THE earthquake which rocked Japan today was 8,000 times more powerful than the one that devastated Christchurch in New Zealand last month, experts said.

The quake, measuring 8.9 on the Richter scale, unleashed a huge tsunami that crashed into Japan's eastern coastline, sweeping boats, cars, buildings and tons of debris miles inland.

The tsunami has now reached Hawaii, with Kauai the first in the Hawaiian Island chain to be hit.

And a state of emergency has been declared at one of the country’s nuclear power plants- the Fukshima reactor – after it suffered a failure in its cooling system.

Chief Cabinet Secretary Yukio Edano, the top government spokesman, said the nuclear power plant developed a mechanical failure in the system needed to cool the reactor after it was shut down.

He said the measure was a precaution as there was no radiation leak and the facility was not in immediate danger.

Around 2,800 people have been evacuated.

The Queen has sent a message of sympathy to the people of Japan following the massive earthquake, which shook the country earlier today.

Her message came as Foreign Secretary William Hague said the UK could send humanitarian assistance and search and rescue teams to help with the aftermath of the quake, which triggered tsunami waves around the Pacific.

The quake was the largest to hit Japan since records began 140 years ago.

Television pictures showed walls of water over 13ft high swamping vast areas of low-lying farmland with motorists racing to escape the deluge.

In downtown Tokyo - hundreds of miles away from the quake - large buildings shook violently and workers poured into the streets for safety.

The quake struck at 2.46pm local time (0546 GMT) 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.

Waves of muddy water swept over farmland near the city of Sendai, carrying buildings, some on fire, inland as cars attempted to drive away.

Sendai airport was inundated with vehicles and thick mud deposited over its runways.

Fires spread through a section of the city.

The tsunami roared over embankments, washing cars, houses and farm equipment inland before reversing direction and carrying them out to sea.

Flames shot from some of the houses, probably from burst gas pipes.

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.

The quake struck at a depth of six miles (10km), about 80 miles (125km) off the eastern coast, Japan's meteorological agency said.

The area is 240 miles (380km) north east of Tokyo. Japan’s Prime Minister Naoto Kan said the quake caused “major damage in broad areas'' but nuclear power plants in the area were not affected.

The government was 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 100ft (30m) flames whipping into the sky.

Brian Baptie, a seismologist from the British Geoological 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."

UK airlines cancelled flights to Tokyo following the devastating earthquake.