Remember the mantle is hot and the crust sits of the mantle. When the mantle moves the crust moves. When it moves mantle starts to form new crust and the crust is above the water. When the crust moves or slides past each other it bumps each other. What causes earthquake and when the new crust is coming one goes over and one goes under and the under crust will melt. Because the mantle is hot now I will tell you what happens is it`s a content on top.When the crust moves the content moves that what makes a earthquake.