That is where the movement occurs!
A subduction zone is where one tectonic plate is sliding under another one. It is not usually a continuous smooth motion but an intermittent cycle of building pressure of one plate against the other and then a slip or movement that relieves the pressure by moving the plates.
This movement is an earthquake. The impact of the pressure/motion waves through the earth actually are felt across the entire planet. But only the areas close to the actual movement feel a distinct – sometimes catastrophic – shock.
Thus the areas near a subduction zone will feel the effects of this movement as an earthquake more frequently and severely than other parts of the world.