The earthquake that rocked Japan was like dropping a boulder the size of the Isle of Wight into the ocean, scientists have said.

It was upto a 1,000 times more powerful than the one that hit New Zealand last month and almost half as powerful as the 2004 Sumatra tsunami that devastated south east Asia.
Yet damage to Japan was minimised because the country is so used to quakes and has built up infrastructure and warning systems second to none in the world.
Japan - which suffers hundreds of tremors a year - sits at one of the most seismically active points of one of the most volatile areas of the world - the Pacific Ring of Fire.
This is a 30,000 mile stretch of fault lines and volcanoes stretching in a horseshoe from New Zealand, north through Alaska and back down to Chile.
Even for this volatile region, Japan is particularly vulnerable to earthquakes as it is positioned above three shifting plates that cover the earth.
These are tens of miles thick and thousands of miles across and ride on molten liquid deep below the earth's crust.
As the plates push against each other pressure builds up until one or the other gives or shifts under the other.
It releases massive amounts of pent up energy which in this case was equivalent to more than 1.5 billion tonnes of explosives.
As the movement was underwater it caused a massive ripple effect which radiated out in all directions - much like when a stone hits water.
Dr Simon Boxall of the University of Southampton's Oceanography department, said: "It is like dropping a great big boulder into the sea - a boulder the size of the Isle of Wight.
"The splash is huge and creates a wave around 200 miles along that ripples out in all directions at 500 mph.
"In deep water it's not very big, so shipping isn't affected but when it hits shallow water, the front of the wave slows down, effectively causing water to pile up behind it.
"This is why we see waves changing from a foot high to 30 to 40-feet high."
Dr Boxall said that small islands just two or three feet above sea level would be "swamped" by the wave but islands that are higher would be fine.
read more