Japan, for the first time ever, issued a warning for a "higher than usual" massive earthquake around the Nankai Trough that runs along the Pacific coast.

The advisory -- issued by the Japanese weather agency -- asked citizens to review their quake preparedness and to stay alert for about a week, Kyodo news agency reported.

This came just hours after a 7.1 magnitude earthquake struck southwestern Japan, reportedly causing no injuries or serious damage. But the quake's epicentre, which was at the edge of the Nankai Trough, led the experts to be on heightened alert.

Nankai Trough is an area of seismic activity which stretches along the Japanese Pacific coast.

As per the Japan meteorological agency, at worst, a powerful tremor could shake a wide area of the country. This could be from Tokyo's Kanto region to the southwestern Kyushu region, wherein high tsunami waves could also cover the coastal areas of Kanto to Okinawa.

The Japanese government reportedly has predicted a 70 to 80 per cent chance of 8 to 9 magnitude earthquake hitting the area along the Nankai Trough within the next 30 years, with the likely death toll being at a point up to 3,23,000, as per a 2012 estimate.

Head of the advisory body to the weather agency, Naoshi Hirata -- while assessing the latest quake -- said that the probability of a 'major earthquake' along the Nankai Trough has now increased by "several times" in comparison to the usual level.

The Nankai Trough, notably, is extended at a distance of about 800 kilometres, a point where Eurasian and Philippine Sea tectonic plates intersect, Kyodo news agency said.

Noting that he cannot predict the exact areas that might be the post effected and that need disaster preparedness, Hirata called for caution against any possible situation.

There is no need to refrain from going swimming in the sea, however, Hirata advised beachgoers to check for places to take shelter in as contingency in case a tsunami warning is issued.

Previous earthquakes along the Nankai Trough have left several thousands dead, with megaquakes being recorded once every 90 to 200 years. The last such quake hit struck in 1946.

The weather agency also began a system to release special information about the Nankai Trough earthquake in 2017. This system is reportedly activated when a quake of 6.8 magnitude or above hits areas with megaquake predictions or when plat boundaries witness crustal movements.

The meteorological agency official, Shinya Tsukada, pressed that while "there is a relatively higher chance of another major earthquake compared to normal times" they also did not say that it would for sure happen "in a certain period of time".

Officials have advised people to be cautious but continue doing the works for their everyday lives, while regularly reviewing the evacuation routes and preparedness.