Science / Tsunami

Tsunami

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Autor:  anton  08 March 2011
Tags:  Tsunami
Words: 2077   |   Pages: 9
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The tsunami is a series of ocean waves of very great length and period generated by impulsive disturbances of the earth's crust. Large earthquakes with epicenters under or near the ocean and with a net vertical displacement of the ocean floor are the cause of the most catastrophic tsunami. Volcanic eruptions and submarine landslides are also responsible for tsunami generation but their effects are usually localized. Although infrequent, tsunami are among the most terrifying and complex physical phenomena and have been responsible for great loss of life and extensive destruction to property. Because of their destructiveness, tsunami have important impact on the human, social and economic sectors of our societies. Historical records show that enormous destruction of coastal communities throughout the world has taken place and that the socioeconomic impact of tsunami in the past has been enormous. In the Pacific Ocean where the majority of these waves have been generated, the historical record shows tremendous destruction with extensive loss of life and property. In Japan, which has one of the most populated coastal regions in the world and a long history of earthquake activity, tsunami have destroyed entire coastal populations. There is also a history of tsunami destruction in Alaska, in the Hawaiian Islands, and in South America, although records for these areas are not extensive. The last major Pacific-wide tsunami occurred in 1960. Others have also occurred but their effects were localized.

We have witnessed in the last twenty years rapid growth and development of the coastal areas in most of the developing or developed Pacific nations. This is the result of a population explosion and of technological and economic developments that have made the use of the coastal zone more necessary than before. Fortunately, tsunami are not frequent events and therefore their effects have not been felt recently in all developing areas of the Pacific. History, however, has proved that although infrequent, destructive tsunami indeed do occur. A major Pacific-wide tsunami is likely to occur in the near future. Among the countries bordering on the Pacific, a number are not prepared for such an event while others have let their guard down. The social and economic impact of future tsunami, therefore, cannot be overlooked. The purpose of this paper is to provide an overview of the social and economic impact of past, recent and future tsunami, to examine tsunami hazard management, and to indicate the need for future planning, at least for the Pacific Ocean where tsunami frequency is high.

Historical record of destructive tsunami

The impact of tsunami on human society can be traced back in written history to 1480 B.C., in the eastern Mediterranean, when the Minoan civilization was wiped out by such waves. (photo of ancient city of Knossos, the capital of the Minoan civilization).

Japanese records documenting such catastrophes extend back to A.D. 684.(1) North and South American records have dated such events back to 1788 for Alaska and 1562 for Chile. Records of Hawaiian tsunami go back to 1821.

While most of the destructive tsunami have occurred in the Pacific Ocean, devastating tsunami have also occurred in the Atlantic and Indian Oceans, as well as the Mediterranean Sea. A large tsunami accompanied the earthquakes of Lisbon in 1755, that of the Mona Passage off Puerto Rico in 1918, and at the Grand Banks of Canada in 1929.

Most of the people in the Pacific countries live on or quite near the coast since the interior is often mountainous and most of the good flatland is in the form of coastal plains. Many of these countries have populations with a natural maritime orientation. For many of these countries, foreign trade is a necessity and some maintain large fleets of ships and have major port facilities. Many of the Pacific island countries and those with extensive continental coastlines depend also on transport by small coastal ships necessitating many small ports to facilitate inter-island and coastal trade as well. Countries like Japan, for example, maintain many ports and have extensive shipbuilding facilities, electric plants, refineries and other important structures.

Similarly, many of the other developing and developed countries of the Pacific have harbors as bases for their large fishing industries. Peru, for example, at the port of Callao near Lima, maintains a large fleet for anchovy fishing. Callao is located near a strong seismic and potentially tsunamigenic region. Finally, when we also note that a number of coastal sites throughout the Pacific have begun aqua cultural industries and canneries, we can only conclude that this combination of factors makes these developed and developing Pacific islands and continental Pacific nations socially and economically vulnerable to the threat of tsunami. The extensive coastal boundaries, the number of islands, the long coastlines of Pacific countries containing a number of vulnerable engineering structures, the numerous large ports, the productive fishing and aqua cultural industries and the great density of population in coastal areas can only place these countries in a very vulnerable position.

Planning for the tsunami hazard

There is very little that can be done to prevent the occurrence of natural hazards. Floods, droughts, earthquakes, hurricanes, volcanic eruptions and tsunami cannot be prevented. But humankind, being as adaptable as it is, has learned to live with all these hazards. In the past, we have taken a passive approach to hazards, justifying them as acts of God or nature about which we could do very little.

But while these natural disasters cannot be prevented, their results, such as loss of life and property, can be reduced by proper planning. To plan for the tsunami hazard, however, we must have a good understanding not only of the physical nature of the phenomenon and its manifestation in each geographical locality, but also of that area's combined physical, social and cultural factors. Some of these areas are more vulnerable to tsunami than others. Because tsunami frequency in the Pacific Ocean is high, most efforts in hazard management have concentrated in this area of the world. No matter how remote, the likelihood of a tsunami should be considered in developing coastal zone management and land use. While some degree of risk is acceptable, government agencies should promote new development and population growth in areas of greater safety and less potential risk. These agencies should formulate land-use regulations for a given coastal area with the tsunami risk potential in mind, particularly if such an area is known to have sustained damage in the past.

