Off the southern coast of Japan, from Shizuoka to Kyushu, there is a massive ocean-floor trench. Along this trench, the Philippine Sea Plate is sinking beneath the Eurasian Plate. As the two plates grind against one another, friction builds and builds ... and builds.
Friction has been accumulating at this unfortunate meeting point, known as the Nankai Trough, for 75 years. History has recorded nine major Nankai Trough earthquakes, over the course of more than a millennium. These are no ordinary shakes, either, and a quake there could directly affect well over a third of Japan’s population, from Kagoshima to Yokohama.