Thought experiments in combination with analogue models have elucidated that the cause of the east-west contraction and crustal deformation of the Japanese islands in the Quaternary Period is movement of the Philippine Sea Plate, not movement of the Pacific Plate as previously thought.

Conceptual diagram of the dynamic framework in crustal deformation of the Japanese islands

The Japan Trench, the Izu-Ogasawara Trench, and the Nankai Trough are all plate subduction boundaries. In the offing to the east of the Boso Peninsula is the point at which these three trenches meet: the world’s only trench-trench-trench triple junction. Thought experiments on the location of this triple junction over time using the analogue models showed that the east-west contraction of a main island of Japan (Honshu) and inland earthquakes that are a part of the contraction are caused by migration of the location of the subduction of the Pacific Plate (the Japan Trench). Westerly migration of the Japan Trench is caused by movement of the Philippine Sea Plate, and it was concluded that the cause of the east-west contraction is not movement of the Pacific Plate itself, as conventionally thought, but rather movement of the Philippine Sea Plate.

While the east-west contraction in the Quaternary Period created negative natural phenomena such as seismic disasters, it also created the land where the Japanese people live. It has also had a strong impact on the unique culture and aesthetics of Japan. Uncovering that cause goes beyond disaster prevention and geologic issues; it leads to understanding of the history of Japan and the Japanese.

The researcher will show that various phenomena surrounding the Japanese islands can be explained based on the relationship among the northwesterly movement of the Philippine Sea Plate, the migration to the West of northeastern Japan, and the crustal deformation on the eastern edge of the Japan Sea, which has now been uncovered.