How a Tsunami Works

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How a tsunami works by Jonny O'Callaghan, 11 March 2011 As Japan is hit by a deadly tsunami we explain how these dangerous walls of water form 1 Email 52,940 views 3comments Tsunami formation Tsunamis form through a complex, multi-stage process that emanates from the massive energy release of a submarine earthquake, underwater or coastal landslide, or volcanic eruption. The first stage in this formation begins when the tectonic upthrust caused by the quake or impulse event causes massive amoun
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  How a tsunami works  by Jonny O'Callaghan , 11 March 2011 As Japan is hit by a deadly tsunami we explain howthese dangerous walls of water form 1Email52,940 views 3comments Tsunami formation Tsunamis form through a complex, multi-stage process that emanates from the massiveenergy release of a submarine earthquake, underwater or coastal landslide, or volcaniceruption.The first stage in this formation begins when the tectonic upthrust caused by the quake or impulse event causes massive amounts of ocean water to be displaced almost  instantaneously. This action kick-starts a simple series of progressive and oscillatorywaves that travel out from the event’s epicentre in ever-widening circles throughout thedeep ocean. Due to severe levels of energy propagated from the impulse, the waves buildin speed very quickly, reaching up to an incredible 500mph. However, due to the depth of water, the speed of the waves is not visible as they expand to have incredibly longwavelengths that can stretch between 60-120 miles. Because of this, the wave amplitudes(the wave height) are also very small as the wave is extremely spread out, only typicallymeasuring 30-60 centimetres. These long periods between wave crests – coupled withtheir very low amplitude – also mean that they are particularly difficult to detect whenout at sea.Once generated, the tsunami’s waves then continue to build in speed and force beforefinally approaching a landmass. Here the depth of the ocean slowly begins to reduce asthe land begins to slope up towards the coastline. This sloping of the seabed acts as a braking mechanism for the high-velocity tsunami waves, reducing their speed throughcolossal friction between the water and the rising earth. This dramatic reduction in speed – which typically takes the velocity of the tsunami to 1/10th of its srcinal speed – alsohas the effect of reducing the length of its waves, bunching them up and increasing their amplitude significantly. Indeed, at this point coastal waters can be forced to raise as muchas 30 metres above normal sea level in little over ten minutes.Following this rise in sea level above the continental shelf (a shallow submarine terraceof continental crust that forms at the edge of a continental landmass) the oscillatorymotions carried by the tsunami are transferred into its waters, being compressed in the process. These oscillations under the pressure of the approaching water are then forcedforwards towards the coast, causing a series of low level but incredibly fast run-ups of seawater, capable of propelling and dragging cars, trees, buildings and people over greatdistances. In fact, these run-ups are often responsible for a large proportion of thetsunami’s damage, not the giant following waves. Regardless, however, following therun-ups the tsunami’s high-amplitude waves continue to slow and bunch into fewer andfewer megawaves before breaking at heights between five and ten metres over theimmediate coastline, causing great damage and finally releasing its stored energy.  Cause and effect Cause Tsunamis initiate when an earthquake causes the seabed to rupture (bottom centre),which leads to a rapid decrease in sea surface height directly above it.  Effect As the tsunami reaches the shore the shallow, long and exceedingly fast waves pile up,reducing the wavelength and increasing their height dramatically.
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