What is the point inside earth where rock breaks and triggers an earthquake?

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It along plate margins. Where plates collide, run along side each other or if one subducts under another, If it gets caught/jammed, over time pressure builds and eventually it gives way, BOOM! The pressure is released as an earthquake. I guess inside the earth, the mantle is included but not hugely. Plates subducting might may cause friction to build up and cause the rock to melt and/or when it subducts into the mantle. But as it does, the pressure builds up again which will in tern cause a volcanic eruption. 

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