“Japan earthquake: Tsunami hits north-east”
Source: BBC 2, Horizon Special
Professor Ian Stewart, Geologist
Broadcast on 27th March 2011
In this programme, Ian Stewart examines the powerful geological forces that caused the devastating Japanese earthquake. 30% of all earthquakes across the world happen in Japan. Earthquakes are caused when the tectonic plates (giant slabs of rock) grind together causing the build up of huge forces. When the pressure gets too much, the edges of the plate suddenly slip and an earthquake results. In Japan, there were two types of shock waves generated by the earthquake: P waves (primary) push and pull the rocks; S waves (secondary) shear the rocks. P waves travel much faster than S waves (10 times the speed of sound), S waves travel 60% slower but cause more damage. Scientists have labelled it as a “Mega-Thrust earthquake”, which measured 9 on the magnitude scale. The huge forces shifted the earth’s axis by up to 25 cm. It has been calculated that the energy released was 600 million times greater than that released in the nuclear bomb that destroyed Hiroshima. Japan’s east coast has lurched four metres out towards the Pacific Ocean and sunk by a metre. The earthquake triggered a Tsunami, a giant wave that began deep under the sea. The huge energy released by the earthquake forced the sea bed up 10 metres, driving a column of water up above the surface of the ocean. This sent a Tsunami racing at 200km per hour, east across the Pacific Ocean and west towards Japan. It hit the north east coast 24 minutes after the earthquake which itself had lasted 5 minutes from start to finish. By the time the individual waves merged in the shallow water they became higher and much more dangerous. The final major wave hit Japan about three and a half hours after the main earthquake. The Tsunami then triggered a near meltdown of one of the country’s nuclear power stations. The disaster claimed over 100,000 lives and almost twice as many are...
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