Tokyo: A major 7.3-magnitude offshore earthquake rattled Japan on Wednesday, swaying Tokyo buildings, triggering a small tsunami and reminding the nation of the ever-present threat of seismic disaster.

Police reported no casualties or property damage, and operators of nuclear power plants and Shinkansen bullet trains quickly gave the all-clear, while the wave hitting the Pacific coast measured just 60 centimetres.

The tremor struck in the late morning about 160 kilometres offshore and 430 kilometres northeast of Tokyo, at a shallow depth of 10 kilometres below the Pacific seafloor, authorities said.

In greater Tokyo -- the world's most populous urban area with more than 30 million people -- the earthquake and a succession of tremors that quickly followed were uncomfortably felt as they shook buildings.

Television channels immediately cancelled their programming to transmit information on the quake and a coastal tsunami advisory.

It soon became clear the quake had left Japan unscathed, but it was yet another uncomfortable reminder that the threat of "the Big One" is a reality of daily life.

Japan, is located on the "Pacific Ring of Fire" and dotted with volcanoes, and Tokyo is situated in one of its most dangerous areas.

The mega-city sits on the intersection of three continental plates -- the Eurasian, Pacific and Philippine Sea plates -- which are slowly grinding against each other, building up enormous seismic pressure.

The government's Earthquake Research Committee warns of a 70 per cent chance that a great, magnitude-eight quake will strike within the next 30 years in the Kanto plains, home to Tokyo's vast urban sprawl.

The last time a "Big One" hit Tokyo was in 1923, when the Great Kanto Earthquake claimed more than 140,000 lives, many of them in fires. In 1855, the Ansei Edo quake also devastated the city.

More recently, the 1995 Kobe earthquake killed more than 6,400 people.