Scientists in Edinburgh say that the earthquake that has hit Japan is 8,000 times more powerful than the one that devastated Christchurch in New Zealand last month.
The Japanese earthquake, which measured 8.9 on the Richter Scale, struck at 2.46pm local time and was followed by 12 powerful aftershocks, seven of them at least 6.3 on the Richter scale, the size of the quake which struck New Zealand on February 22.
The quake struck at a depth of six miles, about 80 miles off the eastern coast, Japan's meteorological agency said. The area is 240 miles north east of Tokyo.
The earthquake set off a huge tsunami that hit Japan's eastern coastline, sweeping boats, cars, buildings and tons of debris miles inland.
Television pictures showed walls of water over 13 feet high swamping vast areas of low-lying farmland with motorists racing to escape the deluge, and waves sweeping across the runway of Sendai airport, north of Tokyo.
A tsunami alert was issued covering a vast area of the globe, including areas as far apart as New Zealand, Latin American and eastern Russia, as well as Hawaii and the west coast of the USA.
In downtown Tokyo - hundreds of miles away from the quake - large buildings shook violently and workers poured into the streets for safety
The Japanese government is preparing to send troops to the quake-hit areas to help relief efforts.
A large fire erupted at the Cosmo oil refinery in Ichihara city near Tokyo and was burning out of control with flames billowing 100 feet in to the air.
Brian Baptie, a seismologist from the British Geological Survey in Edinburgh, said: "This is a earthquake measuring 8.9 on the Richter scale.
"To put that into some sort of context, it's 8,000 times larger than the one that destroyed Christchurch last month, and on a similar scale to the Chile earthquake in February last year.
"An earthquake of this scale is capable of causing huge damage and destruction, and a tsunami that high will cause complete devastation."
The seismologist said it was difficult to judge the scale of the tsunami, but he said this earthquake was smaller than the one that struck off northern Sumatra in 2004, a magnitude 9.3 quake which set off the Boxing Day tsunami, killing thousands of people.
A Foreign Office spokesman in London said: "Our thoughts are with the people of Japan at this time. We have a large embassy in Japan which is rapidly assessing the situation and reacting to what is an enormous earthquake.
"They are working to provide urgent consular assistance to any British nationals who might need it."
There have not yet been any reports of British fatalities, the Foreign Office added.
Scottish Development International reports that its eight members of staff at its Tokyo office are all safe and well.
The Foreign and Commonwealth Office has set up a helpline for anyone concerned about the safety of relatives or friends in Japan: 020 7008 0000