Does Japan's Megaquake Warning Mean the 'Big One' is Coming?

After a magnitude 7.5 earthquake struck north-eastern Japan on Monday, authorities again warned of the possibility of a future megaquake.

It means that thoughts in Japan are turning to the "big one" - a once-in-a-century quake.

In September, Japan's earthquake investigation panel said there was a 60-90% chance that a megaquake would occur in the Nankai Trough - an area of seismic activity which stretches along Japan's Pacific coast - within the next 30 years.

In April authorities had warned that a megaquake had the potential to trigger a tsunami of more than 20m (66ft) which could hit parts of Tokyo and other prefectures. They predicted that there would be around 300,000 deaths and trillions of dollars in economic damage.

So, what is the "big one", can it be predicted - and is it likely to strike any time soon?

What did the latest warning say?

Officials urged residents in seven prefectures from Hokkaido in the north to Chiba in central Japan to stay on high alert for a potential megaquake.

This is a vast area with millions of people.

A government official said there was a possibility that "a large-scale earthquake with a magnitude of 8 or higher could occur as a follow-up earthquake" in the region.

Authorities also told people to check evacuation routes, secure furniture, and prepare emergency kits, including food, water and portable toilets.

However, an evacuation order was not issued.

Japan's director for disaster management said at a news conference that global earthquake data suggests there's a possibility, not a prediction, of a larger tremor to come.

- Authors: Shaimaa Khalil (BBC News, Tokyo) and Flora Drury (BBC News, London)