Does Japan's megaquake warning mean the 'big one' is coming? 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. AP The last megaquake occured along the Nankai Trough almost 80 years agoJapan is a country used to earthquakes. Earthquakes along the Nankai Trough have already been responsible for thousands of deaths. When authorities issued a megaquake warning last year, after a 7.1 quake struck southern Japan, Prof Geller told the BBC that the warning had "almost nothing to do with science".