Under that layer is the mantle which extends down 1789 miles. The mantle changes in density as it gets further down due to pressure and heat. At the center of the earth is an inner and outer metallic core totalling 2166 miles in radius. A very rough approximation would be a golf ball inside a softball which has a thin film of water on it. We live on that thin film.
Most earthquakes are breaks in the earth's crust, caused by the slow movement of mantle rock.
The mantle is driven by heat generated within the earth's core and mantle, and the very slow flow of mantle rock pushes and pulls the continental plates. These forces cause the plates to crack, which shakes the ground we live on. Thus the causes of earthquakes lay ultimately at the core of the earth. The continents are like rock icebergs afloat on a sea of slow moving magma.