Watch two oceanic plates push together, then see what the collision builds. When plates collide like this, the place where they meet is called a convergent plate boundary.
Cross-section about 350 miles wide and 200 miles deep. Heights are exaggerated so the islands are easy to see. The real motion is slow, about as fast as a fingernail grows (about 2 inches a year), so this animation speeds up time. Yellow dots are places where earthquakes can start inside the subducting plate. Orange blobs are magma.
Predict first
The two plates stick together while they keep moving, so stress builds. When it overcomes the friction, the boundary slips and releases the stored energy as an earthquake. Each whole step in magnitude is about 32 times more energy.
Schematic model for teaching. Not to scale.