The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet. The seismic waves produced by an earthquake are monitored and tracked.
The Earth is almost a sphere. These are its main layers, starting with the outermost:
Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.