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The word Tsunami comes from two Japanese words, tsu meaning port of harbour and nami meaning wave or sea. Tsunamis are sometimes referred to as tidal waves but this is incorrect as they have nothing to do with the tides of the sea. Most normal waves in the sea are created by the wind blowing across the sea top and this is where Tsunamis differ as they are created by submarine tectonic activity or other movement in the sea such as landslides. Tsunamis are not caused by the tides or the wind.
The classification of Tsunamis can vary. If a causal classification is used then the tsunami is dependant upon the occurrence that caused it for its classification. For example, if a submarine earthquake caused the Tsunami than it can be classified as a tectonic hazard, while if it was caused by a large landslide it would become a mass movement hazard. As water is the predominant medium that is in force during the hazard then it can also be classified as a Hydrological hazard. Thus Tsunamis can be classified in various waves.
As I have already mentioned Tsunamis can be caused in various waves but not by the wind or tides. Most Tsunamis result from tectonic displacement of the sea bed associated with large, shallow focus earthquakes under the oceans but they can also be caused by exploding volcanic islands (e.g. Krakatoa in 1883) and large rock falls into confined bays.
Not all earthquakes generate Tsunamis but the most destructive Tsunamis are generated by earthquakes. Typical Tsunamis result from a rapid displacement of the sea floor over many of hundreds of square kilometres. In turn, this displaces a column of sea water, which then travels outward from the epicentre at speeds of 500 km/hr and more. It is the vertical oscillatory movement of the water that creates the tsunami, sending out waves as the water rebounds from the sea bed. Similar displacements of the ocean floor can also be produced by volcanic eruptions and submarine avalanches, or submarine landslides. However, these sources are considered as point sources and , although destructive locally, the energy of the waves is rapidly dissipated as they travel across the ocean.
A final possible cause of Tsunamis is the impact of a meteorite or comet. About 65 million years ago a meteorite is supposed to have hit the earth creating a crater at least 100 miles wide. If the meteorite had landed in the sea it would have created a massive Tsunami as the meteorite would have acted as the same way as dropping a stone into a pool of water would, only the ripples created would have been tremendous.
In deep water out to sea these waves are very long (100 - 200 km) apart and very low (only about 0.5 m high). Very often a Tsunami can pass a ship unnoticed and are not visible from the air. The deeper the water the lower the wave and the faster it can travel. In the open ocean it can travel of speed upto 600 miles per hour.
As the tsunami enters the shoaling water near the coast, its velocity decreases and its height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of more than 30-50 metres and strike with devastating force.
Finally, terminal height or run-up of the tsunami at the point of impact will depend on how the energy is focused, the travel path of the waves, the coastal configuration, and the offshore topography. Tsunami run-up is the vertical distance between the maximum height reached by the water on shore and the mean sea level surface. Tsunamis are among the most terrifying natural hazards known to man. They have been responsible for tremendous loss of life and property throughout history.
Tsunamis can reach tremendous heights, as just mentioned upto 50 metres, and hit the coast with tremendous force, with speeds of upto 600 mph. The size and destructive power of tsunamis is illustrated in the many case studies of tsunamis that have occurred around the world.
The Great 1964 Alaska earthquake generated catastrophic tsunami waves that devastated many towns in the Prince William Sound area of Alaska, along the Gulf of Alaska, along the West Coast of Canada and the United States, and in the Hawaiian islands. In Alaska, the tsunami run-up measurements varied from 6.1 m at Kodiak Island, 9.1 m at Valdez, 24.2 m at Blackstone Bay, and 27.4 m at Chenega. The Great 1964 Alaska earthquake and the tsunami waves resulting from it, took the lives of more than 122 people and caused over $106 million (1964 dollars) in total damage. A total of 119 people lost their lives in Alaska, Oregon and California as a result of tsunami waves generated in the Gulf of Alaska and the locally generated tsunami waves in Prince William Sound. Most of the damage and most of the lives lost in Alaska were due to large local tsunami waves within the Prince William Sound area, rather than to the earthquake itself. Of the 119 deaths attributable to tsunami waves, about one-third were due to the open-ocean tsunami generated in the Gulf of Alaska. Of these 4 occurred at Newport Beach, Oregon; 12 at Crescent City, California; and about 21 in Alaska. The rest of the deaths were caused by tsunami waves generated within Prince William Sound.
