A Japanese Self-Defence Force destroyer has rescued a 60-year-old man from the sea some 15 kilometres off Fukushima prefecture after a massive tsunami swept coastal regions following a magnitude 8.9 earthquake on Friday.

The man, identified as Hiromitsu Shinkawa from the city of Minamisoma, was spotted floating in sea on a piece of roof after being swept along with his house, the defence ministry said.

The deadly wall of seawater surged as far as two kilometres inland, leaving random debris, vehicles and the ruins of wood houses jutting from the mud.

The rescued man is conscious and in good condition, ministry officials said, adding he was transported to a hospital by helicopter.

"I ran away after learning that the tsunami was coming," Mr Shinkawa told rescuers, according to Jiji Press.

"But I turned back to pick up something at home when I was washed away. I was rescued while I was hanging to the roof from my house."

At the city of Fukushima, about 80 kilometres north-west of the stricken nuclear power plant, there have been reports of panic buying at supermarkets and petrol stations running dry.

Japan has committed 100,000 troops - about 40 per cent of the armed forces - to spearhead a mammoth rescue and recovery effort, with hundreds of ships, aircraft and vehicles headed to the Pacific coast area.

The world is rallying behind the disaster-stricken nation, with offers of help even from Japan's traditional rivals.

The massive earthquake, one of the largest in recorded history, appears to have shifted the main Japanese island by about 2.4 metres, the US Geological Survey said.

Two days after it struck about 400 kilometres north-east of Tokyo, aftershocks were still rattling the region, including a strong 6.8-magnitude tremor on Saturday and a 6.3 quake on Sunday.

While the official death toll from the quake and tsunami that followed nears 900, many more are feared dead as thousands remain unaccounted for.

- Kyodo