Now, Japan's Prime Minister, Fumio Kishida, has canceled a planned trip to Central Asia after scientists warned the country needs to prepare for a possible major earthquake. After a 7.1 magnitude tremour off the Southern Coast on Thursday, Japan's meteorological agency has issued its first ever mega earthquake warning. Now, residents living in the heavily populated region that faces the seismically active Nankai Trough have been told to be on high alert. Huge earthquakes along the Nankai Trough on Japan's Pacific Coast do occur between every 90 and 200 years, and they're normally accompanied by big tsunamis. Well, to understand a bit more about what a mega earthquake is, I've been talking to Judith Hubbert, who's a geologist specializing in earthquakes, and the author of the Substack Earthquake Insights.

A megaquake is a new term. It comes from the term mega thrust, which is a fault that would rupture to produce a megaquake. It's a big seduction fault, in this case, in the Nancayotra off offshore Japan. A megaquake is just defined as any earthquake here that's greater than magnitude eight.

What then are the dangers in this for Japan, do you think?

Well, we've known for a long time that the Ninkai Trough is capable of producing earthquakes of this magnitude. It's done it about 12 times since the seventh century. A couple of times in the 20th century. It's really something that's been on our radar for a long time. Right now, we know that the earthquake risk is a little bit elevated because there has been some activity And so that's what this advisory reflects.

It's an advisory which has been strong enough for the Japanese Prime Minister to cancel a planned visit to Central Asia, as I say. But is it possible to say how accurate this warning is, whether it could happen?

This is the first time that an advisory has been issued ever. This advisory system was put in place in 2019, and in part, it was inspired by the Tofoku earthquake in 2011, which was preceded by a magnitude 7.2 earthquake. People and the scientists in Japan really don't want to miss that potential warning again. But the reality is that most large earthquakes are not preceded by magnitude 7 earthquakes, and most magnitude 7 earthquakes are not followed by magnitude 8 earthquakes. This is really... It's a warning. Well, it's an advisory. It's not even a warning. We know the risk is elevated, but a large earthquake probably won't happen. It's the best we have.

Judith Hubbert there.