Japan is situated in a volcanic zone on the Pacific Ring of Fire. Most of the earthquakes following that occurred in Japan happened because of a destructive plate boundary. This is when continental and oceanic plates collide and the heavier, more dense oceanic plate sinks underneath the continental one.
Earthquakes are most common at tectonic plate boundaries where different plates meet called fault line. They were thought to not occur anywhere but near plate boundaries.
Japan sits on or near the boundary of four tectonic plates these massive slabs of the earths crust are endlessly slipping locking up then jolting again
