The tremors follow a major 7.3 quake on Wednesday which swayed buildings in Tokyo and triggered a small tsunami but did not cause any casualties or property damage.

The latest was a 6.8-magnitude quake that hit at 6:24 am (2124 GMT Wednesday), 400 kilometres (250 miles) northeast of Tokyo at a shallow depth of nine kilometres below the Pacific seafloor, the Japan Meteorological Agency said.

The tremor prompted the agency to issue a tsunami warning which was soon lifted.

That came three hours after a 6.2-magnitude quake in the same region and another one measuring 6.1. Wednesday's quake and the early Thursday aftershocks were all in the same area.

There were no reports of major damage or casualties, local police said.

"We do not have to worry about a tsunami now but should be on alert for more aftershocks following yesterday's earthquake," an official at the Japanese agency said.

Around 20 percent of the world's most powerful earthquakes strike Japan, which sits on the "Ring of Fire" surrounding the Pacific Ocean.

Tectonics experts have warned of a 70 percent chance that the "Big One" -- a magnitude-seven earthquake or worse -- will strike the greater Tokyo region, home to around 35 million people, within the next 30 years.

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