Japan has issued its first ever alert for a possible “megaquake” hours after a magnitude 7.1 tremor struck.

The country’s meteorological agency said the chance of a major earthquake striking near the Nankai Trough is higher than usual, and estimated it could be as strong as magnitude 9.1.

While it said the advisory – which is expected to be in place for a week – does not necessarily mean a megaquake will occur, it urged people in the region to be prepared for evacuation if one strikes.

Fumio Kishida, Japan’s prime minister, has cancelled a planned visit to Kazakhstan over the warning because he said “people would be feeling anxious”.

The megaquake alert was issued just over two hours after the tremor struck off the coast of Miyazaki Prefecture. Three people were reportedly injured.

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What is a megaquake?

While there is no formal definition of a megaquake, they are commonly considered to be tremors with a magnitude larger than 8.0.

These seismic incidents – also called megathrust earthquakes – often come in pairs and occur when one tectonic plate is forced under another.

Japan’s largest ever tremor was in 2011, where a magnitude 9.0 quake struck in the Pacific Ocean 45 miles (72km) east of the Oshika Peninsula.

More than 19,000 people died from the quake and ensuing tsunami, which also caused a major incident at the Fukushima Daiichi nuclear power plant.

What is the Nankai Trough?

Japan – one of the world’s most seismically active nations – has seen several tremors and tsunamis come from incidents in the Nankai Trough.

The 560-mile (900km) subduction zone – where two tectonic plates collide and one is forced beneath the other – is off the southwest coast of the country, and runs from Shizuoka to the west of Tokyo over to the southern tip of Kyushu.

Nankai Trough is above the Philippine Sea Plate, which is subducting under the Eurasian Plate, and has been the site of megaquakes every 100 to 200 years.

The last seismic event from the undersea trough was in 1946, where a magnitude 8.0 tremor killed 1,330 people.

How deadly could a megaquake be?

In 2012, the Japanese government predicted there was a 70% to 80% chance of a magnitude 8.0 to 9.0 quake in the Nankai Trough within the next 30 years.

Estimates from that time held 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 that may reach the country’s coastline within minutes and destroy up to 2.38 million buildings.

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

The meteorological agency then issues an advisory or a warning – where residents in affected areas could be evacuated – within two hours.

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