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Tsunami's - A Geologic Hazard - REPORT
Please note this is a "Scientific Report" not an essay please bare this in mind.
The word Tsunami is a Japanese term meaning wave in the harbour. It is the most apt description for the hazard as tidal wave or seismic wave lead to incorrect connotations and misunderstandings. The dictionary definition of the word Tsunami itself means a very large ocean wave caused by an underwater earthquake or volcanic eruption, this is only half accurate, which I will attempt to illustrate in this report.
Although 85 percent of Tsunamis occur in the Pacific Ocean (PTWC, web), it is possible for them to occur in any body of water on any part of the Earth. They are not restricted to large oceans and there is the possibility of a tsunami threat in any enclosed body of water. Tsunamis occur infrequently and often with little or no warning especially in regional areas. They can cause unprecedented loss of human life and billions of dollars worth of damage to property and infrastructure.
A tsunami may be generated by any disturbance that displaces a large body of water from its equilibrium position. Most commonly tsunamis are caused by seismic activity on the ocean floor (Hays, 1981). Less commonly they are caused by sub and super marine landslides, sub and super marine volcanic eruptions and very rarely by meteor impacts in the ocean. Tsunamis travel outwards in all directions from where they are generated and can travel huge distances from their point of origination (USGS, web).
Earthquakes in subduction zones at convergent plate boundaries where there is enormous stress between the plates generate most tsunamis (USGS, web). When there is a rupture the sudden movement causes energy to be released into the overlying water column. Deep ocean troughs are another area that tsunamis may originate this is from the slumping of sediments on the sea floor and from direct surface rupture on a fault trace (Shane, lecture 16).
Collapses of volcanic edifices can also upheave the overlying water as sediment and rock slump downwards and are redistributed across the sea floor. Similarly, a powerful submarine volcanic eruption can create an abrupt force that elevates the water column and generates a tsunami (Hays, 1981). Contrarily, super marine landslides or debris avalanches and meteor impacts displace the water from above, as momentum from falling debris is transferred to the water in which it falls.
Tsunamis travel across the open ocean at tremendous speeds 500 to1000 Km per hour depending on the depth and geomorphology (PTWC, web). Their height at sea may only be up to one metre high; most are undetectable by boat or aeroplane. When they reach shallower water they begin to slow down and build up a crest 5 to 40 metres high. Tsunamis are generally not one single crest but many and there can be as much as 500 to 650 Km between them and between 10 and 45 minutes in wave arrival time. Regional Tsunamis are by far the most common travelling a short distance from their origination (geology.com, web).
Impact on Society
Tsunamis can cause devastation to any coastal and low-lying areas. There has been evidence of Tsunamis causing devastation as far back as 1480BC (Paras-Carayannis, 1986). It is believed that the Minoan Civilisation was wiped out by colossal waves (Paras-Carayannis, 1982). With more and more human development taking place in coastal areas due to population pressures it is evident that warning systems are needed to protect life and property.
In 1992 and 1993 over 2,000 people were killed by tsunamis occurring in Nicaragua, Indonesia and Japan. Damage to property was about one billion dollars. In 1960 an earthquake in Chile spawned a Pacific-wide tsunami that caused widespread death and damage in Chile, Hawaii, Japan as well as other areas in the Pacific (PTWC, web).
In 101 years the Tsunami Laboratory in Novosibirsk recorded 796 tsunamis in the Pacific Ocean (PTWC, web). 117 of these caused death and destruction mostly near their point of origin. Nine of these tsunamis caused large-scale destruction across the Pacific. There was no
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