T S U N A M I . . . .

Arch 613 Neha Sivaprasad Spring 2005

Tsunami………….Japanese word for “harbor wave”

How is a tsunami generated?

Causes by a sudden rise or fall of part of the earth's crust under or near the ocean. Less powerful tsunami waves can also be triggered by volcanic activity. They are most common in the Pacific Ocean. In addition, landslides, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis.

A tsunami is not a single wave, but a series of waves that can travel across the ocean at speeds of more than 800kmh. In the deep ocean, hundreds of kilometers can separate wave crests.

How do earthquakes generate tsunamis?Tsunamis can be generated when the sea floor abruptly shifts and vertically displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation.

How do tsunamis differ from other water waves?Tsunami waves are shallow-water waves with long periods and wave lengths. (A wave is classified a shallow-water wave when the ratio between the water depth and its wavelength gets very small.

Shallow water waves are different from wind-generated waves Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength of about 50 to 600 feet (15 to 200 meters)

A tsunami can have a period in the range of 10 minutes to 1 hour and a wavelength in excess of 700 km (430 miles) .

What happens to a tsunami as it approaches the shore?"As the tsunami wave reaches the shallower water above a continental shelf, friction with the shelf slows the front of the wave. When the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and the wave height increases. It is in these shallow waters that a large tsunami can crest to heights exceeding 100 feet (30 m) and strike with devastating force.

Tsunami Warning…In instances where the leading edge of the tsunami wave is its trough, the sea will recede from the coast half of the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed many hundreds of meters. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the exposed sea bed. Japan has implemented an extensive program of building tsunami walls of up to 4.5m (13.5 ft) high in front of populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunamis

A tsunami warning system is a system to detect tsunamis and issue warnings to prevent loss of life. It consists of two equally important components: a network of sensors to detect tsunamis and a communications infrastructure to issue timely alarms to permit evacuation of coastal areas.

Deep ocean tsunami detectionThe Tsunami Warning stations along the pacific coast give detailed information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom pressure recorder (at a depth of about 6000 m) which detects the passage of a tsunami and transmits the data to a surface buoy via sonar. The surface buoy then radios the information to the PTWC via the GOES satellite system.

Radar detection from space

Radar Satellites can record the height of the tsunami waves after an earth quake. During the 2004 Indian Ocean tsunami, data from four radar satellites recorded the heights of tsunami waves: at two hours after the earthquake. It should be noted that the satellite observations of the Indian Ocean tsunami would not have been of any use in delivering warnings, as the data took at least five hours to process and it was pure chance that the satellites were overhead at that time.

Tsunami on the Indian Ocean…

The total energy released by the earthquake in the Indian Ocean has been estimated as 2.0×1018 joules. This is enough to boil 150 litres (40 US gallons) of water for every person on Earth. It is estimated to have resulted in an oscillation of the Earth's surface of about 20 to 30 cm, equivalent to the effect of the tidal forces caused by the Sunand Moon.The shock waves of the earthquake were felt across the planet - as far away as Oklahoma, vertical movements of 0.12 i

Because the 1,200 km of faultline affected by the quake was in a nearly north-south orientation, the greatest strength of the tsunami waves was in an east-west direction.

Due to the distances involved, the tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastline. The northern regions of the Indonesianisland of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly 90 minutes to two hours later. Thailand was also struck about two hours later, despite being closer to the epicentre, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast

References

www.bbc.com

www.boston.com/news/asia

http://www.mannu.info/tsunami-videos.html

http://en.wikipedia.org/wiki/Tsunami