Where do moving plates happen in?

I recently did a report which included earthquakes, and I did a little research about your question. I'm not sure if what I found is entirely true, but the book I read said that scientists are not really sure. The book said that scientists thought that maybe it came from the mantle, where it is very hot. As the heat from mantle is released, it goes up to the crust in waves. These waves cause the tectonic plates to move with them.