Quakes are extremely hard to predict, but in January a government panel marginally increased the probability of a major jolt in the Nankai Trough off Japan in the next 30 years to 75-82 percent.

The government then released a new estimate in March saying that such a megaquake and subsequent tsunami could cause as many as 298,000 deaths and damages of up to $2 trillion.

In 2014 the Central Disaster Management Council issued a preparedness plan recommending a series of measures that, it was hoped, would reduce deaths by 80 percent.

But the government has said that so far the steps taken would only cut the toll by 20 percent, Kyodo news agency reported, and an updated preparedness plan was issued on Tuesday.

This recommended accelerated efforts including constructing embankments and evacuation buildings as well as more regular drills to improve public readiness.

"It is necessary for the nation, municipalities, companies and non-profits to come together and take measures in order to save as many lives as possible," Prime Minister Shigeru Ishiba told a government meeting, local media reported.

The Nankai Trough is an 800-kilometre (500-mile) undersea gully running parallel to Japan's Pacific coast where one tectonic plate is "subducting" -- slowly slipping -- underneath another.

Over the past 1,400 years, megaquakes in the Nankai Trough have occurred every 100 to 200 years. The last one was in 1946.

The Japan Meteorological Association (JMA) last August issued its first advisory warning that the likelihood had risen but it was lifted again after a week.

- Comic prophesy -

Some foreign tourists have been holding off coming to Japan this summer by unfounded fears spurred on social media that a major quake is imminent.

Causing particular concern is a manga comic reissued in 2021 which predicted a major disaster on July 5, 2025.

Hong Kong-based Greater Bay Airlines reduced flights to Japan because "demand has rapidly decreased", a local tourism official told AFP in May.

The number of visitors to Japan from Hong Kong to Japan fell 11.2 percent in May year-on-year, according to the tourism office.

Those from mainland China soared 44.8 percent, however, while arrivals from South Korea rose 11.8 percent.

"It is impossible with current science to predict earthquakes by specifying the location, time, and magnitude of an earthquake, and to say that an earthquake will or will not occur," Ryoichi Nomura, head of the JMA, said in May.

"We ask the public to take certain steps so that you can cope with earthquakes no matter when they occur. But we also strongly urge the public not make irrational actions driven by anxiety."