Where Earthquakes Occur
Earthquakes occur due to movement of the Earth's crust usually around the boundaries of two tectonic plates. In fact, less than 10% of all earthquakes occur inside plates.
There are three main types of plate boundaries. Spreading zones, transform faults and subduction zones.
Spreading zones occur mostly in oceans. They are where molten rock rises to push plates apart. They usually occur within 30km of the surface.
Transform faults are fault lines where plates slide past each other. Cracks along fault lines cause earthquakes.
Subduction zones are where plates override and subduct each other. When one plate gets pushed down into the mantle, it melts. Volcanos, ocean trenches and deep earthquakes are all signs of subduction zones.
Earthquakes occuring inside plates are much less common, and usually not as strong. these happen when weakened boundaries become part of plate interiors.
A map of the tectonic plates.