What triggers an earthquake at a tectonic plate margin?

Earthquakes are caused by the release of tension that builds up at plate margins. They can be triggered at destructive, constructive and conservative boundaries. At destructive margins, tension builds up as one plate gets 'stuck' whilst moving beneath another plate, towards the mantle. At constructive margins, tension builds along cracks within the plates as they move away from one another. At conservative margins, tension builds up when plates that are grinding past each other get stuck. When the tension at these boundaries reaches a threshold, the plates slip past one another, sending out vibrations that we call an earthquake.