TSUNAMI .... 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 earthscrust under or near the ocean. Less powerful tsunamiwaves can also be triggered by volcanic activity. Theyare most common in the Pacific Ocean. In addition,landslides, explosions, and even the impact of cosmicbodies, such as meteorites, can generate tsunamis.A tsunami is not a single wave, but a series of wavesthat can travel across the ocean at speeds of morethan 800kmh. In the deep ocean, hundreds ofkilometers can separate wave crests.How do earthquakes generate tsunamis?Tsunamis can be generated when the sea floorabruptly shifts and vertically displaces the overlyingwater from its equilibrium position. Waves are formedas the displaced water mass attempts to regain itsequilibrium. The main factor which determines theinitial size of a tsunami is the amount of vertical seafloor deformation.
How do tsunamis differ from other water waves?Tsunami waves are shallow-water waves with longperiods and wave lengths. (A wave is classified a shallow-water wave when the ratio between the water depth andits wavelength gets very small.Shallow water waves are different from wind-generatedwaves Wind-generated waves usually have period (timebetween two successive waves) of five to twenty secondsand a wavelength of about 50 to 600 feet (15 to 200meters)A tsunami can have a period in the range of 10minutes to 1 hour and a wavelength in excess of 700km (430 miles) .What happens to a tsunami as it approaches theshore?"As the tsunami wave reaches the shallower water abovea continental shelf, friction with the shelf slows the front ofthe wave. When the tsunami enters the shoaling water ofcoastlines in its path, the velocity of its waves diminishesand the wave height increases. It is in these shallowwaters that a large tsunami can crest to heights exceeding100 feet (30 m) and strike with devastating force.
Tsunami Warning…In instances where the leading edge of the tsunami waveis its trough, the sea will recede from the coast half ofthe waves period before the waves arrival. If the slope isshallow, this recession can exceed many hundreds ofmeters. People unaware of the danger may remain at theshore due to curiosity, or for collecting fish from theexposed sea bed.Japan has implemented an extensive program of buildingtsunami walls of up to 4.5m (13.5 ft) high in front ofpopulated coastal areas. Other localities have builtfloodgates and channels to redirect the water fromincoming tsunamisA tsunami warning system is a system to detecttsunamis and issue warnings to prevent loss of life. Itconsists of two equally important components: a networkof sensors to detect tsunamis and a communicationsinfrastructure to issue timely alarms to permit evacuationof coastal areas.
Deep ocean tsunami detectionThe Tsunami Warning stations along thepacific coast give detailed informationabout tsunamis while they are still far offshore. Each station consists of a sea-bedbottom pressure recorder (at a depth ofabout 6000 m) which detects the passageof a tsunami and transmits the data to asurface buoy via sonar. The surface buoythen radios the information to the PTWCvia the GOES satellite system.
Radar detection from spaceRadar Satellites can record theheight of the tsunami waves afteran earth quake. During the 2004Indian Ocean tsunami, data fromfour radar satellites recorded theheights of tsunami waves: at twohours after the earthquake. Itshould be noted that the satelliteobservations of the Indian Oceantsunami would not have been ofany use in delivering warnings, asthe data took at least five hours toprocess and it was pure chancethat the satellites were overhead atthat time.
Tsunami on the Indian Ocean…The total energy released by the earthquake in the IndianOcean has been estimated as 2.0×1018 joules. This isenough to boil 150 litres (40 US gallons) of water for everyperson on Earth. It is estimated to have resulted in anoscillation of the Earths 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 feltacross the planet - as far away as Oklahoma, verticalmovements of 0.12 iBecause the 1,200 km of faultline affected by the quake wasin a nearly north-south orientation, the greatest strength ofthe tsunami waves was in an east-west direction.Due to the distances involved, the tsunami took anywherefrom fifteen minutes to seven hours (for Somalia) to reachthe various coastline. The northern regions of the Indonesianisland of Sumatra were hit very quickly, while Sri Lanka andthe east coast of India were hit roughly 90 minutes to twohours later. Thailand was also struck about two hours later,despite being closer to the epicentre, because the tsunamitravelled more slowly in the shallow Andaman Sea off itswestern coast