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