#1. Earthquakes are because of the motion of the tectonic plates.
#2 and #3. Scientists know nothing about the Earth’s structure below a certain depth. Everything else is just speculation (but based on current theories).
We tend to give too much credence to “scientific theories.” They are theories, not necessarily facts.
– For example, Newton’s theory of gravitation was supposed to be the truth at one time.
– But now, it holds only under certain conditions, and Einstein’s general theory of relativity is supposed to hold on a large scale.