Japan earthquake: Urgent alert issued over 'MEGAQUAKE' as strong as 9.1 on ​Richter Scale​

The advisory warning will remain in place for a week.

Experts have said that while it doesn't necessarily mean a megaquake will occur, the country should prepare.

Fumio Kishida, Japan's Prime Minister, has cancelled a planned visit to Kazakhstan because he said "people would be feeling anxious".

The warning comes just over two hours after a tremor struck off the coast of Miyazaki Prefecture.

Three people are said to be injured.

A megaquake is commonly considered to be a tremor with a magnitude of more than 8.0.

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Megathrust earthquakes often come in pairs and happen when one tectonic plate is forced under another.

In 2011, Japan saw its biggest earthquake where a magnitude 9.0 quake struck in the Pacific Ocean 45 miles (72km) east of the Oshika Peninsula.

Around 19,000 died as a result of the quake and tsunami which followed.

Naoshi Hirata, a professor at the University of Tokyo and head of the Nankai Trough quake advisory panel, said on Thursday that the megaquake alert is issued when a tremor with magnitude 6.8 or above strikes.

The Japanese government previously predicted in 2012 that there was a 70 per cent to 80 per cent chance of a magnitude 8.0 to 9.0 quake in the Nankai Trough within the next 30 years.

Estimates from that time suggested the number of deaths from such a megaquake could reach up to 323,000.

It was also estimated one could create tsunamis up to 30m tall.