What is a megaquake? Japan issues first-ever warning of huge earthquake risk

After scientists issued warnings of a "megaquake" off Japan 's southern coast, the country’s Prime Minister, Fumio Kishida, cancelled his high-level diplomatic travel to Central Asia.

Mr Kishida was scheduled to go on diplomatic visits to Kazakhstan, Uzbekistan, and Mongolia, from August 9 to August 12.

He said: “I have decided to stay in the country at least for about a week to make sure that government measures and communication are fully in place.”

But what is the megaquake warning and has it ever been issued elsewhere in the world ?

What is the megaquake warning?

The magnitude 7.1 earthquake that occurred on August 8 on the western side of the Nankai Trough raised the possibility of a greater one, according to the Japan Meteorological Agency. After the earthquake , the possibility of a magnitude 8 or 9 earthquake connected to the Nankai Trough within the next 30 years is "higher than normal", according to the JMA. This probability is between 70 and 80 per cent.

That is not, however, a forecast that a megaquake would occur at any particular time or place, according to Naoshi Hirata, a seismologist from the University of Tokyo and leader of the JMA experts panel. He asked everyone to continue being watchful and organised.

The Nankai Trough is an underwater trench which stretches from Hyuganada, in the waters off Kyushu's southeast coast, to Suruga Bay in central Japan. It runs along the Pacific coast for roughly 800 kilometres (500 miles).According to JMA, the Philippine Sea Plate there gradually presses down on the Eurasian Plate, causing it to periodically snap back, which could result in a megaquake and tsunami .Over 1,300 people were killed in the 1946 Nankai Trough earthquake, which had a preliminary magnitude of 8.0 and struck off the coast of Shikoku.

A government disaster prevention team stated in 2013 that a 9.1-magnitude Nankai Trough earthquake could produce a tsunami that is greater than 10 meters (33 feet) in height in a matter of minutes. This could result in the deaths of up to 323,000 people, the destruction of over 2 million buildings, and economic losses exceeding 220 trillion yen (£1.1 trillion) for large areas of Japan's Pacific coast.

When was Japan's last major earthquake?

Japan's last large earthquake took place on March 11, 2011, when it struck northeastern Japan causing a tsunami, which in turn led to the Fukushima Daiichi nuclear power plant tragedy.

Although some estimates said the final death toll of the disaster was at least 20,000, the official count for those confirmed dead or reported as missing was approximately 18,500.

What are the most recent megaquakes?

The 2004 Indian Ocean earthquake and tsunami was a devastating megaquake with a magnitude of over 9.1. It claimed the lives of 227,898 people.