Gravity is another type of indirect evidence that geophysicists use to learn about the interior of the Earth. The force of gravity between two objects depends on the mass of the object and the distance between them. To study Earth's interior, geologists also use an indirect method. But instead of knocking on walls, they use seismic waves. When earthquakes occur, they produce seismic waves. Geologists record the seismic waves and study how they travel through Earth.