Japan's Nankai Trough megaquake - can you predict it?

An alert issued this week has caught the attention of a country used to earthquake warnings.

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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?