International protective and preventive measures established

Present protective measures involve primarily the use of tsunami warning systems employing advanced technological instrumentation for data collection and for warning communications. Countries like Japan, the Soviet Union, Canada, and the United States have developed sophisticated warning systems and have accepted the responsibility to share warning information with other countries of the Pacific.

In 1965, Unesco's Intergovernmental Oceanographic Commission (UNESCO/IOC) accepted the United States' offer to expand its existing tsunami center in Honolulu to become the Pacific Tsunami Warning Center (PTWC). Also established was an International Co-ordination Group (ICG/ITSU) and the International Tsunami Information Center (ITIC) to review the activities of the International Tsunami Warning System for the Pacific (ITWS). The Pacific Tsunami Warning System has become the nucleus of a truly international system. Twenty-two nations (the number is now 28) are now members of ICG/ITSU: Canada, Chile, China, Colombia, Cook Islands, Ecuador, Fiji, France, Guatemala, Indonesia, Japan, Republic of Korea, Mexico, New Zealand, Peru, Philippines, Singapore, Thailand, the United Kingdom (Hong Kong), the United States, the Soviet Union, and Western Samoa. Several non-member nations and territories maintain stations for the ITWS, and tide observers are also located on a number of Pacific Islands. The present system makes use of twenty-four seismic stations, fifty-three tide stations, and fifty-two dissemination points scattered throughout the Pacific Basin under the varying control of the member states. PTWC in Honolulu, operated by the United States National Weather Service, is the operational center for the system. The objectives of the ITWS are to detect and locate major earthquakes in the Pacific region, determine whether they have generated tsunami, and provide timely and effective information and warnings to the population of the Pacific region in order to minimize the effect of the hazards on life and property.

Functioning of the warning system

Functioning of the system begins with the detection by any participating seismic observatory of an earthquake of sufficient size to trigger the alarm attached to the seismograph at that station. Earthquakes of 6.5 or greater on the Richter scale are investigated. PTWC collects the data and, when sufficient data has been received, locates the earthquake and computes its magnitude. When reports from tide stations show that a tsunami poses a threat to the population in part or all of the Pacific, a warning is transmitted to the dissemination agencies for relaying to the public . The agencies then implement predetermined plans to evacuate people from endangered areas. If the tide station reports indicate that a negligible or no tsunami has been generated, PTWC issues a cancellation. In addition to the International Tsunami Warning System, a number of Regional Warning Systems have been established to warn the population in areas where tsunami frequency is high and where immediate response is necessary. Such regional tsunami warning systems have been established in the Soviet Union, Japan, Alaska and Hawaii. Vast areas exist, however, where tsunami cannot be adequately detected or monitored in time and the populations warned to prevent extensive loss of life.

Because of the rarity of large destructive tsunami, it is difficult to institute successful tsunami-prediction schemes for warning the public. However, we can make people aware of the potential hazard. Tsunami warnings are issued to the public for the purpose of convincing people to evacuate endangered areas. Ample time must be allowed for evacuation, which is a rather difficult procedure. Often the public does not understand the meaning of the warning signals and is not aware of the locations of endangered areas. Most people are reluctant to evacuate their homes and businesses, and their response to warnings in general may not be very good, particularly if a number of false alarms have been issued.

Hazard perception by the public

Tsunami hazard perception by the people of a coastal area is based on education and confidence in government agencies responsible for tsunami prediction. Overwarning, based on inadequate knowledge of the phenomenon or inadequate data on which to base the prediction, often leads to false alarms and lack of compliance with warning and evacuation attempts. Such false alarms result in a loss of faith in the capability of the system and result in reluctance to take action in subsequent tsunami events. Even if a tsunami prediction is based on valid information and data, warning and evacuation may not be sufficient to minimize the impact of tsunami on coastal populations . Hazard perception by the public is based on a technical understanding of the phenomenon, at least at the basic level, and a behavioural response stemming from understanding of the phenomenon and confidence of the public in the authorities. Fortunately, forecasting of tsunami in recent years has been quite good and the image of the tsunami warning system and its credibility has improved considerably. Forecasting, however, is not an exact science as the phenomenon itself is very complex and data on which the forecast is based may often be inadequate for certain areas.

Awareness through public education

A heightened community awareness of the potential threat of tsunami can be achieved through a public education programme. Civil defense authorities in each country can initiate such a public education programme consisting of seminars and workshops for responsible government officials, can publish informational booklets on the hazards of tsunami, and can co-ordinate with the communications media on the announcement of tsunami information. Other government agencies can take action also to mitigate future losses from tsunami. For example, government agencies can develop sound coastal management policies, which include zoning and planning for tsunami-prone coastal areas. Scientific organizations can undertake research and engineering studies in developing evacuation zones or engineering guidelines for building coastal structures. Audio-visual materials can be prepared for educating children in schools and the public in general. Brochures and pamphlets can be printed describing the tsunami warning system and what the public can do in time of tsunami warning. Internally, government agencies can streamline and co-ordinate their operating procedures and communications so they can perform efficiently when the tsunami threat arises. Procedures related to tsunami warnings should be reviewed frequently to define and determine better respective responsibilities between the different governmnent agencies at all levels.



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