The Earth's surface is covered with thin plate-like rocks called plates. The Earth's surface can be thought of as a jigsaw puzzle of many of these plates.
However, each of these plates is moving in various directions. Even if they are moving, they are moving only 2 to 20 cm per year at most, but it is certain that they are still moving, even if only a little.
Because the plates that cover the round surface of the earth move in various directions, they are bound to collide at some point. Where these plates collide, one plate is squeezed under the other plate.
When a plate moves under a plate in this way, it destroys the rock of the plate. When this happens, the Earth's surface shakes. This is what earthquakes are.
This plate is actively moving directly under Japan. The plate is moving toward Japan from the Pacific Ocean, and it is in the process of sinking into the earth. This is why there are so many earthquakes in Japan.