A recent tsunami that hit an ELDC was in Papua New Guinea in July 1998. Two earthquakes occurred here creating even more havoc than a tsunami alone. An earthquake of magnitude 7 on the Richter scale occurred 20 miles below the surface and 12 miles inland. Then, only 20 minutes later, a second, aftershock, occurred 12 miles out to sea with a force of 5.7 on the Richter scale. It was this earthquake which sent the tsunami hurtling towards shore. All those that had survived the terror of the initial earthquake were then subjected to the impact of the tsunami and the flooding it caused. The Australian Defence Force initially estimated that at least 6,000 people had been made homeless. Many survivors of the initial earthquake were injured, mostly suffering broken bones, as their beach homes built on stilts flimsy bush materials were torn into fragments. In the village of Warapu alone 500 people died. Two days later there were 1,200 people confirmed dead with 6,000 still unaccounted for. So many people died through this disaster not only because of the double blow of earthquake and tsunami but because there was inadequate warning as no warning systems were in place due to a lack of money for such measures in ELDC's.
Tsunamis pose a threat to 22 countries in the circum-Pacific region. In the past 100 years, over 50,000 coastal residents have lost their lives. Between 1900 and 1980 370 tsunamis were observed around the Pacific with the most active region being along the Japan-Taiwan island arc where over a quarter of all events were recorded.
A map of recent Tsunami around the world
Japan is especially vulnerable. Along the Sanriku coast or in the Tohoku district of northern Honshu there are many flatlands with coastal embayments where large fishing and aqua culture industries have been established. Throughout history, entire settlements in such areas have been struck and destroyed by tsunami, often requiring their rebuilding and relocation.
The record reads as follows: a total of 65 destructive tsunami struck Japan between A.D. 684 and 1960. As early as 18 July 869 the Sanriku coast was hit by a tsunami resulting in loss of 1,000 lives and the destruction of hundreds of villages. On 3 August 1361, a tsunami destroyed 1,700 houses in this same area. On 20 September 1498 1,000 houses were washed away and 500 deaths resulted from a tsunami which struck the Kii peninsula. On 3 March 1933 a tsunami in the Sanriku area reached a height of about thirty meters and killed over 3,000 people. injured hundreds more and destroyed approximately 9,000 homes and 8,000 boats.
The Pacific coast of the USA also suffers from Tsunamis though not to the extent of Japan and eastern Asia. During the twentieth century over 350 people have been killed in the USA with $500 million damage done to property.
Tsunami are not confined to the Pacific Ocean however and do occur in the Atlantic and Indian Oceans. However the conditions for tsunami creation are not as good in these latter two due to the type and location of fault in the earth's crust. For example the mid-Atlantic fault is not suitable for tsunami creation as it is a constructive plate boundary and so does not create vertical displacement of the water above as violently as other types of tectonic plate boundary.
Tsunami are very unpredictable in their frequency and do not have 'seasons' as some other hazards, such as hurricanes. As tsunamis are dependant on some form of catalyst they are dependant upon the phenomena that create them for their frequency. As earthquakes, volcanic activity, landslides, and meteorites are phenomena with an unpredictable frequency, tsunamis too have and unpredictable frequency.
In eastern Honshu, in Japan a tsunami wave of 10 m has a return period of 10 years, that is a wave of that size will occur once every 10 years. This is one method that is used to measure the frequency of tsunami and is identical to that of flooding, another hydrological hazard.
Tsunamis are complex mechanisms. Much depends on the topography of the ocean floor near the coastline and of the coastline itself. Some break like large waves do pounding the coast, crushing buildings and carrying buildings and ships inland. Other tsunamis may produce more gentle waves that simply lift structures off of their foundations but are followed by a violent backwash which can cause devastation. One short term warning of an impending tsunami is the retreat of the sea from the shore. A short time after the sea retreats an tremendous surge of water occurs extending hundreds of feet inland. When a tsunami struck Madeira in 1755 large quantities of fish were left stranded on the beach as the sea suddenly retreated. Villages rushed down to pick up the fish but were killed as the tsunami crashed onto the beach.
