The June 18 thrust fault rupture (where the crust is being compressed horizontally) occurred on the eastern margin of the Sea of Japan in a seismic zone where numerous active faults accommodate the strain of east-west crustal shortening transmitted from the subduction of the Pacific Plate, says Prof. Toda.

During the past 5-25 million years (the Miocene epoch), this region underwent ‘backarc’ extension (stretching), opening what is now the eastern Sea of Japan. Those tensional faults have now been reactivated, with their sense of slip reversed, as thrust faults. Thus, “the hazard of inland large quakes is always high,” Prof. Toda says.