If a magnitude 8 level earthquake occurs along the Nankai Trough — a structural depression between two tectonic plates that run alongside Japan's western prefectures — then the chance of another "Big One" happening somewhere on the trough within a week increases by 100 to 3,600 times compared with normal circumstances, according to a new study by Japanese researchers.
The odds of such twin earthquakes happening within six hours, a day and a week in the region were estimated at 1% to 53%, 1.4% to 64% and 2.1% to 77%, respectively.
Researchers attributed the huge range in probability of a second quake occurring soon after the first to the limited number of historical examples.