Japan sits on or near the boundary of four tectonic plates: the Pacific, North American, Eurasian and Filipino plates.* These massive slabs of earth's crust are endlessly creeping, slipping, locking up and then jolting again.
I had forgotten a lot of the details, I was sure there was more than one plate involved in Japan but couldn’t remember which ones.
Lots of activity there though, one of the most active spots in the world.