On coasts and islands where the seafloor rises gradually or where barrier islands exist much of the tsunamis energy is spent before it reaches the shore. However where the reverse is true, very deep water just offshore, an oncoming tsunami can build to tremendous heights.
Damage caused by the 1964 Alaska Tsunami
Physical destruction from tsunamis occurs through a variety of mechanisms. Floatation and drag forces can move houses whilst inundation turns floating debris, such as boats and timber, into projectiles which smash into structures. Strong wave currents undermine harbour foundations and lead to the collapse of bridges and sea walls. Fire and pollution often result from the spillage of oil and other toxic materials that are stored in port facilities. Finally, disease can be rife as dead bodies decay and this was the case earlier in the year in Papua New Guinea where residents that survived were evacuated for just this reason.
The human response to an impending tsunami depends on whether it is know if a tsunami is approaching. If it is not, as in the case of Madeira in 1755, inhabitants will continue their lives as normal. If a warning has been given people will need to move quickly to higher ground essentially away from the shore. Once a tsunami has occurred people can often return to their homes on the shore only to be hit by a second, following wave that is just as big. Therefore it is important that once the first tsunami has hit that people continue to stay on higher ground, or if they weren't there in the first place that they move there immediately.
Tsunamis are a natural occurrence and they are only a hazard because people choose to live in areas where they will occur. Therefore the most comprehensive prevention to the hazard is to move away from areas at risk. Ideally tsunamis would be halted at source by not allowing them to occur. However as we cannot stop earthquakes or other submarine tectonic activity this solution is not viable. In some areas of the coastline of Japan gigantic structures have been built to try to reduce the damage that a tsunami can cause but it has yet to be seen how effective these will be.
In areas of the world where tsunamis do not occur frequently they are not thought of as a hazard and little is done in the way of education for what to do when one occurs or for defence and prediction. This complacency can be bad and it should be remembered that Britain has experienced a tsunami. In areas where tsunamis regularly occur however, tsunamis are held with very high regard as it can regularly be seen what devastation they can cause. It was because of this high regard that steps were taken by the USA and Japan to develop a warning system.
Following the disaster caused by the tsunami of 1 April 1946 in the Hawaiian Islands and elsewhere a rudimentary warning system was established in 1948 to provide watch and warning information to the civil authorities and various military headquarters in the Hawaiian Islands. The great destruction caused by the May 1960 Chilean tsunami prompted a large number of countries and territories to Join the TWS. Another catastrophic tsunami generated by the great Alaskan earthquake of 1964 emphasised the need for an International Tsunami Warning System (ITWS).
In 1965, the United Nations Educational Scientific and Cultural Organisation's Intergovernmental Oceanographic Commission accepted the United States' offer to expand its existing Tsunami Centre in Honolulu to become the Pacific Tsunami Warning Centre (PTWC).
Functioning of the System begins with the detection of an earthquake which has a magnitude and location that make it potentially capable of generating a tsunami. The earthquake has to be of sufficient magnitude to trigger the alarm attached to the seismograph at the station where it is being recorded. The alarm thresholds are set so that ground vibrations of the amplitude and duration associated with an earthquake of approximate magnitude 6.5 or greater on the Richter Scale anywhere in the Pacific region will cause them to sound. Personnel at the station immediately interpret their seismographs and send their readings to the Pacific Tsunami Warning Centre (PTWC), in Honolulu, which are the headquarters for the international system (ITWS). Upon receipt of a report from one of the participating seismic observatories or as a consequence of the triggering of their own seismic alarm, PTWC personnel send messages requesting data from the observatories in the system.
Tsunami hazard perception by the people of a coastal area is necessary in reducing loss of life and damage to property. In the USA 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 that understanding and confidence of the public for the authorities responsible for warning.
Over warning, based on 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 a warning system and result in reluctance to take action in subsequent tsunami events.
Fortunately, forecasting of tsunami in recent years has been quite good and the image of the Tsunami Warning System and its credibility have 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 of the Pacific.
University of Washington Geophysics Department Web Site (USA)
The Tsunami Page of Dr. George P.C.