Under pressure, rock will melt into a denser than rock magma. If that magma cools, it will expand. The interior of the earth has liquid rock. That magma is cooling. As it cools it expands. As it expands it
Under pressure, rock will melt into a denser than rock magma. If that magma cools, it will expand. The interior of the earth has liquid rock. That magma is cooling. As it cools it expands. As it expands it puts pressure on the overlying material.
That's where deep earthquakes come from.