Thursday, July 2, 2009

TSUNAMIS

The word “tsunami” comes from Japan where it has always been a frequent visitor and where it originally meant a wave in harbor. Before there was a clear understanding of the tsunami phenomenon, Japanese fishermen would frequently return home from sea and observe their whole harbor area devastated by water. Nothing had been experienced while they were in deeper water so they were very puzzled about the event and described it as a wave in harbor. In Japanese usage , the plural of the word is also tsunami but in English it is tsunamis. Because it is a wave formed when water is displaced in an area of ocean from events such as earthquakes, mass movements above or below the ocean’s surface, volcanic eruptions, or landslides, there is little evidence of a tsunami in deep water. Even if a wave is forty feet high it makes little difference in water that is a thousand feet deep but, as it comes into shallow water, the wave rises higher and higher and finally strikes the shore area with devastating force.

A tsunami can be generated when tectonic plate boundaries at subduction sites abruptly deform and vertically displace the overlying water. This happens in most places as tension builds up between two plates due to some obstruction preventing the natural movement of the plates. The tension builds up until something finally snaps, pushing one of the plates up or down and, by doing so, displacing a quantity of ocean water. Around the Pacific Rim, known as the Ring of Fire because of the many massive earthquakes of this type that occur, these displacements are greater than anywhere else. This is because the main subduction sites are located at the edge of continental areas like Alaska, California, South America, and Indonesia, in all of which the resistance to plate movement is particularly strong. As a result, a build up of tension persists for a long time between the two very big plates and a powerful earthquake then ensues. The displaced mass of water moves under the influence of gravity and radiates across the ocean like ripples on a pond.




Although there have been many examples of huge destructive tsunamis from subduction earthquakes around the Pacific Rim, there is nothing in the historical record like the one that originated in an Indonesian earthquake offshore from the Island of Sumatra on Boxing Day, 2004. The displacement of one of the plates and, therefore, the extent of the seabed that was moved upwards, was more than seven hundred miles. Indonesian fishermen who were at sea at the time, true to Japanese experience, felt nothing and had they stayed at sea for a time their lives could have been saved. Unfortunately, even today, tsunamis are not well understood; both in Indonesia and all over the stretch of ocean from Indonesia to Africa where the tsunami went, few people took advantage of what is known about tsunamis to run to higher ground. The tsunami, traveling at 500 mph as is common for such, devastated shore areas and even whole nations that were generally low in elevation, all the way from Indonesia to Eastern Africa. The earthquake was so powerful and the tsunami so big that the speed of the earth’s rotation was slowed down for a fraction of a second.

Just as this one was part of the Ring of Fire, so were most of the other big tsunamis that occurred globally over the previous century. Krakatau was a volcanic eruption in another part of Indonesia. The tsunami that accompanied it killed 36,000 in the two major islands of Indonesia on either side of the eruption—Java and Sumatra. Japan, thirteen years after Krakatau, experienced a monster tsunami from an offshore earthquake that took the lives of 26,000. In many other places across the Pacific since that time, lethal tsunamis took away many other people and did major damage in Hawaii, Alaska, and Papua New Guinea. While the Rim of Fire was the setting for most tsunamis from the past, Europe and the Eastern Mediterranean Sea have also seen powerful tsunamis in past times when, because of their lack of knowledge of tsunamis, many thousands were killed. The Lisbon tsunami of 1755 that followed an offshore earthquake killed many thousands and, much earlier in time, in Alexandria in the year 365, a tsunami generated by a distant earthquake in Greece killed thousands of Egyptians.

A tsunami wave generated by an earthquake and carried from the epicenter of the quake by gravity is like waves generated by dropping a stone in a pond. More than one wave results from the impact of the stone and its displacement of some water. Similarly, in the greater setting of a tsunami, many waves are generated and they can be close together or widely spaced depending on the volume of water displaced and the shape of the seabed around the epicenter. As the first wave nears shore it encounters friction from the seabed as water depth decreases and the shape of the wave is distorted. It may rise as high as thirty, forty, or more feet at this point. The first wave often retreats back out to sea leaving the seabed dry for great distances from shore. This aspect of tsunamis gave rise to the observation that they are far more dangerous than the earthquakes that generate them. A tsunami might be as long as sixty miles and, since it is not pushing water ahead of it but rather carrying water along with it, strange things like the retreat back out to sea happen when the sixty miles of water hits a small shore.

A tsunami is really a crashing wall of water, a bit like the giant waves off the coast of Hawaii that are so loved by surf riders; however, tsunamis are far more dangerous and not to be played with by these surfers because the tsunami is really a mass of water carried along in a series of waves. It is not just a local wave created by wind or the displacement caused by passing ships. A more accurate picture of tsunamis might be a river overflowing its banks because a high volume of water was added to the river upstream and it can no longer carry it. Some tsunamis come ashore and move huge volumes of water, along with the masses of debris that it collected along the way, far inland. Later, sometimes hours later, it retreats far from shore. As has already been noted, thousands of people were killed, both in the past and in the recent Indonesian tsunami, when they ventured out on to the newly-dried seafloor and were drowned in the next phase of the tsunami. In Thailand, as the Indonesian tsunami of 2004 reached its shores, a young girl was close to the shore when the first wave arrived and then retreated far out to sea. She and her family were on holiday in Thailand. This girl attended a school in Edmonton, Canada, and she had learned about tsunamis in a geography lesson. Immediately she saw the wave retreating she shouted, “run for higher ground” and then ran in that direction. Only a few followed her.

Tragically there was no tsunami warning systems in the Indian Ocean when the Indonesian earthquake and tsunami struck. It had been such a long time since the previous big tsunami had affected the area that authorities became complacent about this danger. Attitudes were still somewhat indifferent even when the earthquake happened as became evident in the responses from some countries in the region. Thus it was the Pacific Tsunami Warning Center in Hawaii that gave the quickest information to the rest of the world. This center along with its associated seismic stations and tidal gauges in places all over the Pacific Ocean, including Chile, New Zealand, Japan, Alaska, and all places in between, was established in the late 1940s after Hawaii experienced severe damage from several tsunamis. It is expected that there will be a similar system in place in the Indian Ocean in the future. Huge tsunamis have struck many places around the Pacific and Indian oceans in the past but little is known about these where historical records go back for only two or three hundred years. The west coast of North America is one of these places and Australia is a second. Research work is needed on the evidences of these past tsunamis, including the oral records of native peoples, so that there is a greater awareness of the possibilities of future mega-tsunamis.

Image source: thinkquest


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