It's plate tectonics, Earthquakes can be cause by ...
Destructive boundaries (where an oceanic plate goes under a continental plate which causes pressure to build up by friction, and when is released, causes an earthquake)
Or Conservative boundaries where 2 plates rub against each other causing pressure to build up and be released as an earthquake.
Volcanos can be made by...
constructive plate boundaries where 2 plates pull away from each other, leaving large cracks in the earth for magma to erupt.
As well as...
Hotspots which are on the plate itself not a boundary.