Answer:
tectonic plates
Step-by-step explanation:
The earthquakes are in general distributed along the boundaries of the tectonic plates. Some tectonic plate boundaries produce more earthquakes than others, as well as stronger ones.
The reason why the earthquakes are roughly aligned along the tectonic plates boundaries is that those places are the most active geologically. The pressure that is created between the plates is enormous. This pressure manages to create big cracks deep into the crust. Once the crust can not bend anymore, it crumbles, and as it does a gap opens up. The opened up gap is than filled in with other crust from above, thus it is a natural way of adjustment.
When the crust adjusts itself, the force created is enormous. This force creates strong vibrations, which are actually what we call earthquake. The place where the earthquake forms in called the hypocenter, while the place vertically above it on the surface is called the epicenter.