifteen minutes to reach Sumatra, 30 minutes to reach the Andaman Islands and 90 minutes to reach Thailand, so even if a tsunami warning system had been in place, as it has been for some time in the Pacific, it is unlikely that it would have saved many lives in those countries. A warning system, which at the time of writing is due to be installed in 2005, may have helped save life in Sri Lanka, where the wave took two hours to arrive and the Maldives where it took three and a half hours. It took seven hours to reach East Africa. Television gave some warning of its approach and only one life was lost on the Kenyan coast. But there was no warning for Somalia and hundreds died.

A tsunami can cross the Pacific in a less than a day. In 1960 a powerful underwater earthquake off Chile, measuring 8.6 on the Richter scale, generated a tsunami that killed many people locally. Fifteen hours later it came ashore at Hilo in Hawaii. Its height was 10.7 metres (35 ft) and despite warnings it killed 61 people. This was mainly because, after the first wave, people began to return to their homes only to be drowned when the second wave struck the shore. In Hilo alone this tsunami caused damage costing $US24 million.

As the tsunami waves reach shallow water, the drag of the seabed slows them so their heights build up in excess of 10 metres (33 ft), even so they can be still travelling at speeds of over 100 kph (63 mph). They come ashore as a great river of water, so their destructive power is awesome. If they are funnelled into harbours or bays they can resonate back and forth like sound in a musical instrument, which increases their destructiveness. One interesting observation of the impact of the 2004 Indian Ocean tsunami was that coastlines with intact mangrove swamps and offshore coral reefs were far less seriously affected than those where the mangroves had been cleared and the coral reefs degraded, so the degradation of coastal habits led to greater damage and loss of life (see also environmental issues).

Tsunamis give little warning of their approach, but often the sea ebbs away from shallow bays leaving boats high and dry before the arrival of the first destructive wave within about half an hour. This first wave is usually followed by several more, and, as was experienced in Thailand on Boxing Day 2004, it is not always the first of the waves that is the largest. This tsunami immediately killed over 200,000 people and rendered millions homeless and exposed them to serious health risks and economic ruin. Previous to that the most destructive tsunami on record resulted from the explosion of Krakatoa, a volcanic island in the Sunda Strait between the south of Sumatra and Java. A large volcano had collapsed in ad 416, forming an underwater caldera—a cauldron-like cavity—and leaving a few remnant islands, one of which was Krakatoa. On 27 September 1883, after a long series of volcanic eruptions, the island exploded. It is believed that the eruptions had cracked open the magma chamber beneath the volcano and water had flowed in. The resulting explosion had a force estimated to be 20,000 times that of the Hiroshima atom-bomb, and caused the island to collapse back into the caldera. The resulting tsunami that came ashore in Sumatra and Java reached an estimated 40 metres (130 ft) in height, and it wiped out all the coastal communities along the shoreline, killing an estimated 30,000 people.

Another destructive tsunami was the one which hit Japan in 1896. Fishermen fishing out at sea off Sanriku were unaware that it had passed beneath them, but when they returned to port they found 28,000 people had been killed by the waves. Between 1995 and 2005 about 2,000 people were killed by tsunamis around the Pacific.

Recently concern has been expressed about the lack of stability of Cumbre Vieja, a volcano on the island of La Palma in the Canaries. Some, but not all, geologists, have speculated that if this volcano erupts its flanks may collapse and triggered a major underwater landslip. This might generate a mega-tsunami that would have a devastating impact on the eastern seaboard of North America. No such mega-tsunami has been experienced during historical times, but about 5,000 years ago during the Bronze Age there is archaeological evidence that a giant wave over-topped a number of Scottish islands wiping out settlements at least 65 metres (200 ft) above sea-level. This was probably triggered by a massive failure of the continental shelf off south-west Norway.

M. V. Angel

Bibliography

Bryant, E. , Tsunami: The Underrated Hazard (2001).
Simon Winchester, S. , Krakatoa: The Day the World exploded, August 27 1883 (2003).